Neutra1 Hydrogen in NGC 2613: Probing the Dynamic Gaseous Environment of Spiral Galaxies

Tara A. Chaves

A thesis submitted to the Department of Physics

in conformity wit h the requirements for

the degree of Master of Science

Queen's University

Kingston, Ontario, Canada

Jdy, 2001

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The author has granted a non- L'auteur a accordé une licence non exclusive licence aliowing the exciusive permettant à la Nztional Library of Canada to BIhiiotheque nationale du Canada de reproduce, loan, distribute or sell reproduire, prêter, distribuer ou copies of this thesis in microform, vendre des copies de cette thèse sous paper or electronic formats. la forme de microfiche/nIm, de reproduction sur papier ou sur format électronique.

The author retains ownership of the L'auteur conserve la propriété du copyright in this thesis. Neither the droit d'auteur qui protège cette thèse. thesis nor substantial extracts fiom it Ni la thèse ni des extraits substantiels may be printed or otherwise de celle-ci ne doivent être imprimés reproduced without the author's ou autrement reproduits sans son permission autorisation. This thesis presents a detailed study of the disk and halo regions of NGC 2613. an

edge-on spiral galaxy 26 41pc away. The observations are in neutral hydrogen (HI.

2 1 cm) and were taken wit h the Very Large brray. There are four data sets presented:

one at CnB array, two different sets at D may and one which is a combination of

the CnB and September 2000 D amay data. Globally, it was found that SGC 2613

is an extremely massive galaxy with a nearby companion galaxy (ES0 495- G 017).

NGC 2613 appears to be mildly interacting with its companion galauy, which is seen

by its warped disk and two small tidd tails in D anay.

In the main data set of the CnB array, three high latitude features have been found

aàove the piane, symmetric in projection with three beiow the piane in NGC 2613.

Correlations have been seen in the D array data for some of the features, suggesting

that two of the features seen in CnB array may actually be part of one larger feature

on broader scales. The radio continuum emission shows two large extensions asso-

ciated with a strong continuum source in the disk. These are the highest latitude

discrete features clearly connected to a spiral galaxy yet detected in radio continuum.

Moreover, two of the HI features seen in CnB may show a stunning correlation with

these large continuum extensions, with the continuum ananged on the perimeter of

the HI.

There is a velocity gradient with height observeci for dl six features, where the rotational velocity decreases toward systemic with increasing height above the plane.

This is an important result and it is postulated that the lag is due to gaseous drag iii

hma pre-existing low density hot hdo witk s rotation lower than thst of thdisk.

This halo. if it exists, has an estimated mass of 3 x 10~Mo (assuming a spherical

distribution of radius 33 kpc) and may have formed dong with the disk, out of

the dark matter halo. Other velocity trends that the features share include several

velocity spurs within the features and the presence of a nurnber of high velocity detached components.

It is argued that these features were formed by some mechanism interna1 to the galaxy. Correlated supernovae (SNe) is one possible model, however it has a weakness in that 104 to 105 SNe would be needed at large galactocentric distances to form these features, which seems unreosonable. hnother possible formation mode1 is that of a relativistic jet passing through the IS41 of the galaxy where the jet would have fiareci up when it reached large scaie density perturbations. This modei accounts for the symmetry and large energy requirements of the features, however it also has a weakness in requiring a compact object in this gai-, as well as a jet aligned with the plane of the galaxy. It shoutd be possible to distinguish between these two models by using future observations to search for evidence of star formation at the base of the features or to look for signs of a compact object at the nucleus of the galaxy. ACKNOWLEDGEMENTS

Thanks be to Cod, for calling me to behold the wondea of His creation.

!vIany thanks to my supervisor, Judith Irwin, whose genuine enthusiasm and passion for uncovering the rnysterics of the cosmos is both infectious and inspiring.

1 could not have a stronger role mode1 on the path of science.

Thanks to Kristine Spekkens and Siow- Wang Lee, my sisters-in-science. Thanks to Joey MacMillan, for being the best officemate in the astro wing and for introducing me to StarCraft. To Kathy Perrett, our matriarch, who has always had everything

I've asked to borrow and managed to answer al1 of my pesky questions... Thank you.

Thanks to al1 the astrekids... to the Steves: Butterworth, Bickerton, and Toews. to

Dam SM, Tum Merrail, arid Rupinder Brar, tu Mustapha islid, Jasou hIwhiii1, and Ian Lepage. Thanks also to Mikey Seymour. Thanks to Yahmoud Abunhan...

I miss Our Monday afternoons together.

To Douglas blcXei1, for leading me when 1 was blind ... Thank you.

Thanks to the people of St. George's Cathedral, Kingston, for truly welcoming me in Christ's name. Thanks especially to Father George and Father Kris for their prayers and guidance and for not kicking me out of their offices when I went in shouting about revolution. Thanks also to the tremendous Cathedra1 Choir for calling me away from my desk and allowing me to exercise that part of me that science has no hold on.

Finally, a humble and inadequate thank you to my parents, Salvador and Claudette

Chaves. I certainly would not be who 1 am today were it not for their constant and expansive love, support, guidance and üviog examples of how we should al1 live our lives. Thank you so much, Mom and Dad, so much of what I do is an effort to give back a fraction of al1 chat you have given me. Thanks to al1 my family and friends for their love and support!

Technical StuE Thanks to the telescope operators, especially Greg Taylor, at the

VLA. Thanks to the editorial st& at the Astrophysical Journal. This research has made use of the NASX/IPd4C Extragalactic Database (NED), operated by the Jet

Propulsion Laboratory under contract with SASA, the .\strophysics Data System

Abstract Service (also run by YASA), and the Digitized Sky Survey of the Space

Telescope Science Institute. This research has been supported by the Naturd Sci- ences and Engineering Research Council of Canada ('ISERC). CONTENTS

.. Abstract ...... ii

Acknowledgernents ...... iv

Table of Contents ...... vi

List of Tables ...... ix

List of Figures ...... x

1. Introduction ......

1.1 The Galactic Fountain and Chimney Models ......

1.2 Supershell Formation Models ......

1.2.1 Interna1 Generation Mechanisms ......

1.2.2 External Generation Mechanisms ...... 12

1.3 Motivation ...... 14

1.4 Outline of Thesis ...... 17

2. Details of Observations and Data Reduction ...... 18

2.1 CnB Array Data ...... 19

2.2 May 1999 D .br ay Data ...... 20 Contena vii

2.3 March 2000 D Xrray Data ...... 22

2.4 Combined CnB and 2000 D hrray Data ...... 23

3. CnB Array Results ...... 25

3.1 The HI Distribution ...... 25

3.3 The Radio Continuum Distribution ...... 36

3.3 Analysis of the High Latitude Features ...... 37

3.3.1 The Velocity Fields ...... 37

3.3.2 Physical Parameters ...... 45

4 . May 1999 D Array Results ...... 49

5. September 2000 D Array Results ...... 56

5.1 Xeutral Hydrogen ...... 56

5.2 Radio Continuum ...... 62

6. The Combined Cd3 and September 2000 D Array Data ..... 65

6.1 Xeutrai Hydrogen ...... 65

6.2 RadioContiauum ...... 70

7. The Global Properties of NGC 2613 and ES0 495 G 017 .... 73

8. Discussion ...... 78

8.1 The Global Distribution of Neutral Hydrogen and Radio Continuum

in and hound XGC 2613 and ES0 495- G 017 .-...... 78

8.2 The High Latitude Features .KI and Continuum ...... 83 Contents viii

8.2.1 Velocity Çpurs and homatons Vefoeity Stmctares ...... 85

8.2.2 The Velocity Decline With z and the Inference of a Lagging Halo 87

8.2.3 The Formation of High Latitude HI Features in YGC 2613 . . 93

9 . Conclusions and fiture Research ...... 101

9.1 Major Findings ...... 101

9.2 Possible Future Miork on YGC 2613 ...... 103

A .Radio Telescope Theory ...... 112

.A .1 Single Dish Antenna Theory ...... 112

A.2 Interferometers ...... 117

A.3 Synthesis Imaging ...... 119

.4 .1 Cl~aningand In! Wiighting Within AIPS ...... 121 LIST OF TABLES

1.1 Typical Supershell Parameters ...... 9

1.2 Optically Determined Properties of XGC 2613 ...... 16

2.1 ObseMng and Map Parameters ...... 24

3.1 Parameters of High Latitude Features ...... 48

7.1 Global Properties of YGC 2613 Derived from the Data Sets ..... 76

7.2 Global Properties of ES0 495 G- 017 Derived from Al1 Data Sets . . 76

7.3 Comparison of Global Parameters of YGC 2613 with Typical Values

initsclass ...... 77 LIST OF FIGURES

The fountain and chimney models of galactic circulation . 6

Supershells in XGC 3536 ...... 8

Supershells in NGC 5775...... 8

Simulations of high velocity cloud impacts...... 13

20 cm C and D array continuum maps of NGC 2613 ...... 15

Integrated intensity map of NGC 2613 £rom Bottema (1989)...... 15

Optical image of NGC 2613 and its cornpanion, ES0 496 G 017... 16

The global profile of NGC 2613 from the CnB data...... 26

The global profile of ES0 495- G 017 fiom the CnB data...... 26

The channel maps Erom the CnB may data set ...... 28

The integrated intensity map from the CnB array data...... 30

First moment map of NGC 2613 made from the CnB data...... 32

Position-velocity slice dong the major axis of ZIGC 2613. made from the CnB data...... 33

Fiat moment map of ES0 498 G O17 fiom the CnB data ...... 34

Position vs velocity dong the major axis of ES0 495- G 017 fiom the

CnBdata...... 35

CnB continuum, overlaid on an optical image...... 38 List of Figures xi

3.10 CnF continuum . ove~laidon the moment O mrtp...... 39 3.11 The CnB moment O map. rotated by 23'...... 40

3.12 Position-velocity slices. paralle1 to the minor &S. of F1 and F2 .... 41

3.13 Position-velocity slices of F3 and F4 ...... 42

3.14 Position-velocity slices of F5 and F6 ...... 13

4.1 The global profile made fIom the 1999 D array data ...... JO

4.2 The 1999 D axray channel maps...... 51

4.3 The 1999 D array moment O map...... 52

4.1 The moment 1 map from the 1999 D array data...... 54

4.5 The averaged D array continuum map ...... 5.5

5.1 The global profile of NGC 2613 from the 2000 D array data ...... 57

5.3 The global profile of ES0 495- G 017 from the 2000 D data...... 58

5.3 The channel maps made from the 2000 D array data ...... 59

5.4 The 2000 D array moment O map ...... 60

5.5 The 2000 D array data moment O map overlaid on the CnB array

moment O map ...... 61

5.6 The moment 1 map fiom the 2000 D array...... 62

5.7 The rotation curve of YGC 2613 from the 2000 D array ...... 63

5.8 The 2000 D array radio continuum map ...... 64

6.1 The global profile of NGC 2613 made from the combined CnB and

2000 D array data...... 66 List of Figmes mï

6.2 The global profile of the cornpanion made from the cornbineci data set . 66

6.3 The channel maps for the combined data set ...... 68

6.4 The moment O map from the combined data set ...... 69

6.5 The moment 1 map for the combined data set ...... 70

6.6 The rotation curve for the combined data set ...... 71

6.7 The radio continuum. from the combined data set ...... 72

.A .1 .\ Bat 1 D aperture radiating into the far.fiefd ...... 114

-1.2 The Fourier Transform pairs of the aperture distribution and the an-

gular spectrum...... 115

A.3 The est-west projected baselice ...... 119

A.4 Components of synthesis imaging...... 122 1. INTRODUCTION

Spiral galaxies, like the Universe on the whole, are well structured, dynamical and

beautiful. They consist of two main regions. The first is the disk which holds the

vast majority of stars. Other components of the disk include dust, cosmic rays,

atomic, molecular and ionized gas (collectively known as the interstellar medium or

ISM). The ISM itself is complex and dynamical and is shaped by the star formation

activity in the galaxy. The curent widely accepted view of the structure of the ISM

is the Three Phase Mode1 (McKee & Ostriker, 1977). The ISM consists of three components: the cold neutrd medium (CNM, T - 80 K) which includes the atomic and molecular gas, the warm medium (T - 8000 K) including both the wmneutral medium (WNM) and the warm ionized medium (WIM), and the hot, low density, ionized medium (HIM, T - 5 x 105 K),which takes up most of the volume of the

ISM. The CNM is thought to be coocentrated in elumps or clou&, surrounded by a thin shell of WXM and an outer layer of WIM. The WMand WIM dso exist as clouds or nebulae. These clouds are then embedded in the HIM which permeates the disk. The t hree phases have different vertical distributions as weil. In our galaxy, the moiecular gas has a Caussian distribution with a small scale height of .- 60 (McKee,

1990). The cool atornic gas has been modeled with two Gaussian and one exponential distributions with a combined full width at half maximum of - 300 pc (Lockman & Gehman, 1991). The WSM has bobh a GausMao end euponential component, of

scale heights 250 and 480, respectively. The WM,also known as the Reynolds layer

(Reynolds, l989),is the sum of two exponential distributions of scale heights < 70 pc

and -- 1 kpc. The HIY extends the furthest, having an exponential distribution with a scale height of 3 kpc. For comprehensive reviews on the ISM. see McKee (1990) and Spitzer (1990).

The second major component of a spiral galaxy is the halo, a somewhat mys- terious. huge sphencal region which contains the globular star clusters as well as the undetectable dark matter. The warm medium extends to the base of the halo region (defined to be roughly 1 kpc from the midplane), while the HI41 is found in the disk and the halo. When viewed at radio wavelengths, one can observe the interplay brtwrru the disk aiid hdu regious. Ili yarticular, wLeu observecl in ueutrd atomic hydrogen (HI, u = 1420.406 MHz)', discrete features flowing out from the disk into the halo region have been observed in many star-forming spiral galaxies.

These features Uiclude shells, supeaheLls, holes, worms, and chimneys and were first discovered in Our own Milky Way (Heiles 1979, 1984). Supenhells. or superbubbles, are large expaading shell-like structures in the interstellar media of galaxies, while

and chimneys are longer, filamentary outflow features. The Galactic shells

(i.e. those in the Milky Way) are typically 10 pc to 2 kpc in radii, with expansion velocities of 10 - 24 km s-' and require input energies of 105* - 10% ergs. assuming the shells are inflated by supernovae. HeiIes termed the mast energetic shells as The 21 cm line of KI results hm a transition between the hyperhe structure leveis of the ground state, 2S1/2,F = 1 - 0. This "spin-fiip" transition is a magnetic dipole transition and is termed "forbiddenn since the spin of the eiectron is much more Wyto be changed by collisional excitement rather than spontaneousiy. *supershellsP,having input ene~gies2 3 x lOS2 ergs. The nearest spiral galaxy to

our own. M31, has also been observed to have over a hundred HI holes, which are

interpreted as open-topped shells seen faceon (Brinks & Bajaja. 1986). The fint

galaxy, other than our own, found to have outflows from the disk into the halo in

HI was NGC 3079 (Invin & Seaquist, 1990). The list of galaxies with discrete disk

outflow features is srnail but growing, including NGC 4631 (Rand & van der Hulst.

1993), YGC 5775 (Irwin, 1994; Lee et ai. 2000), ZIGC 1313 (Ryder et al.. 1993,

NGC 3044 (Lee & Irwin, 1997) and NGC 3536 (King & Irwin, 1997).

We do not yet know if such outflows are cornmon to every spiral galaxy or even

to every starforming spiral gai-uy. in this thesis, 1 will focus on spiral galaxies that

have outflows from the disk, without presuming that al1 spirals do. CVhat is the fate

ulgits ~hatLas teeu expelleri from the di&? Either the gas wiii retum CO the disk and

replenish the ISM or it will escape the galaxy altogether and form the intergalactic

medium (IGM). For the moment, let us just deal with a starforming galaxy with

no outtlows from the disk. A typical spiral galaxy will cornpletely deplete its gas supply in approxirnately 3 Gyr, assuming there is no replenishment by any meam

(Kennicutt, Tamblyn & Congdon, 1994). The beginning of star formation in the disk of the MIky Way happened 9.3 Gyr ago2(Carroll & Ostlie, 1996). There must then be some mechanism of gas replenishment. Supernovae may enrich the ISM,just as newborn stars deplete it. When such stellar recycling of gas is included, the gas depletion time for a typical spiral rises to 3-13 Gyr. Sow, if we include outflows of This age is derived from the luminosity distribution of white dwarfstars. The globular clusters in our Gaiaxy are much older, implying they formed at an earlier epoch. 1. Introduction 4 gas from the gaiaxy, these depletion bimes MUbe decreased. Thus recyciing by stars may continue to fuel some galaxies' gas supply but not dl. Generally. the late-type morphology3 galaxies have small gas depletion timescales and a global circulation of gas inflow and outflow must be invoked to explain the fact that we do see such galaxies of older ages with a healthy supply of gas. We will now consider two such galact ic circulation models.

1.1 The Galactic Fountain and Chimney Models

The galactic fountain model. first proposeci by Shapiro & Field (1976) and lster modified by Bregman (1980),suggests a circulation of material from the disk to the halo and back to the disk. The cycle begins with supernova (SX) explosions heating the disk gas. This gas then rises out of the disk where it rernains bound to the galavy as a hot corona (the HIM). As gas nses from the disk, the radial component of the gravitational force diminishes and the gas then Bows radially outward as well as upward. The gas then slows down in accordance with consenation of angular momentum. Neutra1 clouds condense near the top of the corona where thermal instabilities occur. With no pressure support, these donds then fafl bdisticdfy dom and radially inward, retuming to their point of origin in the disk. Bregman's calculations for the Galaxy suggest that gaç must travel up to a height of 5 - 10 kpc and outward to 2 - 3 times its original position before condensation occurç.

The galactic fountain model is modifieci slightly in Xorman t Ikeuchios (1989) The optical morphology of spuai galaxies is classifieci by Jeariy-type" galaxies which have large bulges and smaii, clm1y knit spiral arms and "late-typen gahcïes, which have smd buiges and large, distinct spiral arms. The late-type spirals generalty have bigher star formation activity than the eariy-types. The terms "earlyn and %ten were used by Hubble in his original classification scheme but have no reievance to the actual evolution of galaxies. chimney mdel. In ahis model, elustered supernovae @Xe) from OB a99ociations

(loosely clustered Young, massive O and B stars) create superbubbles of hot gas which expand rapidly and break out of the disk. Vt'hen this "blow out" has been achieved. a chimney structure is created which allows mas, energy. momentum and magnetic flux to be convected upward into the halo. Thus, gas escapes the disk in a highly concentrated flow through a chimney rather than over the entire disk, as in the fountain model. The high latitude escaped gas then cools and condenses into clouds which rain back down on the disk. 'lote that in this model, Norman &L

Ikeuchi are studying discrete outflows on smaller scales than Bregman, and so they predict that chimneys eject material straight upward and they do not note any radial migration effects in the ejected material. The clouds then fa11 back down at some other point in the disk which chen might cause a burst of star forniatiou (SFj there and the process could begin again.

Figure 1.1 illustrates the differences between the galactic fountain and the chimney models. In the chimney model, the HIM is localised and distributed mainly dong the disk. whereas the HIM is uniformly distributed in the fountain model. Norman &

Ikeuchi (1989) then suggest that the ISM of galaxies may be structured in two-phase

(cool and wa.rm cornponents only), three-phase or chimney models, depending on the star formation activity of the galaxy. For example, the ISM of a galaxy undergoing active starbursting or nuclear activity may be in the three-phase stage with the HIM having a large fiuing factor, whereas a low-activity galaxy might be in the two-phase stage with a very small hot component. 1. lntrod uction 6

Figure 1.1: Two models of galactic circulation. (a)The galactic fountain mode1 (Bregman, 1980) where hot gas (dashed Lines) traveis upward and outward throughout the disk, then condenses into clouds which fall back to thek place of origin in the disk. (b) The chimney mode1 (Norman & Ikeuchi, 1989) where hot gas escapes the disk t hrough localiseci chimneys. The gas t hen condenses into clouds which min down randody on the âisk, possibly causing more localised SF and additional iihimneys to be formed. 1. Introduction 7

The largest and most energetic of the disk-halo connection features seen thus far

have been HI supershells. NGC 3536. an isolateci galaxy, has two large supershells at

similar galactocentric distances, one at either end of the galauy. These features can

be seen in Fig. 1.2 while Table 1.1 lists the properties of these supenhells. Another

galaxy with prominent supershells is NGC 5775, a starburst and interacting gaiauy,

shown in Fig. 1.3. The three largest features are labelleci and their parameters are

also listed in Table 1.1. Notice how F1 and F2 seem to be paired symmetrically

across the midplane, while F3 lies at a similar galactocentric distance as FI. It is

interesting to note t hat Irwin (1994) did point out the possible existence of a fourth

feature, across the plane from F3, but it was not parameterized further because of

its proximity to the southem bndge connecting NGC 5775 to its nearby cornpanion.

As seen from Table 1.1, these features require extremely large input energies. on the order of lOs5 to 10~~ergs. The mystery then lies in Bnding the mechanism that could be powerful enough to expel the gas out of the disk with these energies. These models faIl into two general categories: mechanisms intemal to the host galq and rnechanisms wit h external sources. Figure 1.2: Superposition of two HI chameh of NGC 3556 taken fkom King & hin(1997) at velocities of 633 km s-l (east side) and 757 km s-' (west side). (a) shows the HI emission in contours (levels at 0.9, 1, 1.25, 1.7, and 3 mJy beam-' ) and a truncated gray

Figure 1.3: Total HI intensity rnap of NGC 5775 (edge-on) and NGC 5774 (Eace-on to the LW).The three largeat features are marked F 1 through F3. Contour levels are 1, 5, 10,20,40, and 80 x 1020 cm-*.The beam is shown in the lower left. The image is fiom Ii.win (1994). 1. Introduction 9

Table 1.1: Typicai Supershell Parameters

Feature Ka Rb nic T~ Ei, km s-l kpc cm-3 x 107 yr x 10'' ergs YGC 3556 LV loop 41.4 1.9 0.26 4.4 3.4 E loop 1 3.2 0.26 6.0 26.0 XGC 5775 Fl 41.1 2.2 0.05 2.6 2.4 F2 52.2 2.0 0.08 1.9 4.2 F3 52.2 1.7 0.14 1.6 4.7 " Half of the velocity range over which the feature is observed. Radius of the feature. Density in the disk below the featiire. prior to the formation of the shell. This quantity was determined from the mode1 of Irwin St Seaquist (1990). The lifetime of the feature. given by r = R/L;,. One time input energy given by

(Chevalier . 1974).

1.2.1 Internai Generation Mechanisms

Current 1- the most widely discussed interna1 generation scheme is t hat of mu1 t iple supernovae. In this rnechanism, many SNe. concentrated in one area in the disk. with the help of stellar winds could inflate shells in the ISM. These shells would then expand and may achieve blowout when they cross the disk-halo interface (eg. Lehnert

S; Heckman 1996). This mechanism implies that a shell is the natural outcome of an

08 association. As well as being spatially correlated, the SNe must be ternporally correlated in order to produce a large coherent structure. In addition. a supemoia- star formation-supernova "chah reaction" is also possible, where the shock front of a

SX compresses the surrounding material enough to induce star formation. It has also 1. Introduction IO been shown that propsgttting Slje coutd presem the mtegrity of the superbubbk in the presence of shear (Frei. 1997). This helps to extend the effective size of an OB association. The main difficulty the SNe model faces is its failtire to produce the energies required for the largest structures observed. -4 typical SN explosion reieases

.- 10" ergs of energv. Comparing this with the input energies in Table 1.1 shows that t hese feat ures would require 10" - 105 OB stars. a sornewhat unrealistic expectation.

Super star clusters (generally found in the nuclear vicinity) in some starburst galaxies contain 10' OB stars (bleurer et al.. 1995). In the cases of NGC 3356 and NGC 5775. the supershells have formed at large galactocentric distances and would require large

OB associations at the estremes of the disk. Another difficulty with the SNe model is the apparent lack of correlation between starforming regions and some high latitude

HI: fcaturcs or in-disk HI holcs (see esamples in Efreniuv et al. 1998).

Hypernovae. the results of collapses of rapidly spinning stellar mas black holes

(Paczynski. 1998) which rnay lead to gamma ray bursts. are theorized to expel as much as 10" ergs of energy per event though the exact value is not ive11 known.

Although these energies suggest hypernovae might be a better candidate for inflating supershells (Perna Sr Raymond, 2000). their properties, if they exist at all. are not well known and they are relatively rare, occurring 10" - 10' times Iess frequently than supernovae.

Slagnetic fields could substantially assist SNe in ejecting gas into the halo via

Parker Instabilities (Kamaya et al., 1996). In this case, supernova explosions disturb the parallel component of the magnetic field of the gdaxy, causing the fields lines to undulate vertically. The gas then responds by siiding dom the crests of the field I. Introduction 11

kines to the tmughs. rnagnfing the ci-isturbance. This couhi cause the field Iines to

become buoyant in the IShI and expand out of the plane in loops. possibly pulling some of the partially ionized gas and dust up into the halo. It is possible For blowout

to occur in this way.

Another interna1 generation mode1 has recently been proposed by Gopal-Krishna

9- Invin (2000). In this scheme. sets of HI features, synmetric about the galactic centre, could be inflated out of the disk by the localised fiaring of a pair of radio lobes ejected from the nucleus. nearly parallel to the plane. These features would then remain visible after the jet has faded amy (- 10' pars). This mode1 requires that the galauy host a compact object that has ejected a jet at a srnail angle to the midplane. It has recently been determined from sarnples of Seyfert galaxies that jet; are randunil! urieiited with respect tu the galactic disk (Nagar k Wilson IY'J'J.

Kinney et al. 2000) rather than perpendicular to the disk. as previously thought.

Furthermore, there are several examples of spiral galaxies hosting jets that are apparently interacting with the ISSI. For example, YGC 1068 shows a radio lobe with a bow-shock morphology in the disk. coinciding with its jet (Wilson S; Cilvestad, 1987).

SI51 (Ford et al.. 1983) and XGC 4258 (Cecil et al.. 2000) both show two suggested

*'bubbles" paired across the nuclei in the direction of the jets and these regions also show signs of shock-excited optical emission Iines. The shocked regions in these galaxies then suggest that the jets might be plowing through the ISàI in the galactic disks. This mode1 naturally meets the large energy requirements and accommodates the large galactocentric distances of some HI supershells and their smmetry about both the major and minor axes. However, this mode1 too has a shortfall in that it 1. Introduction 12 rernains a statistical rarity to observe such preeise alignment between the jet and t fie disk.

1.2.2 Externd Generation Mechanisms

The external generation scheme involves the impact of high velocity clouds (HVCs) ont0 the disk (see Tenorio-Tagle Sr Bodenheimer 1988). In al1 cases. the cloud collides with the disk. sending one shock front down into the disk and one up through the cloud. The iipward rnoving shock front will sweep up the ISM and spherically erpand out of the disk. possibly leading to a high latitude shell structure. Depending on the cloud's velocity and density. the stmuiûtions of Tenorio-Tagle Sr Bodenheimer

(1986) show that the cloud may pass through the disk and create another high latitude feature on the other side. However. Santillan et al. (1999) find that when the magnetic field is taken into account. it creates an added pressure which will not allow an average cloud to penetrate the disk (see Fig. 1.4 for an example). Apparent associations of HVCs with HI supershells in Our own galaxy gain merit for this model. This scheme also eliminates the energy problem since the infalling cloud can have an arbitrarily large mass. However. this mechanism irnplies the presence of an interaction between the parent and a cornpanion galaxy, or at least the close proximity of sufficiently massive clouds, and cannot account for supershells in isolated galaxies such as NGC 3556 (King L Irwin, 1997) and NGC 3044 (Lee & invin, 1997). 1. Introduction 13

Figure 1.4: MHD simulation of a cloud 210 x 105 pc with a mass of 3.5 x 105 M,? and an infall velocity of 200 km s-~,impacting perpendicular to the galactic disk. The magnetic field Lines (shown as thin soiid Lines) are pardlel to the midplane and the field at the midplane is 5 pG. The density of the disk material is shown in gray while the velocity fields are represented by arrows. The midplane is located at z = O and the distance between the tick marks is 500 pc. The system has been grown adiabatically and is shown at 3.2,6.3, 9.5, and 15.9 Myr. In this simulation, then, the collision does not pas the midplane and an outflow resuits instead as the compressed field Lines flatten out again and propel material upward. This image is from Santilh et al. (1999). 1. Introduction 14

Motivation

Although high latitude structures have been observed in every ISbI tracer. HI obser-

vations provide detailed velocity information which is crucial to the understanding

of the dynamics and formation of t hese features. Discrete outflow features, such as

supershells and chirnneys. are best explored by observing edge-on spiral galaxies so

that the details of the features are not confused with the disk material.

In an effort to identify galaxies with disk-halo connections. Invin. English Sr

Sorathia (1999) found in a Very Large Array (VL.4) radio continuum' survey of

16 edge-on galaxies that every galavy (except one) showed evidence for kpc-scale

high latitude emission. NGC 2613. a highly inclined spiral galaxy 26 Mpc away. was included in this survey Fig. 1.5 shows their 20 cm continuum rnaps. Early work by Bot tema (1989). using the Westerbork Synthesis Radio Telescope (WSRT). showed evidence in HI for two large features well above and below the plane in this galauy. which he thought were rotating along with the disk gas. Figure 1.6 shows his integrated intensity map.

This prompted us to observe NGC 2613 in HI using the VLA in the hopes of seeing large supershells. It was hoped that the sensitivity of the VLA would provide a bet ter signal-to-noise ratio than that avaiIable with the previous Westerbork data and allow us to pick up broad scale structures at low intensity levels. An optical image of XGC 2613 is shom in Fig. 1.7 and Table 1.2 lists its basic properties.

NGC 2613 is in a small group of 4 galaxies (Sulentic & Tint, 1973). "The radio continuum ernission in spiral galaxies results mostly Erom non-thermal synchrotron radiation from highly accelerated electrons spiralling around magnetic field lines. A srnaii fraction is also from thermal Bremsstrahlung emitting particles in the disk. 2. introduction 15

Figure 1.5: 20 cm C array and D array continuum maps of NGC 2613. taken from Irwin. English Sc Sorathia (1999). The C array data have contour levels of 0.134 x (1, 2. 4. 6, 8: 10, 12, 14, 16. 18, 20, 22, 24, 26, 28, 30) mJy beam-'. The D array data have contours of 0.246 x (1, 2, 3, 5, 7. 10. 15, 20. 25, 30, 35, 40, 45. 50, 55. 60) mJy beam-l. Note that in both maps, the lowest contour level is at 20 and the beams are shown in the lower left of each image.

Figure 1.6: Integrated intcnsity map fi-om Bottema (1989) superirnposed on an optical pbotograph (J-band).Contours range from 7.8 to 46.8 x 10'' cm-2 in steps of 7.8 x loL9cm-*. The notable high latitude features are at a = 08~31~15~and 6 = -22O47' and a = 08~31~05~and d = -22'51'. 1. Introduction 16

Figure 1.7: Optical image of NGC 2613 and its cornpanion, ES0 495- G 017. takeo Grom the Digitized Sky Survey (DSS) of the Space Telescope Science Institute. Note that at a distance of 25.9 Mpc, lf'=126 pc.

Table 1.2: Opt ically Determined Propert ies of NGC 2613 Property Valuea Opticai centre (J2000j: Right Ascension (h ') 08 33 22.6 Declination ( " ' ") -22 58 19.1 SIorphoIogy SA (s)b ~istance~(SIpc) 25.9

Position angle (O) 133 Optical major avis (') 7.2 Optical minor avis (') 1.8

Maximum inclinationC(O) 85

Minimum inclinationd (O ) 76 Propert ies taken from the 'rlASA/IPAC Extragalactic Database (XED),unless otherwise noted. The distance was calculated using the recessional velocity relative to the 3 K background, as stated in de Vaucouleurs et al. (1991), and Ho = 75 km s-1 Mpc-'. Since the value of is not well known, this implies some error in the distance which is propagated through in any calcula tion involving the distance. Taken from Tully (1988), which as sumes a thick disk (c/a = 0.2). Assumes a thin disk (cos i = bla). Chapter 2 contains the details of the three observation runs and the data reduction

performed. The main results. that of the CnB array data. are presented in Chapter 3.

including the calculated global parameters of the galaxies and a detailed analysis

of the high latitude features found. The 1999 set of D array results are covered

in Chapter 4, while the 2000 set of D array observations are found in Chapter 5.

Chapter 6 shows the results of the CuB array data combined with the 2000 D array

data. Chapter i then sumrnarizes the global parameters calculated From each of the data sets. Chapter 8 contains a discussion of the results, concentrating ori the vclocity

fields of the high latitude features as well as possible methods of t heir formation. The

major conclusions of the research rs well as possible future work on NGC 2613 cm

be found in Chapter 9. 2. DETAILS OF OBSERVATIONS AND DATA REDUCTION

.\II data were obtained using the Very Large Array (VLA), operated by the National

Radio ;\stronomy Observatoryl near Socorro, 'iew Mexico. The VLA has 27 anten-

nas arranged in a Wye pattern. The antennas are movable. allowing for four main configurations. .A array (highest resolution with a maximum antenna separation of

36.4 km) through D array (lowest lesolution with a mauimiirn antenna separation of 1.03 km). The CnB array refers to a hybrid C configuration where the north arm is in B configuration while the SE and SW arms remain in the C configuration: hybrid configurations are useful when observing low declination sources. For more information on interferometric techniques see Appendiv A.

In order to observe using the VLA. 1 had to wnte an observing program.* which gives the telescope pointing and timing information. prior to the observation runs.

The actual data were then taken remotely; obsenring runs were monitored by telescope operators at the VLA. The data were then saved to exabyte tapes and shipped to me. 1 reduced the data at Queen's University. as descnbed below. The National Radio Xstronorny Observatory is a facility of the Xational Science Foundation operated under cooperative agreement by Associatecl Universities, Inc. I did not mite the May 1999 observing program. 2. Details of Observations and Data Reduction 19

The main data set was taken with the VLA in the CnB configuration. The largest

angular scale detectable by this array for the L (20 cm) band is 15' ? The obseriing

took place over three days. Each session consisted of 12 or 14 scans, 2 minutes in

length, of the phase calibrator. 0837-198 and 10 or 22 scans of YGC 2613 in 30

minute intemals. as well as two 5 minute scans of 3C 147 and one of 3C 286. the flux and bandpass calibrators . On-line Hanriing smoothing was appliedJ. Each data point. called a visibility, gives a Ruand phase value (see Appendix A). For strong point sources. such as the calibrators. the fluxes should remain constant for a11 times and baselines while the phases should remain at zero degrees. Data points in the calibrators are then deerned unsatisfactory if there are high amplitudes or phases.

For this data set, calibrator visibility fluxes with discrepancies > 15% of the average calibrator flux and phases higher than 10" were edited out. The Rux data were then calibrated against the known calibrator fluxes. .A bandpass calibration \vas also carried out. A spectrum was plotted next, to determine the Iine-free channels. The line-free channels (excluding the noisy end ones) were averaged together to form a continuum data set and this was then subtracted from the original cube to give a line-only data cube. This cube was t hen CLEXNed (Clark, 1980) tvit h bot h a natural and uniform Ll; weighting (see Appendix A). Al1 maps presented here were made The VLX is an interferorneter and so is limited to angular scales allowed by the given range of fringe spacings, which in turn is restricted by the dlowed baseline lengths. The angular resolution is 8 = XID. where D is the baseline and X is the observing wavelength. Thus the resolution will have a maximum and a minimum value given by the minimum and maximum baselines. respectively. This is a smoothing done in frequency space, where a charnel and the channels on either side of it are convolved with a triangle. This is done to smooth out ringing effects which are a result of the hite time lags used to form the bandpass. 2. Details of Observations and Data Reduction 20 from the data cube CXXANed wiah rt UV taper of 10 kA with naturd weighting? as this weighting best brought out the interesting broad scale structure. Table 3.1 gives a summary of the observing and rnap parameters for al1 of the data sets used in t his work. Calibrations. editing and imaging were performed wit h the .-\stronomical

Image Processing System (AIPS. release lJAPR99). running under Linitx. on an

Intel-based personal cornputer. Some images were also generated by WIP (Morgan.

1995). Please note that throughout this thesis. al1 of the maps were made frorn data sets that were uncorrected for the prima- beam (see Appendix A). hoivever al1 calculat ions were made from data cubes wit h the primary beam correct ion applied. unless otherwise stated. This is becaiise the correction for the primary beam is required in order to measure accurate flu but also makes for poorer map displays due to the increase in the noise !crcl may from thc ccntrc of t hc ficld.

2.2 May 1999 D Array Data

The next tnro data sets were taken with the VL.4 in D configuration. The largest detectable angular scale for D array is also 15'. The sarne calibrators used in the

CnB observations were used in this observing session as well. However. this obsefving run was much shorter. consisting of 4 scans of YGC 2613 and 0837-198, one scan of 3C 147 and two scans of 3C 286. The May 1999 D array data set was plagued with problems. The first problem was high Ninds during the observation which caused several antennas to be stowed and data to be lost. The next major problem was a short bandwidth. The whole band obsemed (32 frequency channels) covered the line spectrum. meaning there were no line-free channels from which to extract 2. Details of Observations and Data Reduction 21

the continuum radiation. This made continuum subtraction very difficult. 'iIany

approaches were tried to get around this problem. The most successful method will

now be discussed. It was noted that in the first channel of the line plus continuum data cube. the HI emission msconcentrated on the east side of the galaq-. displaced from the continuum emission which was concentrated in the center of the galauy.

Similarly. the HI emission is displaced to the west of the continuum emission in the last few channels. Therefore. taking the west side of channel 1 and the east side of channel 30" allowed a rough determination of the continuum emission. This emission was theri mapped and averaged with an independent D array continuum rnap (from

Irwin, English Sr Sorathia. 1999). Both maps were smoothed to the sarne beam size for proper cornparison and al1 negative fluxes were blanked out. The flux of the blanked continuum rnap averaged hmchannels 1 and 30 agreed with the flux of this rnap averaged with the independent continuum map. within errors. The dirty6 map of the total averaged continuum rnap was then subtracted from the dirty line plus continuum cube. leaving us with the desired line-only data cube. This dirty cube was then cleaned. under a natural UV weighting. This process introduces an additional 0.55 mJy beamdl into the rms noise of the clean line-only map. This error was determined by subtracting the clean 1999 D array continuum rnap from the clean independent D array data rnap and rneasuring the resulting rms.

There was one other problem with this data set. There is a low level stripe running through the images. The stripe should have been eliminated during the WhanneIs 31 and 32 were too noisy to be usefui. See Xppendiv A. 2. Details of Observations and Da ta Reduction 22

editing protess but was not. The cauve of the sbripe was not known at the tirne but

the same problem arose in the next data set and is discussed in the next section.

The value of these data is questionable and its inclusion in this thesis is meant to serve as a qualitative cornparison to the CnB array data only.

2.3 March 2000 D Array Data

The second D array observing run consisted of one scan of the phase and Rus calibrator 3C4S. followed by 7 scans of 3 minutes each on the phase calibrator. 0837-198: alternating with 6 scans of NGC 2613 in - 25 minute slots. The reduction process was similar to that of the C array data however and the above mentioned stripe in the previous D array data resurfaced in this data set. It ivas foundi that the striping was due to solar interference. The Sun was 95% and 5Ooï away from NGC 2613 in the sky over the entire course of the 1999 and 2000 D array observations, respectively

This is not normally problematic at the VLA. however we are currently in a year of maximum solar activity. Solar interference caused the phase information from the small baselines to "wind" or rapidly increase and cycle through from -T to ?r. Inter- ference from the active sun particularly afFects the 20 cm band at srnall baselines at the VLA. This effect is more pronounced for smaller baselines since the Sun takes up a large angle of the sky. Larger baselines are less affected as their resolution is better and time and bandwidth averaging effects tend to smear out the interference. This explains why we only saw this effect in D array and not in C array. Therefore. the data derived from the inner UV plane in the 2000 D array set were examined very ' SIanj- thanks to Greg Taylor at the \LA! 2, Detaiis of Observations and Da ta Reduction 23 carefully and a- phase winding seen in the source (NGC 2613) as mit as phases greater than 20" in the phase calibrator were edited out. 'iote that this editing was in addition to the usual editing out of abnormally high amplitude data.

It is important to note that different source structure can be highlighted bv different synthesized beams. UV weightings. and shortest projected baselines. Thus. two different observations taken at the same VLA configuration might show different

Features of the source being observed.

2.4 Combined CnB and 2000 D Array Data

In addition to iising different UV weightings to bring out source structure. one can combine data taken at different configurations. This has the advantages of providing an increased number of visibilities. which should reduce the rms noise. as well as creating a different synthesized beam, which may reveal different structure in the source. To this end. I have combined the CnB array data with the September 2000

D array data. The data were combined in the uv plane (before any imaging was done) by the AIPS task. CONCAT. No extra calibrations or editing were done as each data set was individually celibrated. This new data set wm then cleaned under a natural UV weighting. The data used in the cleaning process were constrained to baselines less than 5 kX (the maximum baseline length in the D array). This does reduce the rms noise to 0.45 mJy beam-l, a factor of tivo smaller than the 2000 D array data but it is still larger than the C array noise level (see Table 2.1). 2. Details of Observations and Data Rednction 24

Table 2.1: Observing and Map Parameters Parameter CnB 2999 D 2000 D CnB + 2000 D Observing date 2000 Mar 4. 6. 8 1999 May 3 2000 Sept 15 On source timea (hours) 15.3 1.1 2.4 17.7 Synt hesized beam (") 26.1 x 20.2 95.0 x 50.0 96.5 x 42.9 47.1 x 32.1 9 - at position angle (O) - r .3 -6.1 -21.G -8.2 Number of veIocity channels 63 32 63 63 Velocity resolution (km s- ') 20.85 20.84 20.84 20.84 TotaI bandwidth (MHz) 6.250 3.125 6.250 6.250 RMS noise (mJy beam-') 0.32 1.22 0.93 0.45 RMS noise (K) 0.37 0.16 O. 14 O. 18 a Before editing. 3. CNB ARRAY RESULTS

3.1 The EU Distribution

The global profile, which is a plot of total flux density versus heliocentric velocity. for NGC 2613 is shown in Fig. 3.1. The error bars on the fluxes are given b-:

as, = (N)Lb where 6Sv is the error in flux density N is the number of beams across the measured area and a is the root mean square noise (rms, 0.41 mJy beam-' in this case). This error arises from the flux being integrated over the beam area and will be different for different sizes of measuring area. The "double-homed shape arises from the manner in which gas in a differentially rotating disk sums into different velocity intervals. The profile shape is typical of inclined spiral galaxies. The profile for the cornpanion galaxy. ES0 495- G 017. (Fig. 3.2) is much less defined as the beam size is comparable to the size of the galauy itself. These profiles were made from the tapered, natural weighting data cube after a correction for the prirnary beam was applied. '

The line-only channei maps can be seen in Fig. 3.3. HI emission in NGC 2613 was detected from 1360.3 to 1985.6 km sel. The eastem side of the galaq is redshifted Note that the primary beam correction increases the noise level with increasing distance from the obsening centre. Figure 3.1: The global profile of NGC 2613.

Figure 3.2: The global profile of ES0 495- G 0 17. ( receding), while the western side is bluesbifted f app~oaching},inditat ing that the

galaxy is rotating clockwise, relative to an observer above the galaxy. Due to the

tilt of the galaxy (with the south side closer to us), any spiral arms that might exist

would have an inverted integral sign shape. The companion galauy, which can be seen to the northwest of NGC 2613, was detected over a velocity range of 1464.4 to

1389.3 km s-'. This companion galaxy is blueshifted with respect to the systemic velocity of NGC 2613 and is on the blueshifted side of the galauy indicating its motion is prograde. There are some high latitude features noticeable in NGC 2613

(ie. see u = 1923 km s-' at a = 08~33~40~and 6 = -22'58') as well as a possibly detached feature between the two galaxies at G = 08~33~05~and 6 = -22O5.6' in channeis u = 1401.9 km s-' and u = 1382.1 km s-'.

Fig. 3.4 shows the intcgûtcd intcnsity (zcroth marncnt map) of bath ;\'CC 2613 and ES0 495- G 017. overlaid on an optical DSS image. The integrated intensity,

S I,dv in mJy km s-', can be converted to a column density. !VH in cm-*. via the formula:

where 0' and O2 are the beam dimensions in arcsec and v is the centre of the frequency band in Hz. We note the presence of three features on the north side of

ZiGC 2613. symmetric (in projection) with three features on the soutb side, as well as the interesting curvature at the eastern tip of the gal. These features (except for the eastern tip curvature) are labelled FI through F6 for clarit. This map also shows that the HI occurs in a ring with a deficiency in the central region of the Figure 3. gai- There is no detectable warp in the disk in these data. The north part of the companion's HI distribution also shows a small extension. There are several HI peaks in the moment O map that form an interesting arc or Stream around the two galaxies. far from the disk (see 6 = -22'52' to b = -22O5.L'). Such arcs might be produced by beam sidelobes which are not cleaned correctly. It was found that each of the peaks corresponds with a sidelobe peak in the beam in a few channels. although it should be noted that the sidelobe pattern is not oriented in the same sense as the arc seen in the moment O map. These peaks occur at the second sidelobe. which has an intensity of 10% of the central peak. before cleaning. The iincleaned sidelobe level would be 3 times the rms noise per channel and would be less after cleaning. The three peaks at a = 08~33~35'and 6 = -22O52'. a = 08~33~05~and

6 = -22O54'. and a = 08~33~10~and 5 = -23'03' can clcar!:; bc idcntificd in at least two consecutive channel maps at the 2a level. The others cannot be seen in the channel maps or are seen in only a single channel. We cannot clearly attribute the arc to rnap errors but given the low intensity of the features, we will treat this arc as a tentative detection only Further observations will be needed to verify whether these features are real.

The velocity field or first moment map of NGC 2613 is show in Fig. 3.5. The contours are levels of constant line of sight velocity. For an inclined spiral galaxy? one would expect isovelocity contours to curve away hom the centre on either side of the disk and remain straight in the centre, since for any given line of sight the largest velocity component will be from gas lying dong the major auis. As t his is precisely what is seen, there are no obvious signs of any perturbation in the velocity field on OB3400 33 45 30 15 RIGMASCENSION (52000)

Figure 3.4: Moment O map. overlaid on an opticai DSS image. Contour Ievels are at 0.3. 0.7. 1.4. 2.3. 2.9. 4.4, 8.8, 14.6, 17.6, 20.5, 23.0. 26.6. 29.3 x 1oZ0 cmA2. Note that the contours decline toward the centre of the galaxy. 3. CnB Amy Results 31

the scales debectable by t his arr- Xote. hotvever. tkat the kinernatical minor axis

(meaning the contour corresponding to the systemic velocity or zero rotation) is not

perpendicular to the kinernatical major avis (the line that would connect points of

maximum curvature of the contours levels) which could indicate an oval distortion

in the disk (possibly a bar) or a warp in the disk.

Fig. 3.6 shows the rotation curve. a plot of position vs. velocity taken through a

Y.'j wide slice dong the major ais of XGC 2613. The rotation curve of the galaxy

is very well behaved. The solid body portion. in which the rotation velocity is

proportional to the radius. is well defined out to a radius of about 13 kpc. while the

curve then becomes flat. indicating differential rotation of the disk. out to a radius

of 38 kpc. slightly dipping at the ends. What is particularly impressive is the large

velocity spread of the gcrlxq (about 700 km s-'), indicating the galasy is iiiiüsive since a larger mass is required for the gravitational force to balance the centripetal

acceleration due to the large rotation velocity.

The moment 1 rnap of ES0 495- G 017 is shom in Fig. 3.7. The eastern side of the galauy is receding and the west approaching. Therefore the companion is

rotating in the same sense as NGC 2613. Interestingly, it would appear as though the

northeastem feature (noted in the zeroth moment rnap) is blueshifted with respect

to the underlying gas and yet is on the redshifted side of the gala-. It is not clear whether this feature lies in the plane or above it or whether it might be a tidal relic

from an interaction with YGC 2613. Fig. 3.8 shows a position velocity slice dong

the major axis of the companion. We note an asymmet~,in that most of the gas lies on the blueshifted side of the galaxy. The gas on the redshifted side (closest to Figure 3.5: Moment 1 map of NGC 2613. Contour levels are as indicated. Units are km s". The beam is shown in the lower Left corner.

XGC 2613) may have been stripped due to tidal interactions with NGC 2613. I I - 1 I I I I 1 I 30 200 100 O -100 -2m 300 Distarice 9- centre (arcsec)

Figure 3.6: Position-velocity slice dong the major axis of NGC 2613. The contours are at 0.64 (201, 1. 2. 3, 5. 7, 10, 13, 17. 20. 24 mJy bearn-L and the gray scale ranges from O to 24.26 mJy beam- l. The velocities are relative to the systemic velocity with negative values indicat ing a blueshift (i.e. the approaching side). Note that at 25.9 Mpc. 1" = 126 pc. Figure 3.7: First moment map of ES0 495 G 017. Contour levels are as indicated. in km s-l and the beam is shown at lower Ieft. Figure 3.8: Position vs velocity dong the major axis of ES0 495- G 017. Contour levels are at 0.64 (20): 1, 1.5. 2. 2.5. 3, 3.5. 4. 4.5 mJy beam-' and the gray scaie ranges kom O to 4.8 mJy beam-l. The systemic velocity is drawn as a horizontal line. 3. CuB Array Results 36

3.2 The Radio Continuum Distribution

Yearly al1 of the radio continuum emission in a spiral gala~yis due to non-thermal synchrotron radiation of relativistic elect rons spiralling around magnet ic field iines.

Wit hout knowledge of the spectral index of the continuum emission in this galau. one cannot discount the contribution of thermal emission. However. the non- t herrnal component will Iikely dominate the continuum emission. especially in areas far from the plane of the disk.

A continuum map was formed from the line-free channels and is shown in contours overlaid on the optical image (Fig. 3.9) and the HI integrated intensity map

(Fig. 3.10). The continuum does not extend radially past the optical disk. There ap pears to be a ring of continuum emission at the centre of the galôuy. extending out to a radius of 6 kpc. This probably signifies a ring of star formation. We note one bright source in the disk of the galauy at cr = 08~33~27sand 6 = -22O.59' and emission extending to the north and south of this source. The northern continuum feature, extending up to -220561 or 18.8 kpc above the midplane, appears to have a loop like structure which beautifully surrounds one of the high latitude HI features (F3). The southern continuum feature which reaches nearly to -23'02' or 22.5 kpc from the rnidplane. also lies on the edge of a high latitude HI feature (F4, at cr = 08~33~23~ and 6 = -23'00'). This is the highest latitude radio continuum emission yet detected which is clearly connected to the disk of a spiral galaxy, exceeding even the features seen in the starburst galaxy NGC 5775 (Duric. Irwin Sr Bloemen, 1998).

Perhaps the most interesting correlation of the continuum with the HI can be seen on the west side of the galauy- There is a depression in the HI disk seen at about

CL = 08~33~17from b = -22'58' to b = -22"56!5 which coincides with an extension in the continuum emission. If we follow these features northward. we reach one of the large high latitude HI features (F5) coincident ivit.h a dip in the continuum emission at a = 08~33~25~and b = -2236'. and then further above the HI featurc. ive see a small half arc of continuum emission. Correlat ions between high latit ucIe HI featurcs and extensions in the radio continuum have recently been seen in NGC 5775 and

XGC 3044 (Collins et al.. 2000). hlso interesting to note in the continuum map is the collection of small detached radio continuum components to the West of the large vertical extensions at 6 = -23O01' and 6 = -22"55'. These appear to form two very large shell or arc structures. one above and below the plane. There are also several point soimes in the field !sep Fig. 3.9) whirh are likely harkgroiinrl qiiaeari. Ycte. however. that the point source at (a = 08~32~53',d = -2Z057') appears to have emission extending northwestward up to another point source. There also seems to be a southern extension, as well as two additional point sources to the southeast

(lower-left ). It is possible that these peaks could make up an extended radio source in the foreground.

The companion gaiaxy shows no continuum ernission above the 20 lewl.

3.3 Analysis of the High Latitude Features

3.3.1 The Vefocity Fields

Fig. 3.11 shows the same moment O map as Fig. 3.4, rotated such that the major a'ùs lies horizont dly, wit h high latitude features labelled. Three posit ion-velocity (PV) Figure 3.9: Cont inuurn map (contours) overlaid on the optical image (grayscale). Contour levels are 0.30 (20), 0.5. 0.75. 1. 1.5, 2, 2.5: 3, 3.5. 5. 10 mJy beam-'. Figure 3.10: Continuum map (contours) overlaid on the zeroth moment rnap (colours). The contour levels are 0.23 (1.5aj, 0.30 (24, 0.5, 0.75, 1, 1.5, 2, 2.5. 3. 3.5, 5, 10 mJy beam-'. The colour scale for the moment O map ranges from O to 2.94 x IO*' cm-* and resdings of 500 and 10M) correspond to 1.05 and 2.09 x lo2l cmd2, respectively. ARC SEC

Figure 211: Moment O map. rotated by 23'. The features and the positions of the position- velocity slices are indicated. Contour levels are at 0.6. 1.5. 3.1, 4.8. 6.1. 9.2. 18.4, 30.7. 36.8, 42.9. 48.1. 55.6, 61.3 x IO*' cm-* and the gray scale ranges from O to 61.3 x lo20 cm-2. Note that 100" = 12.6 kpc. slices. parallel to the minor auis. were taken of each feature. the positions of which are as indicated in Fig. 3.11. Slices for F1 and F2 are seen in Fig. 3.12. F3 and F-4 in

Fig. 3.13 and F5 and F6 in Fig. 3.14. A11 of the PV plots show a striking gradient in velocit- such that the velocities tend toward the systemic veloci ty wit h increasing vertical height from the plane. Physical interpretations of this trend will be discussed in 5 8.2.2. There are also two other trends the features share: one is the presence of several velocity spurs. a newly seen phenornenon if they are al1 associated with the same feature and the other is a large ournber of smalle~detached components. with wide ranging velocities. Possible physical interpretations of these velocity structures are discussed in 5 8.2.1.

Fig. 3.12 shows that F1 has four distinct velocity spurs in panel (a). shom by red tick marks. The three at (180 km s-l. 60"), (260 km s-l. 55"). and (340 km s-l

30") face outward and another spur. a lower extension of the first. at (155 km sdL.

10") faces toward the disk. It appears, in all three slices, as though there is a sizable Velocity (km/s)

Velocity (km/s)

Figure 3.12: Position-velocity slices, pardel to the minor axis, through the a) le&. b) centre and c) righr of F1 and the d) left, e) centre and f) right of F2. The velocit ies are relative to systemic, while the distances are in arcsec and are ret ative to the major axis. Contom are at 0.64 (20), 1,3' 5. 10, 15 mJy beam- - . Velocity -spurs2 are marked in the htpanel. 3. CnB Array Results 42

Velocity (km/s)

-300 O 300 -300 O 300 -300 O 300

Velocity (krn/s)

Figure 3.13: Position-velocity slices t hrough the a) left. b) centre and c) right of F3 and the d) left, e) centre and f) right of F4. Velocity (km/s)

Ve locity (krn/s)

Figure 3.14: Position-velocity slices through the a) Lefi. b) centre and c) right of F5 and the d) left, e) centre and f) right of F6. 3. CnB Array Results 44 amount of gas lagging behiod the rnajority of the high latitude gas by 84 km s-'. The veloci ty gradient mentioned previously is clearly seen here. Yumerical values for the gradients in this and other features are given in Table 3.1. Fig. 3.12 also shows small detached structures around F1 (most easily seen in (a)),mostly blueshifted between

Ot'and 30". These features exist at the 30 level. F2 (also seen in Fig. 12) shows two spurs in panels (d) and (e) at (225 km s-'. -60") and (184 km s-l. -65") (in (d)) and three in panel (f), one which faces upward. toward the disk at (170 km s-l. -40") and two that face away from the disk at (150 km s-'. -80") and (250 km s-'. -60").

Again. we see that there could be a substantial amount of gas lagging behind most of the high latitude gas. F2 also shows a large number of small. scattered componmts. again being both red and blue shifted. Most of these seem to exist farther from the plane than -30" (- 10 kpc).

The velocity structure of F3 is shown in Fig. 3.13. There are three spurs in panel

(a), two facing away from the plane at (150 km s-l, 65") and (250 km s-'. -40") and one facing south, apparently in the disk, at (83 km s-', -5"). Three spurs. al1 facing away from the plane are seen in panels (b) and (c) at (100 km s-' , 65"). (180 km s-'.

60") and (270 km s-'. 20"). -4s opposed to F1 and F2. the spur at 180 km s-' in panel (b) seems to suggest that there is gas leading most of the high latitude gas by 80 km s-l. Yote. however! that this component still lags the disk gas by about

100 km s- ' . We also note that two smaller concentrations seem to be leading the disk gas in panel (a) (20"to 40"). F4 shows three spurs in panels (d) and (e).al1 facing away froni the disk at (40 km s-'. -95'7, (165 km s-'? -65") (225 km s-'. -50") (in panel (e)). Again, many smaller structures are seen over a large range of velocities. 3. CnB Array Results 65

These seem to lie below -100" (-12.6 kpc) and probablp make up the detached feature below F4 (see Fig. 3.11. x = 70". y = - 125"). F4 shows the largest velocity gradient of al1 the features.

FJ (Fig. 3.14) shows four distinct velocity spurs in panel (a). three facing away from the plane at (-290 km s-l, 25"). (-210 km s-'. 80ft),and (-145 km s-l. Tot') and one facing south at (-130 km s-l. 5"). Similar to F3. the third upward spur

(-210 km s-') seems to be a component of the high latitude gas which leacls the rnajority by 64 km s-'. while still showing an overall lag behind the disk gas by

8-4 km s-'. For F6. four spurs can be seen in panel (d). three facing away from the disk at (-330 km s-l. -30"). (-250 km s-'. -55"). and (-165 km s-'. -55") and one facing the disk at (-145 km s-'. -15"). Only two spurs are seen in panel (e). both king swv from thc disk at (-250 km S-', -3"j and (-185 km s-!, -6Wj. In ai1 three panels. ive see many individual components covering almost the whole band in velocity. These detached features both lead and lag with respect to the underlying disk gas. Again, it is suggested that the detached feature seen below F6 in the moment O map at r = -180". 9 = -100" (Fig.3.11) is actually made up of these smaller cornponents with a large velocity dispersion. In panel (f). however. we see three structures closer to the disk (-SOM),one which Ieads most of the halo emission and twvo that lag behind it. These structures, if real, could be part of F6.

3.3.2 Physicai Parameters

The physical parameters of t he main features are presented in Table 3.1. A11 distances were measured hmthe moment O map (Fig. 3.11). Al1 of the features have a multiple 3. CnB Array Results 46

velocity structure. Since there is no confmion agahst the disk at these latitudes.

we infer an expansion from this. although we do not propose t hat the structures are

shells since shell-like structure is not seen in the total intensity map (Fig. 3.11). \Ve

can calculate a kinematical age of the features using

where RF is the radius of the feature and I&, is one half of the total velocity range over which the feature is observed. -4s ive are unsure if al1 the spurs correspond to a single feature. we have measured a maximum and a minimum expansion velocity corresponding to the largest and smallest separation betwen velocity spurs. leading to a minimum and maximum kinematical age. rpspectively. The HI masses. .\IF.were calculated using equation 7.1. The ambient density, n,, in the disk at the proiected galactocentnc radius below each feature was found from preliminary modelling. fol- lowing Irwin Sr Seaquist (1991). (1993). More detailed results from this mode1 will be presented elsewhere.

We can also calculate several energies for these hatures. These energies are compared in Table 3.1. The sirnpte kinetic energy, presuming these features are expanding in some way. is given by

If these features were formed by supernovae. nie can estimate the required input energy 3. CnB Array Resdts 47 where n, is the initial density Rp iç the radius (pc) and 1.;- is the expansion velocity (km s-'1. This equation assumes that the energy is deposited instanta- neously (Chevalier. 1974). Note t hat the kinetic energy considers only the currently observed expansion energy and does not take into account the energy required to reach a certain height above the plane or any efficiency factor to account For energv losses from the formation mechanism. The supernova niodel does include such an efficiency. which is mode1 dependent. but also does not include the energv required to reach a height above the plane. TabIe 3.1: Parameters of High Latitude Features Fea t ure F1 F2 F3 F4 Fa F6 =F~(kpc) 10.5 18.0 9.9 14.2- - 11.2 8.2 DF~(~Pc) 20.9 26.9 7.4 3.3 17.5- - 21.4 RF' (kpc) 3.3 5 2 4.8 4.8 a. i ?5.4 .uFd( ~OW~, ) 5.4 O.9.2 f 0.1 13.0 * 1.4 4.1 * 0.3 10O. 1.8 I0.1 P (10-2 cm-2) 9.1 5.4 4.6 3.1 121 5.8 9.1 Erd: (km s-' kpc-') 9.3- 12.2 23.2 26.4 17.5 23.8 at :(kpc) 3.1- 4.4 7.9 7.6 7.9 4.3 (L-z~)~~~~(km S-7 84 52 84 92 84 92 (&&,,n (km S-') 42 --33 40 12 32 32 9 - rmmh(loi y) 1.1 23.1 11.7 39.1 17.4 16.5 - c. rmIn(loi y) 3.8 9.8 5.6 5.1 6.6 3.1 (Eh.),.,' (10'~ergs) 3.81 2.47 9.14 3.47 7.48 1.48 (Eh-),,, ergs) 9.5 4.4 20.1 0.6 10.9 1.8 (E,,),,,J (10~~ergs) 1.71 1.99 2.52 2.27 8.99 8.95 (Et,),,, (IO= ergs) 6.5 1.-4 8.9 1.3 23.3 20.4 a SIauimum vertical distance From the midplane. measured to the 20 contour level. using an estimated inclination of 80". -411 distances were measured from the rotated Moment O map and have an error of 0.2 kpc. Projected galactocentric distance. ' Radius ûf the Eeature. une lialf the IeiigtL uf th halurr. parallei ro the major auis. of the feature. Errors in mas are given by the random errors in Aux density which are dominated by measuring the flux over differently sized regions. In plane density the density in the disk below the current feature. prior to the formation of the feature. The change in velocity wit h increasing vertical distance from the plane. Lfeasured from the centre of the highest peak at the midplane to the lowest contour level (20) of the the most prominent upper spur. The maximum height at which this measurement was made is given in the next line. Hatf of the vetoci- range owr mhich the feature is observed. The vetocities have errors of 5 km s-' (one quarter of the channel width). The age of the feature. The kinetic energies of the proposed expansion of the feature.

J The instantaneous supernova input energy. See text for the formula. 4. MAY 1999 D ARRAY RESULTS

The global profile for SGC 2613. made from the May 1999 D array data is shown

in Figure 4.1. The short bandwidth problem. described in Section 2.2. is evident

in this figure. The profile is fairly similar to the C array profile (Fig. 3.1). The

peak point in the C array profile is 174 m.Jy beam-l at v = 1928 km s-l. while the

flux is 135 mJy beam-' at the sarne velocity in the D array profile. There is line

emission in every channel which car. also be seen in the channel maps. al1 of which are

shown in Fig. 4.2. The striping effect mentioned in 5 2.2 is present at a low level of - 0.6 mJy beam-L in half of the channels. There are two interesting extensions seen in the channels maps. for example at (a = 08~33~35~,d = -22'57') in the = 1991.0.

1949.2. and 1928.3 km s-' panels and at (a = 08~33~10~.6 = -23'00') at u = 1490.5.

1448.9. 1428.1. and 1386.5 km s-l. Although these features only appear at the 20

level, their continued presence in more than two charnels suggests that they are real

detections. These two features seem to correspond with the features labelled F 1 and

F6 in the CnB data. with the D array features extending 2' further from the plane

in both cases. The companion galaxy is detected over the same velocity range as

in CnB array. within errors (1469.7 to 1394.6 km s-l). The extended features in

the companion galaxy seen at C array are not seen in these D array channel maps.

This is likely due to the beam size being comparable to the size of the companion. 4. May 1999 D Array Results 50

MmmcVeIoaty (km's)

Figure 4.1: The global profile made from the 1999 D array data. as well as the higher rms noise. There is also an interesting detached component at

(a= 08~33~55~d = -23O02') seen in most of the D array channels (best seen in the u = 1511.3 km s-' panel). However. the reality of this feature is unclear. as this data set is questionable and as this feature is not seen above the 2a level in the C array channel maps.

The integrated intensity map of NGC 2613 and its companion, ES0 495- G 017. is shown in Fig. 4.3. The ring of HI through the disk, noted in the C array moment O rnap (Fig. 3.4) is also seen here. There are five high latitude features noticeable in

NGC 2613. Two are on the north side of the galaxy. at either end of the optical disk at (a = 08~33~35~,6 = -2205i1) and (a = 08~33~15".6 = -22O54'). There is a feature across the plane at (a= 08~33~1~.6 = -23001t) and then two very large southern features at û = 08~33~25~and a = 08~33~35'.These latter two features extend approximately 45 kpc below the midplane (measured from the towest contour 4. May 2999 D Array Resuits 51

Ob3400 3330 00 Riom ASCENSION (J2000)

ll + D 33 30 00 063400 3330 00 RlGHT ASCENSION (J2000)

Figure 4.2: The 1999 D array channel maps. Contour levels are 1.8 (1.50), 2.4 (20): 7? 15, 30: 50, 70 mJy beam-l. The velocity is stated in the top right corner of each panel and the beam is shown at the bottom left of the btpanel. 4. May 1999 D Amoy Resuits 52

Figure 4.3: The integrated intensity (moment 0) map of NGC 2613 and its companion. kom the 1999 D array data. overlaid on the optical image. The contour levels are at 0.1. 0.2. 0.3, 0.4, 0.6, 1.0. 1.9, 3.8, 5.7, 7.6: 9.5, 11.5, 13.5, 15.3. 17.2, 19.1 x 1020 cmv2. The beam is shown in the Iower left. level). The companion galaq shows a northern extension, as it does in the C array data. The detached feature to the east of NGC 2613, mentioned in the channel maps. is clearly seen in the integrated intensity plot. This feature has a peak column density of 1.9 x 10~' This feature does appear in the C array moment O map. at a peak of 2.1 x IO*' cm-' but to a smaller spatial extent.

The velocity fields of both galaxies are seen in Fig. 4.4. The isovelocity contour 4. May 2999 D Array Results 53 lines of NGC 2613 are not as curved as th- are in the C arrap data and this is due

to the larger beam size over which several different velocities are averaged. .-\gain. it is noted that the kinematical rninor avis is not perpendicular to the kinematical major mis. The fact that this is seen on a larger scale in D array suggests that it is not due to a smaller structure. such as a bar. If these data are to be believed. it may suggest a warp in the disk. One of the very large high latitude features is seen in this map. on the south side of the galaxy (at the 1575 km s-' contour). This feature is on the redshifted side of the gala~yand its velocities are blueshifted with respect to the disk gas above it . This suggests that t his feature. if real. is lagging the disk gas. which is consistent with the C array findings. The velocity contours of the cornpanion galaxy have somewhat of a turnover in the 1525 and 1550 km s-' contours which could indicate rotation. Sirnilar to the C array data, it rvcjulcl appear iw ~liuugli the northern extension is lagging the disk gas underneath it (see the 1500 km s-' contour to the northeast). Finally. the anomalous feature to the east of NGC 2613 is also seen in this plot and would be blueshifted (at a peak heliocentric wlocity of

1550 km s-l) on the redshifted side of the galaxy. if this feature is real.

The averaged continuum map (see discussion in 5 2.2) is shown in Fig. 4.5.

NGC 2613 shows a possible extension on the north side of the galauy at (a =

08~33"25', 6 = -2Z056'). There are &O severai point sources around the gaI- aq in this plot. which are likely background radio sources such as quasars. Note that the two point sources at (a = 08~33~55'.8 = -22'52') and (a = 08~32~50~, b = -22"55') bot h have extensions to the southeast (lower left ) . It is unclear whet her these are in fact extended radio sources as the continuum subtraction was not done 4. May 1999 D hayResuits 54

I I I I I l 34 00 33 45 30 15 00 32 45 RlGHT ASCENSION (J200a)

Figure 4.4: The velocity field (moment 1 map) hmthe 1999 D array data. Contours are isovelocity levels and are as shown (units in km s-l). I I I I 1 08 34 W 33 45 30 15 00 32 45 RlGHT ASCENSION (J2000)

Figure 4.5: The continuum map made from continuum information derived from this data set averaged with the Irwin, Engiish Sr Sorathia (1999) D array 20 cm con-

tinuum data. Contour !CFCIS arc at 00.5 (1.5~),0.60 (23). :? 2. 3, 4. :. 7 mJy bearn-'. as accurately as possible for this data set.

No further analysis and no quantitative rneasurements were done with this data set as its merit is questionable due to the short bandwidth. making continuum subtraction difficult. and because of the stripe permeating the data cube. 5. SEPTEMBER 2000 D ARRAY RESULTS

5.1 Neutrd Hydrogen

The global profile for NGC 2613 made from the 3000 D array data is shown in

Figure 5.1. This is again the expected "double-horned" profile from a rotating spiral

galavy and agrees well with the C array profile (Fig. 3.1). The fluxes are. however.

lower than the C array values overall. The rms error in fi LX for this data set is

1.09 mJy beam-' (primary bearn corrected data cube) aiid the total flux errors were

calculated in the sarne rnanner as the C array flux errors (recall 3.1). It is expected

that the flux values would be higher at D array than at C array. as the resolution

is lower in D array and therefore more broadscale emission should be detected. It

is possible that some real low level ernission might fa11 below the higher rms noise

level in the D array data. It also seems as though a negative sidelobe has not been cleaned out properiy and is causing a minimum in the Auxes (ie. the average Rux density in an "empty" patch of sky is negative). This deficiency in flux is apparent

in the lower HI mas. The global parameters derived from this data set are listed in

Chapter 7 (see Table Tl). The profile for the companion galaxy is shown in Fig. 5.2 and is quite erratic. 1 report this as an insignificant detection in these data. as the

non-emission channels vary greatly in Buand seem comparable with the emission channels. Thus, no global parameters were calculated for the companion galaxy from 5. September 2000 D Array Resuits 57

Figure 5.1: The global profile of NGC 2613 made from the 2000 D array data. these data.

The channel maps for the 2000 D array data are shown in Figure 5.3. Line emission is detected from 1469.7 to 1991.0 km s-'. There are some interesting extensions from both ends of the disk noticeable at a = 08~33~40~.6 = -23O02' in the u = 1865.7 km s-l panel and at cr = 08~33~20~,6 = -22O56' in the u = 1824.0 km s-' panel. The cornpanion galaxy is seen from 1469.7 to 1394.7 km s-l. to the northwest of XGC 2613. 'lote that the stripe mentioned in Section 2.3 was not apparent in the grayscale of the individual channels. however the integration of these channels in the moment O map (Fig. 5.4) has brought out the stripe and it is seen at the

380 mJy km s-' (1.0 x 1020 cm-*) level.

The integrated intensity map (Fig. 5.4) shows interesting features on the eastern side of the galaxy, at u = 08~33~45~from d = -22'57' to 6 = 43'02'. and on the 5. September 2000 D Array Results 58

Figure 5.2: The global profile of ES0 495- G 017. western side of the galaxy at a = 08~33~00~.6 = -22'58'. These features rnay be caused by tidal effects from the conipanion gala~y.There is also a pair of interesting high latitude features at a = 08~33~30',d = -22'3 and a = 08~33~32".6 =

-23O02'. This lattcr fcaturc roughlj- corresponds ulth what 1 have called F2 in the

C array moment O rnap (recall Fig. 3.1 1). This can clearly be seen in the overlay of the 2000 D array moment O rnap with that of the C array rnap (Fig. 5.5). From this map, it also appears as though F1 and F3 in the C array rnap are part of one larger feature in the D array rnap (see the NE part of the gala.). Yote also that the large knot east of the galaxy seen in the 1999 D array rnap (at a = 08~33~55'. d = -23O02') is only hinted at in this data set. appearing as a few peaks at the

1.0 x 10" cm-2 level in the moment O rnap (see Figs. 5.4 and 5.5).

The moment 1 rnap is shown in Fig. 5.6. It is especially obvious here that the kinematical minor avis is not perpendicular to the major aviso as was also seen in the C and 1999 D array data sets. Further than that, the isovelocity contours at the 5. September 2000 D Array Results 59

O 16'&bm..>678a~vs O 0

08 33 30 00 08 33 30 00 RlGHt ASCENSION (52000)

Figure 5.3: The channel maps made ftom the 2000 D array data. Contour levels are at 1.86 (20), 5, 10, 20,40, 60 mJy beam-'. Velocities are stated at the top of the panels and the beam is shown in the Iower left corner of the ktpanel. 5. September 2000 D Array Resuits 60

J I 1 I q 33 45 30 15 00 326 RlGHï ASCENSION (52000)

Figure 5.4: The integrated intensity map. Contour levels are at 1.0, 1.7. 2.6. 3.5. 5.2. 8.7. 12.2, 15.7 x 1020 cm-', while the gray scale ranges from 0.9 to 17.3 x IO*' cm-?. Figure 5.5: The 2000 D array data moment O map (contours) overlaid on the moment O map (colours). The contour levels are the same as in the image. The colour scale ranges kom O to 2.94 x IO*' and rei 500 and 1000 on the wedge correspond to 1.05 and 2.09 x 102' cme2 Figure 5.6: The moment 1 map. Contour leveb are in km s- and are as labelleci.

ends of the disk are curved upwards (see especially the 1400 km s-' contour) which strongly suggests that the disk is incleed warped. Also note that the eastern and

western extensions from the disk mentioned above are seen in this map. The eastern extension is blueshifted on the redshifted side of the galaxy (by 230 to 450 km s-')'

while the western extension is redshifted on the blueshifted side of the galaxy (by

150 km s-l). This suggests that both of these features are lagging behind the disk gas.

This trend is consistent with the velocity fields of the high latitude features found in the C array data. The position-vebcity siice along the major axis of yGC 2613

(Fig. .S.'i) agrees well with the C array PV slice (Fig. 3.6), with a large velocity spread of approximately 700 km s-'.

5.2 Radio Continuum

The continuum map made from the 2000 D array data is shom in Fig. 5.8. There is a double peak seen in the central regions of YGC 2613, which is probably the 5. Sep tember 2000 D Array Results 63

Distance Along the Midplane (arcsec)

Figure 5.7: A position-velouty &ce. L3" widc along the major axis of NGC 2613. Contour leveis are at 1.86 (20), 5, 10, 20, 30, 40, 50 mJy beam-l. while the grayscale runs from 1.40 (1.5 O) to 50 mJy beam-l. 5. September 2000 D Array Results 64

Figure 5.8: The 2000 D array radio continuum map. Contour Ievels are at 0.48 ( 1 .k),0.64 (20): 1. 2, 3. 5. 7. 10, 12, 15 mJy beam-'.

ring already reported in the C array data (recall Section 3.2). The eastern end of the gala~yis particularly extended. When compared with the C array continuum

map (Fig. 3.9): it is seen that there is one point source just to the east (Left) of the gai- at (a = 08~33~33~.6 = -22O58'). However. bearn smearing of this source is not enough to explain the emission extending out to a = 08~33~40~.Thus. it is likely that these D array data are revealing a real extension to the disk, in the radio continuum. This may be a result of star forming regions in the outer portions of the disk.

The same background point sources seen at C array are also seen here. In particu- 8 lar. the point source at (a = 08~32~50~.6 = -22'55') also shows an elongation in the southeast direction, as was seen at C array, witb the other three sources mentioned in 5 3.2 resolved out into one extension. 6. THE COMBINED CNB AND SEPTEMBER 2000 D ARRAY DATA

6.1 Neutrd Hydrogen

The C array data were concatenated with the 2000 D array data to attempt to improve upon the signal-to-noise. These data were cleaned using a natural weighting and only baselines of length les than 5 kX (the maximum D array baseline) were used. Figure 6.1 shows the global profile for XGC 2613 made from this combined data set while the global profile for the cornpanion is shown in Fig. 6.2. The rms for the Primary beam corrected cube is 0.77 mJy beam-L and total flux errors were determined in the same manner as the C array Bu?< errors (recall 3.1). The global parameters derived from these data are presented in Chapter 7.

The channel maps are shown in Fig. 6.3. The solar interference stripe permeating the D array data is not apparent in the grayscale of the individual channels in this combined data set. XGC 2613 is detected from 1345 to 1991 km s-'. wbile the cornpanion gdavy is detected from 1449 to 1374 km s-l. These ranges agree with those reported from the CnB data. There is evidence for high latitude features in

YGC 2613, for example at a = 08~33~35=,6 = -23"01r at v = 1886.6 km s-' and at a = 08~33~25~.b = -22O59' at o = 1407.3 km s-LeThere appears to be four large extensions (two northern and two southem) from the disk in the 1365.7 km s-L panel. 6. The Combiwd Cdand September 2000 D Array Data 66

Figure 6.1: The global profile of NGC 2613 made from the combined C and D array data.

Figure 6.2: The global profile of the cornpanion made hm the combined C and D array data. 6. The Combined CnB and September 2000 D Amy Data 67

The cornpanion galaxy ahshows miderice for a socrthm high latitude feat use at a = 08~33~00~.CS = -22"33' (see panels at 1573.8 and 1190.5 km s-l).

The moment O rnap made from the concatenated data set is shown in Fig. 6.4.

Here ive see the same six features noted in the CnB array moment O rnap IFig. 3.4 with two additional features at (n = 08~33~29',d = -23OOOI5) and (a = 08~33~05~,

5 = -2Z056'). The features are labelled as they were in the C array rnap with the additional features labelled F2a and F7, respectively. It is unclear whether F2 and F2a are two separate features or whether they make up the walls of one very large supershell (of radius 3.7 kpc), as the lotvmt contour level shown would suggest.

There is also an emission peak attached. at the Iawest contour level. to the south of F2 at a = 08~33~35~,d = -23'03'. There is a trace of the larger feature joining

Pl and F3, seen in the 2000 D Wray data (red Fig. 5.41, iii ~iiecoriiliried data set

(see the detached components at cr = 08~33~275.6 = -22'5615 and a = 08~33~35'.

6 = -22"57!5). There is no sign of the large western disk extension seen in the 2000

D array data. The disturbance of the eastem (left) tip of the disk could however be part of the larger extension seen in the 2000 D array. The cornpanion does not show the northern extension that was seen in the C array moment O map. nor the southern extension noted in the channef maps of the combined data set. There is a minimum

ALV cut-off applied when integrating the flux to make the moment O map and this is probably the cause of this southern feature appearing in the channel maps and not the integrated intensity plot.

The velocity field for the combined data set is shom in Fig. 6.5. This rnap looks rnuch closer to the CnB array rnap (Fig. 3.5) than the 2000 D array rnap (Fig. 5.6). 6. The Combined CnB and September 2000 D Array Data 68

08 33 30 w RlQHT ASCENSION (J2000)

08 33 30 00 RlGHf ASCENSION (52000)

Figure 6.3: The channel maps for the combined data set. Contours are at 0.9 (20). 2. 5. 10: 20, 30. 40 mJy beam-'. Velocities are found at the top of the panels and the beam is shown in the lower left corner of the first panel. 6. The Combined Cdaad September 2000 D Array Dota 69

P 1 O 33 4 30 15 RiGm ASCENSION (52000)

Figure 6.4: The integrated intensity map made hm the combineci C and D anay data. Contour levels are at 2.1, 2.5, 3.0, 3.8, 5.1, 7.6, 20.1, 12.6. 15.2. 17.7. 20.2. 22.7 x 1020 cma2 while the grayscale ranges Erom 1.8 to 25.5 x 10'O cm-2. 6. The Combineci CnB and September 2WO D Array Data 70

I I I I 11 08 33 45 30 15 00 RlGHT ASCENSION (J2000)

Figure 6.5: The moment 1 map for the combined data set. Contour levels are in km s- 'and are as labelled. l'his velocity tield is apparently well behaved with only a srnaIl trace of a disk warp suggested by the kinematical minor avis not being perpendicular to the kinematical major auis. The rotation curye for this data set is shown in Fig. 6.6. This looks quite similar to the CnB rotation curve. with a large velocity spread of 700 km s-'. It is expected that this cornbined data set would resemble the CnB data more than the

2000 D data. as there are many more data points contributed from the CnB array under n natural UV weighting. .As a result, the beam in this data set is much closer in size to the CnB array beam than the D array beam.

6.2 Radio Continuum

The continuum map is presented in Fig. 6.7. This map again more closely resembles the C array map (Figs. 3.9 and 3.10) than the 2000 D array map (Fig. 3.8). While the 6. The Combined CnB and September 2000 D Arrav Data 71

Figure 6.6: A position-velocity slice, 5'' wide, along the major axis of NGC 2613, barn the concatenated data set. Contour levels are at 0.9 (2a),3, 5. 10, 20. 30. 10 mJy beam-', while the grayscale ranges fiom 0.67 (1.5 O) to 40 mJy bearn-'.

large southern feature noted in the C array is seen to a Iesser extent in the combined data set (at a = 08~33~25~,from 6 = -23O00' to 5 = -23"01t). the associated northern feature is not seen in this data set. The discrepancies of the appearance of

these large features between different data sets may simply be due to differences in beam sizes. depending on how the ernission is coupted to the beam. The strong knot in the disk, previously noted at (a = 08~33~27,6 = -22O59') is also seen in these data. There are also detached emission peaks seen in an arc shape to the south of the galaxy? which seem to stem from the southern extended feature. This southern

"arc". along with a northern counterpart, was noted in the CnB array data (recall

5 3.2). The two distinct peaks to the east (left) of the galaxy at (a = 08~33~37~,

6 = -22'59') and (a= 08~33~33'.6 = -22"38!5) seem to be unresolved into one 6. The Combineci Cd3 and September 2000 D AmyData 72

(3 @-

(JO a O -

Q O

0 O u I 1 1 I I * 08 33 40 35 30 25 20 15 10 OS RlGHT ASCENSION (52000)

Figure 6.7: The 2 1 cm radici continuum, from the cürubiud data set. Cuutuur Ieveis art: at 0.19 (1.5 a).0.26 (20).0.5. 1. 1.5. 2, 3, 5, 7 mJy bearn-'. The beam is shown at the lower left. extended feature in the 3000 D array map (recall Fig. 5.8). 7. THE GLOBAL PROPERTIES OF NGC 2613 AND ES0 495 G- 017

This chapter presents al1 of the global properties derived from the different data sets and compares them. Table 7.1 shows the parameters derived from al1 of the data sets, excluding the 1999 D array on which no calculations were performed. for

XGC 2613. Table 7.2 presents the properties derived from the companion galauy.

Note that in this case. the companion ads stated as a non-signifiant d~tectionin the 2000 D array data arid so this data set is not included.

The systemic velocity, u,,., is determined by averaging the centre of the velocity ranges corresponding to Ritues of 20% and 50% of the line peak. The velocity width of the profile, CC*, is also taken to be the average of the velocity range at 20% and

50% of the line peak. The systematic error for v,,. and CV is one half of the channel width (10.4 km s-~)while the randorn errors are given by one half of the difference in the 20% and 50% values. The area under the global profile (Figs. 3.1. 5.1. 6.1) was integrated to find the total HI flux for XGC 2613. The total KI mass (assuming optical thinness) is then given by

in units of solar masses. M,30 where Sc(~)dvis the flux density integrated over the line in Jy km s-' and D is the distance in Mpc (Giovanelli k Ha~es,1988).

73 7. The Global Properties of NGC 2613 and ES0 495 G-017 74

The errors for the individual flu~esmm added to get the total error in j' SC(u)du.

The fractional error in S,(C)~,Uwas then applied to the HI mas. The assumption of optical thinness. together with the fact that structure larger than 15' rnq be resolved out in both C and D array. suggests that this will be a minimum HI mass for this galaxy This equation was also used to find the total hydrogen mas of the cornpanion. The total dynamical mass for each galavy \vas derived from the major avis position-velocity plots (Figs. 3.6 and 3.8). Taking the extreme points of the rotation curve I*(r).the total mass contained within a radius r is

in units of M,. with V(r)in km s-' and r in kpc (Giovanelli & Haynes, 1988).

We have not corrected for any internal velocity dispersion. as the rhann~lwidth is typical of the dispersions normally seen in spiral galaxies. This equation assumes a pureiy spherical mas distribution for the halo inside of r and a spherically syrnnietric distribution outside of r. Note that the accuracy of the total mass is more affected by errors in the heliocentric velocity, the inclination and the distance t han by the choice of mas mode1 for the halo (Lequeux. 1983). The dynarnical masses have been calculated for the redshifted and blueshifted sides of each galavy and then averaged together to give the table values. Again. the systematic error in velocity is propagated through to give the systematic error in iLk while the random error is given by one half of the differences in the red and blueshifted sides. The inclination of the cornpanion galaxy was calculated to be 66t7, using the thin disk approximation.' cos i = b/a, where i is the angle of inclination where O0 is face-on, and a and b are the optical major and minor axes, respectively. 7. The Giobcrl Properties of NGC 2623 and ES0 495 G- 02 7 75

The rrns noise of the prima- beam mrrected data cubes are f3.41, 0.77, and

1.09 mJy bearn-'. for the CnB, 2000 D, and combined data, respectively. Note that t hese rms noise values are higher than those of the uncorrected data cubes, listed in

Table 2.1. as the primary beam correction causes the noise Ievel to rise with distance from the observation centre in the sky. There is a random error involved in flux rneasurements which arises from measuring the flux within different sized areas. The error in the flux measured in any given channei is (N)% where N is the number of beams over the measured area and o is the rms noise.

The 2000 D array integrated flux. SC(v)dvand, consequent ly, the HI mas. MHI. do not agree with either the Cni3 or the combined data values. The D array fluxes are lower in general and this may be attributed to Ruthat has fallen below the largcr rrns noise levcl and/or to a negative sidelobe ahich svaç not removed prcjperly.

AI1 of the other parameters agree between data sets. In determining final accepted global parameters for NGC 2613, the D array values will be discounted on account of the stripe through the data, the higher rms noise, and the negative sidelobe which is present. Final accepted values are chosen to be those values with the smallest errors and these are usually the CnB values. 7. The Global Properties of MC2613 and ES0 495 G-01 7 76

Table 7.1: Global Properties of NGC 2613 Derived Eiom the Data Sets

Pro~ertv* - CnB Data 2000 D Data Combined Data Final Value UJgra (km S-') 1660.5 + 10.5 1672.2 k 10.4 166.k 1.4 1665.4 f 10.4 Wb (km s-') 620.4 + 15.2 621.5 f 14.9 621.5 =t 16.6 621.3 & 14.9 J Sc(~)dvC(Jy km s-') 55.2 dz 2.0 50.1 f 2.3 57.1 k 2.4 35.2 k 2.0 ( log 11,~) 5.73 * 0.32 7.92 2 0.37 9.03 k 0.38 8.73 * 0.32 AIT" (10" Ml=,) 7.50 st 0.87 8.88 k 2.75 1.37 & 0.57 7.37 31 0.57 Systernic velocity. This pas determined by averaging the centre of the velocity ranges corresponding to fluxes of20% and 50%of the line peak. These velocities are uncorrected for the inclinations of the galaxies. Velocity width of global profile. These values are not corrected for the inclinations of the galaxy Flux density integrated over the global profile. HI mass of galy See text for formula. Dynamical mass of gala~y.See text for formula.

Table 7.2: Global Properties of ES0 495 G- 017 Derived from Al1 Data Sets Property CnB Data Combined Data Final Value v,,, (km s-l) 1517.1 =t: 10.6 1513.0 k 10.4 1513.0 zt 10.3 CY (km s-l) 129.9 * 16.9 135.2 k 13.3 135.2 k 23.3 S&j&u (Jy km s- !j 1.62 i0.19 1-275 0.27 i.62 Iû.19 i\fH1 (1o8 kIG) 2-56 10.30 2.01 k 0.43 2.56 & 0.30 MT (logMa) 5.19 k 1.06 7.12 & 1.14 5.19 + 1.06 YGC 2613 has been observeci with large beam, single dish radio telescopes by

BottineIli. Gouguenheim. % Paturel (1982) and Fisher Sr Tully (1981). who have found flux densities of 83 and 94 Jy km s-'. respectively. The flux density from these data of 55 Jy km s-Lhas discrepancies of 35% and 41% with t hese previously published values. respectively. It is likely that the smaller beams used in these observations is resolving out some of the flux from NGC 2613 which the large single dish antennas are picking up. Bottema (1989), however, found a flu density of

29.4 Jy km s-', using the Westerbork Sflthesis Radio Telescope (WSRT)T):a value

47% less than ours. Note though that Bottema listed the low declination of the source as a possible problem causing a low flux density value. 7. The Global Properties of NGC 261 3 and ES0 495 G- 01 7 77

Table 7.3 presents the globtd parameters of HGC 2613 and compares these mith the median values for a selection of Sab, Sb galaxies from the Third Reference Cat- alog of Bright galaxies (RC3) and the Uppsala General Catalogue (UGC) sample of Roberts St Haynes (1994). Note that their masses have been recalculated for

Ho = --(a km s-' hlpc-'. Here we see that 'IGC 2613 is unquestionably a very massive galauy. The HI mass is higher than the median value but does fa11 within the 75th percentile. The dynamical mass is well above the 75% value of the sample population.

Table 7.3: Cornparison of Global Parameters of NGC 2613 with Typicd Values in its Class Paramet er NGC 2613 Median 25 Xa 75 %

a This refers to the 25th percentile value of the sample population taken by Roberts Sr Haynes (1994). That is, 25% of the galaxies in the sample have a parameter value equal to or less than the 25th percentile value. As not al1 the distributions mre Gaussian. the mors are best represented by 25 th and 75th percentile values. Note. however, that on average the median (50 %) dues were not significantly different from the mean values. The optical (blue) luminosity. Taken from Invin. English Sr Sorathia (1999). The far infrared luminosity. Taken from Irwin! English Sr Sorathia (1999). 8. DISCUSSION

The main results of this thesis, those of the CnB array data. were presented in

Chapter 3. Two different data sets, the 1999 and 2000 D array data, were presented

in Chapters 4 and 5 respectively, and are meant to serve as a cornparison to the CnB data findings. The CnB and 2000 D array data were combined in such a way as to simulate D array data and the results of these data were shown in Chapter 6.

This discussion is broken into two maiii parts: the global distribution of HI and continuum in the NGC 2613 environment (in 5 8.1) and the discrete high latitude

HI features (in 3 8.2). The consideration of the features will concentrate on the CnB data and is divided into topics of the velocity fields and their implications (5 8.2.2 and J 8.2.1) and possible formation mechanisms of these features (5 8.2.3).

8.1 The Global Distribution of Neutral Hydrogen and Radio Continuum in and Around NGC 2613 and ES0 495- G 01 7

We have seen that NGC 2613 is unquestionably a very massive gala-. wîth its dynamical mass of 7.37 x 10'' 410 being over twice the 75th percentile value of a large sample of galaxies of the same morphological class. The HI mass is also comparatively high. The dynamical mass of the cornpanion galaxs ES0 495- G 017, is two orders of magnitude srnaller than that of NGC 2613. It is moving in a prograde 8. Discussion 79

fasashion with respect bo XGC 2613. meaning it is blwshifted and position& on the

blueshifted side of XGC 2613; both galaxies are also rotating in the same sense.

NGC 2613 is reportedly in a small group of four gala-ies. however ES0 495- G 017

is the only other galaxy we are detecting within our field of view (27 in D array j.

The HI is arranged in a ring of radius 17 kpc (in projection) with a central

deficiency in YGC 2613 (recall Fig. 3.4). This kind of HI ring is fairly common in

spiral galaxies (for example. hl31 has a 12 kpc ring). There is also a smaller ring

of radius 6 kpc seen in the radio continuum. This could be a ring of starbiirsts.

the likes of which have been seen in many spiral galaxies, including our own. The

average radii of these observed HI1 (ionized hydrogen) rings rypically range from

300 pc (Alonso-Herrero et al.. 2001) to 1 kpc (Reunanen et al.. 2000). as measured

from optical line emission. Such starforming rings arc thought to Se a rcsult of

orbital resonances with rotating oval or bar asymrnetries within the stellar disks.

The continuum radiation also shows a strong knot in the disk. This knot will be

further discussed in § 8.2.3. The continuum ernission in the CnB data shows no

radial extensions. There is an eastern extension of the disk seen in the 2000 D array data (displaced slightly northward), while the combined data set shows several detached peaks on the eastern side of the galaxy, at a low level. It is likely t hat the estension seen in D array is caused by real emission at the end of the disk, as simple beam smearing cannot account for the radial length of the eautension. This extension niay be related to the tidal tail seen in the moment O rnap, although the continuum does not extend as far as the HI tail. In any event, this eastern continuum extension may suggest the presence of starforming activity to the northeast of the stellar disk. 8. Discussion 80 - --

The pressing question is whether MGC 2613 is interacting with its cornpanion or

not. The velocity field of NGC 2613 from the CnB array data (Fig. 3.5) suggests

either a bar or a warp in the disk. From the first moment map of the 2000 D

array data (Fig. 5.6)!it is concluded that the disk is indeed warped. As the warp is

clearly seen in D array but not obvious in the CnB data or in the optical picture.

the warp must be present in the outer HI disk. If the inner disk were warped as

well. it would have been more apparent in the C array data. There are aiso large

extensions seen in the D array integrated intensity map (Fig. 5.4, on either end of

the disk. These appear to be radial extensions of the disk. rather than high latitude

extensions and it may be that thesr are small tidal tails. The companion galaxy

shows more evidence for an interaction, given its strange velocity field (Fig. 3.7)

and its asymrnetric dhtributiûn vf III away fmii XGC 2613 (Fig. 3.8 j. From these

arguments then. it is likely that XGC 2613 and ES0 495- G 017 are undergoing a

mild interaction. This interaction seems to be affecting the lower mass companion

galaxy more than the parent galaxy. NGC 2613 seems to be affected only on larger

scales.

There are a nurnber of srnall detached emission peaks, seen in HI and continuum,

around the two galaxies. 1 have stated the large "arcs" of emission of HI above and

below NGC 2613 as tentative detections only (recall Fig. 3.4 and discussion in 5 3.1.

There are also two smaller "arcs" seen above and below NGC 2613 in the continuum

(in Figures 3.9 and 6.7). These arcs may indicate a circulation of the relativistic electrons, originating from the continuum bot in the disk, nsing out into the halo, t hen falling back down on the western side of the disk. Alternatively, t hese continuum 8. Discussion 81 arcs could be tracing the shape of the gdanic magnetic field. However, it is quite difficult to determine the reality of the continuum and HI arcs given their small signal-to-noise. If real, the HI arcs may be tidal streams related to the interaction of

NGC 2613 with its companion, similar to the Magellanic Stream in our own galauv.

CVe do not see more coherent arching structures in D anay which might suggest that these peaks are not real. It should also be noted that there is no evidence in the HI velocity fields for strearning motions betveen the galaxies. The reality of such arcs or streams in this group will have to wait for more sensitive observations.

.\ simple calculation of the tidal force on the companion galavy due to NGC 2613 will now be done. The tidal force, on a test mass, mt, at a distance of one radius, r' from the centre of the companion galaxy (with mas m,) due to the parent galauy of mass dl, a distance R may [centre tu centre) is givrii by:

Xote that the projected separation will be used for R. however the actual separation could be substantially larger. The gravitational force, F,' on the same test mass at radius r due to the companion galaxy is:

where G is the gravitational constant (6.6'72 x cm3 g-' s-*) in both cases.

Taking the ratio of the tidal force on one end of the companion galaxy disk to the gravitational force on one end of the companion ga1a-q disk gives: 8. Discussion 82

The ratio of the masses is 100 mtrife the distance betffeen the tmo gatstxies is about

10 times the radius of the companion galaxy. This gives a ratio of:

Thus the companion galaxy should suffer strong tidal forces that may affect its structural integrity.

A rough calculation may also be done to determine whether the companion galaxy will be captured by the parent or simply pass by. For simplicity, let us assume that t here is only the gravitational force from the parent galavy acting on the companion.

Let us furt her assunie t hat the corn panion is exhibiting uniform circular mot ion

(which is most likely false). F, is now given by:

and the centripetal force. Fc' needed to keep the companion in its circular orbit is given by:

where u is the velocity tangent to the orbit. Since we only have information on the line of sight velocity and not on any velocity components in the plane of the sky let us also naively assume that there is a transverse velocity component rvhich is equal to the radial velocity. Then . = (2)Ii2vr where we take v, to be the difference in the systemic velocities of NGC 2613 and its companion. Cornparing these equations gives: 8. Discussion 83

The projected sepration of the galmituie~ (rneasured opticdly. centre to mm) is

58 kpc while v. the difference in the two systemic velocities' is 152 km s-'. Since the projected separation R is a minimum? this leads to a maximum ratio of:

indicating that the galaxies will likely merge.

8.2 The High Latitude Features - HI and Continuum

We have detected only the largest HI features in NGC 2613 due to Our beam sizes and there could be smaller extensions from the disk into the halo that we have not detected. There are six high latitude features seen in the CnB data (Fig. 3.11), which appear to be paired across the midplane of the galaxy and thus may be grouped into thrcc pairs (i.c. Fl/F?? F3:F-L. F5iF6). The 2000 D array data suggest that

F1 and F3 could be part of one larger feature. Also. the combined data show an additional feature next to F2 (Fig. 6.4). This could be one large supershell. It is then possible that Fl/F2. F3/F4 are al1 associated with each other and with the continuum extensions and knot in the disk. More large scale observations would be needed to concretely determine this.

Fig. 3.10 shows t hat t here is a strong continuum source in the disk and tm large extensions to the north and south of it. These continuum extensions appear to be associated with two of the HI features (F3 and F4, with the continuum situated around the outside of the HI. Xote that in the combined data set, only the southern high latitude continuum feature is seen and this is further displaced from F4 than what is seen in the CnB data. The correlation seen in the CnB array between the 8. Discussion 84

HI and continuum strongiy suggests that F3 and F4 are indeed physically associated

with each other and with the continuum knot in the disk. F5 also appears to be

associated with a continuum extension in CnB array (coincident with a deficiency

in the HI) in the disk. This. however. suggests that F5 is not physically associated

with F6 since F6 does not appear to extend from the same point in the disk as the continuum/HI correlation.

Why is the continuum emission displaced from the neutral gas in the outflotvsY

The continuum emission is rnost likely non-thermal synchrotron radiation. due to relativistic electrons spiralling around rnagnctic field lines. Recall that the 2 1 cm HI line emission arises from a cornpleteiy different mechanism. the spin-fi ip transition of the hydrogen atom. 1 have suggested that the large continuum features. along with the HI features F3jF-I. are hjsxiated with raçli utlier aiid wilh the contiriuuni knot in the disk. If the features were caused by some disturbance in the disk, it is perfectly natural for the lighter, faster electron component to be leading the heavier. colder hydrogen in an outflow. .Ut hough t his simple logic is qualitat ively consistent with the data. theoretical models of outflows do not predict in what way the different components of the ISSI will be affected.

Recall that YGC 2613 was previously obsewed with the Westerbork Synthesis

Radio Telescope (Bottenia, 1989). Fig. 1.6 shows his integrated intensity map in which we see two large emission peaks in the HI. These peaks do not correspond with any of the high latitude features found in the VLA data presented here. It could be that his features are real and they were not seen in the VL.4 data due to a particular beam coupling. It is more likely, however, that the problem lies nrith the Westerbork observations. given that the CnB data presented hmhave an obseroing time twice as long as Bottema's data and a rms noise level 3 times smaller. In addition. Bot tema did mention synthesis problems arising from the low declination of the galauy. Thus. it is the CnB data presented here that should be more accurate.

For now. we will discuss the sk high latitude features seen in the CnB array data. These features share several trends in velocity space: one is the tendency of the features to Iag behind the disk gas (discussed in 5 8.2.2) and the other is the number of velocity spurs and detached high velocity features (discussed in 3 8.2.1).

Finallv? 8.2.3 discusses the results as they relate to the formation niechanisms introducerl in Chapter 1.

8.2.1 Velocity Spurs and Anomaious Velocity Structures

.As noted in fj 3.3.1. the presence of multiple velocity spurs in the PV slices of the features is a newly seen effect. if they al1 belong to the feature in question. What sort of structure would give rise to this velocity behaviour'? The simplest expianation seems to be that the rising, expanding feature develops instabilities which themselves rise or fa11 and are observed as spurs. Such instabilities may well be shocks within the features. Alternatively, if the spurs al1 belong to a more stable feature, a candidate might be a 'mushroom cloudn7such as has been seen in our Gzl- on a much smaller scale (English et al.. 2000). Several velocity spurs, representative of a central stem. an upper cap, and two lobes on either side of the stem occur in such a feature and may be more or less prominent, dependent on the line of sight tilt.

We turn now to the matter of the srnaIl detached cornponents seen in velocity 8. Discussion 86 space. Shese knots are seen at both rd and btueshifted wiocities. with respect

to the underlying disk and the main features themselves, and exist at the 30 level.

Similar detached structures are also seen randomly dist ributed at ot her positions in the gala^ not associated with the high latitude features at similar inteiisities suggesting that these could be noise peaks. The knots si-cuated around the features. though. seem to be less randomly oriented and able to form coherent structures in the integrated intensity rnap and occur at heights in the range 10 to 20 kpc. Also. it is not expected that noise peaks would form such "lines" in position-velocity space. as these components do (see Figs. 3.12. 3.13. 3.14). Sirnilar srnell high velocity components have recently been seen in the halo of NGC 5775 (Lee et al., 2000), again at both red and blueshifted velocities. In this case. Lee et al. postdate that when the expanding

HI features reach a certain height above the plane, they break apart. furniing tliese smaller components. This may be true in NGC 2613 as well. considering that most of these components lie above the disk and even above the high latitude features.

These velocity structures are inconsistent wit h a simple galactic fountain since t hey occur at quite large red and blueshifted velocities on both sides of the galaq. If these velocity features are real. they could be important to understanding the dynamics between the disk and the halo regions of spiral galaxies. For example' there ma- be a transition region $ 10 kpc above which gas velocities more closely resemble those of Population II objects. 8. Discussion 87

8.2.2 The Velocity Dedine Wikh 2 aod 6he Enference of a Lagging

Halo

The most interesting result of these HI observations is perhaps the observed tendency of the high latitude features to slow down toward the systemic velocity with increasing height from the plane. This occurs on both the redshifted and blueshifted sides of the galauy. Therefore the motion of the galaxy through an intergalactic medium, where the streaming motion is greater than the rotational motion, can be ruled out as the cause of this trend since in that case al1 the velocity shifts would either be redshifted or blueshifted. This is an important result as it is thoiight that a hot intergalactic medium exists in most galaxy groups or rlusters and it has been proposed that friction caused by movements of galaxies through this medium (ram pressure stripping) would explain the HI deficiency seen in many spiral galaxies belonging to clusters.

Apparent lagging halos have been observed recently in other edge-on galaxies.

Swaters et al. (1997) note that the HI halo of YGC 891 (5 kpc thick) rotates 25 to

100 km s-L slower than the disk gas. h gradient of 30 km s-l from 2 = 1 to 4.3 kpc has also been observed in Ha for this galavy (Rand, 1997). NGC 2403 (Schaap et al., 2000) and NGC 5775 (Rand, 2000) also exhibit lagging high latitude gas. In the case of NGC 5773. Lee et al. (2000) find discrete lagging high latitude HI features. similar to those seen here in NGC 2613. A discrete outflow which lags the disk in rotation velocity has also been noted in molecular gas in 4182 (Seaquist % Clark.

2001). 8. Discussion 88

It has t raditionally been thougttt that gatavies exhibiteci @indrical rotation, meaning that gas at a certain radius. r! would rotate with the same velocity. independent of its distance from the plane. z. We are now finding t hat t his is not the case.

There are two possible causes of change in velocity with height. One is a piivsical effect such that the higher latitude gas is indeed rotating more slowly (or perhaps not rotating at all) than the lower latitude gas. However, this effect could also rnanifest itself if one is observing through an inclined. thick gaseous disk. In this latter case. a given lincof-sight through the major avis will cut through gas above and below the plane as well and this gas atvay from the plane will have lower line-of-sight velocity components, causing the velocity averaged over the beam to decr~aseslightly.

This effect is heightened when observing above the midplane. Both Swaters et al.

(1937) and Schaap ct al. (2000) conclude that their data are most cûnsistênt witli a physical lagging halo and not geometrical effects. Both groups further suggest that the lagging halo is explainable by Bregman's galactic fountain mode1 whereby gûs rising from the disk would flow radially outward due to the decreased gravitational force and t hen its tangential velocity would decrease to conserve angular momenturn

(recall 5 1.1).

For the features observed in NGC 2613. there is no confusion against the disk at these heights, and therefore any velocity shift is certainly real. 1 have not ptobtained information on the velocity of a global halo. Homever. the velocity gradient observed in t hese discrete features is consistent *th the feat ures experiencing gaseous drag from a preexisting lower density (possibly hot) halo wit h a rotation lower t han t hat of the disk. The observed gradient is also qualitatively consistent with the galactic 8. Discussion 89 - fountain mode1 (Bregman 1980). as describecl sbuve. However. it is inconsistent quantitatively in the sense that in Bregman's model, the gas is expected to travel radially 2 - 3 times its original galactocentric distance as 2 increaçes. whereas the discrete features in YGC 2613 are roughly vertical. Data on NGC 5775 and NGC 891 are also showing velocity gradients with r which are not fully consistent with siich fountains (J. Collins. private communication). The data presented here agree better with the chimney model of Xorman Sr lkeuchi (1989). since there are discrete outflows from the disk rather than a more global movement of gas upward and outward as the fountain model predicts (recall Fig. 1.1).

If there is such a hot, low-density gaseocs halo. hor; did it form and why is it rotating more slowly than the disk gasb?The simplest explanation seems to be that this halo aas formcd at thc same time as the disk. The current popular view of galactic formation is a hierarchical one in which clumps of rnaterial accreted along large filaments of dark rnatter in the early Universe. The dark matter halos then formed at the intersections of these filaments and may have been given some initial spin by torques caused by neighbouring systems (Eggen, Lynden-Bell 91 Sandage.

1962: Fall Sr Efstathiou. 1980). Eventually. the gas in these halos would cool and collapse to form the galactic disks. It is intuitive that the collapsing gas would speed up in rotation to conserve angular momentum. Suppose that instead of al1 the primordial gas collapsing, some of the gas was heated during the collapse stage.

Then this left-over gas would cornpress slightly, into a diffuse halo with a rotation slower than that of the disk. Numerical simulations of cold dark matter (CDM) formation models fail to reproduce the large rotational velocities observed in disk 8. Discussion 90

galaxies (Xç'evarro & Steinmetz, 1997; 20(JO}. For e'rampte, Nawsrro Si Steinrnetz

(2000) lound that to match one of their simulations with observations' that the disk

mass would have to be only 30% of the total halo baryonic mass but must have

about 60% of the available angular momentum. This so-called "angular momenturn

problem" requires some heating mechanism to prevent al1 of the primordial gas from

lorrning a dense. slowly rotating disk early on. It should be noted that to this end. they incorporated both a photoionizing UV background and a star formation feed back mechanism into t heir simulationi2 and still failed to form such quickly rotating disks. They then concluded that some extreme heating mechanism is neecled in galactic formation rnodels that would substantially affect the cooling, accretion and star formation properties of dark matter halos. This "extreme'? heating may thcn have lcft this proposcd hot, lon=dcnsitç gaseous halo behind in the collapse stage. Furthermore. Navarro Sc Steinmetz (2000) also theorize that more massive galactic disks will rotate substantially faster than their dark matter halos. Thus the pre-existing hot, low-density gaseous halo, which lags the disk gas. being inferred from these HI data is not inconsistent with current galactic formation models.

There is evidence in X-rays for hot, diffuse gas in the halos of some starburst galaxies (Pietsch et al. 2000; see also Sofue 8i Vogler. 2001) and. in fact, there has been a soft S-ray upper limit reported in the literature (Burstein et al.. 1997) for

YGC 2613. This X-ray gas has traditionally been thought to be driven into the halo ' Specifically, they were trying to match the zero-point of the Tdy-Fisher relation. ' Navarro & Steinmetz (2000) incorporated the self-gravity of gas. stars. and dark matter. a three-dimensional treatment of the hydrodynamics of the gas, Compton and radiative cooling, a photoionizing UV background, and a star formation feedback rnechanisrn into their numerical sirnulat ions. by galactie winds from starbirrsts in the nudear regions of these galaxies. However, recent Chandra observations have revealed a diffuse X-ray corona. extending 10 kpc in r, around YGC 4631 (Wang et al.. 2000). In this case, the authors deem the ongin of the gas to be frorn chimney outflows from the disk. Thus, it is also possible that the hot gaseous halo may have originated after the formation of the galauy but this scenario does not explain the rotation difference between the disk and halo gas.

-4s a final consideration of the gaseous haIo. we can cornpute a density and a mass for such a halo based on the HI observations3. Let us assume t hen that the HI features cari be thought of as cylinders of radius R, density p' and height z that are moving with a speed u through a gaseoüs halo medium of density ph with a different speed vh. The drag force of the halo on the feature per unit area is then:

where Au is v - uh and ive will take v to be the rotation velocity of the disk directly beneath a given feature. The force of the outflow feature on the halo per unit volume is:

where duldz is the velocit~gradient with height. Dividing the drag force by the feature diameter. 2R. to give the force per unit volume and equating then gives a halo density of:

Xgain, this kind of calculation is only possible because the KI observations allow velocity fields to be probed in great detail. 8. Discussion 92

Noting that the density of the HI features is p = iVH/2Rwhere :VH is the cohimn density from the moment O map (Fig. 3.4) and that Au = z(dv/dz). we can rewrite

(8.7) as:

L'sing the velocity gradients and associated measured heights of the features from

Table 3.1. gives an average density for the gaseous halo of 2.4 x IO-* cm-3. which is an order of magnitude lower than the average HI density in the disk (- 0.10 cm-3. recall Table 3.1). These values are consistent with a density gradient in going from a cold gas disk to a hot lower-density gas halo. We IIOW calculate a rough estimate of the mass of such a halo. It is thought that any gas in the halo region would trace the dark matter. so we will adopt the density distribution profile of the clark niatt~rfor the proposed gas halo. The density of the spherical halo is a function of the radius, r. and is given by Navarro. Frenk Sr White (1996,1997):

where pa is the density at the centre of the system (which tve take to be 0.01 and r, is the scale radius which is determined to be 4.11 kpc when we impose the condition that the density should be 0.024 crü3 at a radius of 6.3 kpc (the average height that the velocity gradients were measured at). Let us now assume that the gas halo extends at least out to the radius of the HI disk (R = 35 kpc)? which will probably give an underestimate of the gas halo mass since the halo likely extends further out than the disk. Integrating (8.9) over the spherical volume elernent. dV = 8. Discussion 93

r2sin BdrdedQ leads to:

and a mass of 3.0 x 108 SIg. SIany assumptions have been invoked to arrive at this value. however it is a reasonable value. being an ~rderof magnitude less than the mass of the HI disk.

In summary. these observations lead to strong indirect evidence of a hot. low- density gaseous halo of mass an order of magnitude lower than the disk mas. Direct evidence will have to tvait unt il high-resolution X-ray observations can be obtained on t his gala~y.

8.2.3 The Formation of Nigb Latitude HI Features in NGC 2613

We will now discuss the possible mechanisms of formation for the high latitude features seen in 3GC 2613. The models that will be considered include the interna1 generation schemes of clustered supernovae and a jet-disk interaction and the exter- na1 scheme of impacting high velocity clouds. Please refer back to Section 1.2 for a detailed description of these models. Xote that these rnodels? for the niost part. describe shell formation mechanisrns. The features obsened in 'iGC 2613 do not appear shell-like (save perhaps F2/F2a in Fig. 6.4) but it is likely that such rnecha- nisrns are applicable to the formation of expanding high latitude features in general.

For example. once supershells have blown out into the halo. its structure is expected to break apart. 8. Discussion 94

Egh Veiocity Cloud Impacts

In the case of NGC 2613. let us first consider whethcr impacting clouds could produce the observed high latitude features. As Ive have seen. there does appear to be a mild interaction going on between 'IGC 1613 and its cornpanion. There is also some evidence for high latitude gas between the galaxies, possibly in an arc or stream (see discussion in 5 8.1). and consequently a source of irnpacting clouds is potentially present. If each pair of features is physically associated. then there would have been three impacts. with clouds travelling straight through the disk. If the pairs of features at the ends of the disk are not symrnetric then we would have expected four non-penetrating impacts to have caused FI, F2, F5 and F6 and one impact rvhich penetrated the disk to cause F3 and F1 and the associated continuum disturbances.

It is debatable whether impacting clouds could pass straight through the disk. For example, Santi1li.n et al. (2000) indicate that impacting ciouds would not penetrate the disk due to the magnetic field pressure caused by field lines paralle1 to the disk.

However. if the disk were porous and the magnetic field lines were perpendicular to the disk, perhaps the clouds could have passed al1 the way through so this should be considered in more detail.

It is important to note that each feature shows a similar velocity structure in a direction away from the plane (discussed in 8.2.2): that is, there is an obsemed decline of rotational velocity with height for al1 six features. Note that any infalling clouds must impact the disk at an inclination which has a line of sight component or we would not expect to observe any velocity difference between the feature and 8. Discussion 95

the underljing disk. This impties that on the reckkifted (east) side of the gafauy

(applicable to Fl/F2 and F31F-I) the approaching high velocity clouds would have

to be redshifted with respect to the disk gas, in order t hat the "splash" from the

impacts reproduce the lag with height (blueshift on this side) observeci in the velocity

fields of the features (see Santillan et al. -000). However, on the blueshifted side of the galaxy (relevant for F5/F6. on the West side) the impacting clouds would have to be blueshifted with respect to the disk gas in order to produce redshifted splash. This then requires two streams of potential infalling material. one in front of the galauy (on the east side) and one behind (on the west side). to reproduce the geometry needed. .Uso. impacts from both streams rnust have occurred at about the same time in order for the time scales for al1 the features to be similar (Table 3.1).

Though possible, this seerns improbable. Sforeover. the case ùf F3 aiid F4, wliich i have argued are associated with each other. would require one cloud to pass through the disk to cause both features. If this is the case, then the cloud has probably passed through a low density region in the disk and it is unlikely that material would be ejected both above and below the plane in a similar rnanner. If ejected material were produced on both sides of the plane. then the ejecta on the impacting side of the disk should be blueshifted but the material ejected on the ot her side of the disk should then be redshifted. This is clearly not what is seen in the PV plots of F3 and F4 (Fig. 3-13)?as both features show an increasing blueshift with distance from the midplane. Finally. it is unlikely that a radio continuum knot i~ouldbe seen in the disk between F3 and F4 if indeed this area of the disk has a low density. Thus. the incompatibility of the observed velocity structures wit h knom impacting cloud 8. Discussion 96

models argues for an interna1 origin of the high taticacte features in NGC 2623.

Correlated Supernovae

Let us then consider an interna1 mechanism of formation. The supernova input ener-

gies for the features are on the order of 10~~to 10j6 ergs (see Table 3.1). This implies

that 10' to 105 spatially correlated supernovae would be needed. in a timespan of be-

tween 10' - 108 pars. to form each feature. Super star clusters in some starburst gal-

axies contain 10" OB stars (Sleurer et al.. 1995). 'IGC 2613 has a star formation rate

for stars between 0.1 and 100 XI, of SF&.l-iai LI, = 5.13 yr-l = 0.57 SFRhr82

(Invin. English Sr Sorathia. 1999) so it is somewvhat less active than a starburst

galaxy. In addition. if the features are symmetric about the midplane then three

starhiirsts woiild be need~d.~arh powprfiil ~nniightn mak~two featurer, meabove

and below the plane. Also, the three starbursts would have al1 occurred around the same time? since the features al1 have similar ages. Furtherrnore, two of them must

have occurred at large galactocentric radii. There does seern to be some evidence

in the 2000 D array radio continuum for in-disk star formation (SF) on the eastern edge of the stellar disk but this continuum emission still does not extend out as far as F1/F2 and there is no such evidence in the radio continuum for star formation on the western side of the disk (where F5/F6 reside). Recall that Figs. 3.9 and 3.10 show a strong continuum source in the disk and high latitude extensions which lie on the perimeter of F3 and F4. This strongly suggests that F3 and F-4 were both caused by one disturbance in the disk. Note that the strong continuum source has also been seen at 843 MHz (Harnett & Reynolds, 1991). This source has a luminosity 8. Discussion 97

of 1.1 x idLW Hz-L(frorn the CnB data). The brightest supernova remnant [SNR)

in LI82 has a radio power (scaled to 1425 MHz) of 1.16 x 1020 W Hz-' while the

brightest SNR in the Galaxy (Cas A) has power of 1.85 x 1oL8W HZ-'(Invin. Saikia

Sr English. 2000). Comparing. ive find that this continuum source could contain be-

tween 10 and 600 SXRs. Obviously. this number is much srnaller than the number

required to form the HI features seen above and below this source. Invin. Saikia

Si English (2000) observed this galaxy in 20 cm continuum using the VLA A array

and did not detect this source. Their noise level was 0.043 mJy beam-l. well helow

the noise level of the CnB array continuiim map (0.15 mJy beani-l). This suggests

that this source is resolved out at A array, leading to the conclusion that the source

must be > 300 pc and is probably indeed a region of SNRs and not one powerful

point source. It is doubtful then that this ccjntinuun~source Ly i&i rryressiits a

disturbance in the disk capable of forming the features seen in the vicinity

Given that one pair of features (F5/F6) at the western end of the disk show no

in-disk SF (in the radio continuum) and one pair (F3/F4) shows an associated in- disk continuum knot that appears energetically unable to generate the high latitude feat ures. two possibilities present t hemselves in the starburst scenario. Eit her t here has been much stronger starburst activity in the past which has died dom in a time less than the age of the features (a few 10' years) or the radio continuum emission is not providing an accurate picture of the SF activity. Even if SF regions could be identified below these features. it would stiil be difficult (especially at large galactocentric radii) to assign such large energies to these regions. It is likely that some additional factor is involved. such as non-linear Parker Instabilities (see 5 1.2.1). 8. Discussion 98

Jet-Disk Interaction

The remaining scenario is that of a radio jet which flows through the ISbI in the plane of the disk. flaring out where there are changes in the IShI density (Gopal-

Krishna & Irwin, 2000). A11 six of the high latitude features in this gala- couid have been caused by one jet pair. Large features near the edge of the optical disk

(see Fig. 3.4). where there is a rapid decline in ISM density, would be expected in this model. FI. F2. F5, and F6 are positioned near the end of the optical ciisk and could have been produced in this fashion. although F5 and F6 seem to be misaligned which cornplicates this scenario. The central features (F3 and F4) could also have been caused by a jet-ISM interaction at the position of the radio continuum knot.

In this case. the knot would not represent many SNRs but rather an extended knot of ernission due to shocks as the jet interacts with the ISM. The plausibility of this scheme depends on the initial orientation of the jet (i.e. in the plane), whether it has sufficient energy to reach the ends of the disk, and whether its radio continuum decay time (t-ypically 106 to 10' years) is shorter than the age of the features (here

- 10' - 108 years, Table 3.1). Since we are seeing these features in projection, ive cannot be sure about their positions along the line of sight. If this model is correct. there must be a compact object in the nucleus of the galaxy. From the CnB rotation curve (Fig. 3.6), a mass of 1.5 x 108 Mo was calculated Rithin 1 kpc of the nucleus.

This would be an upper limit for a compact object in this galaxy, should one exist.

Bower et al. ( 1993) used stellar velocity dispersions in NGC 2613 to look for signs of a supermassive (log Mo)black hole (SMBH) at the centre of the galaxy and concluded 8. Discussion 99 that both th& mode1 that inctuded a SMBH and their constant mas-tdight ratio model (that did not have a SXIBH) agreed with the observations. Currently there is no evidence for a compact object or nuclear activity in this galauy4 although no such evidence has been specifically sought out (excepting Bower et al.. 1993).

Summary of Formation Models

The HI observations presented here have allowed for a detaiied analysis of the velocity fields of the high latitude features as well as the disk gas on the whole. Other ISbI tracers. such as radio continuum or Ha. either do not give velocity information at al1 or give velocities only in thin slices of the sky. The similarity of the velocity fields of the six high latitude features (ie. the velocity gradient with height) strongly

RT~IIPS against impacts hy high v~locityrlotlrls as the rniiw of these fwttlr~s.The continuum/HI correlation also argues that these features were most likeîy formed by a mechanism interna1 to the galauy. If the supernovae/stellar winds explanation is correct. then very large numbers of SWe far from the nucleus would be required.

There is no evidence in the radio continuum to support t his. If the jet-disk interaction model is correct. then there would have to be (or recently have been) a jet ejected at a small angle to the plane of the galaxy and the three pairs of features would have to be nearly aligned with this jet. This information cannot be gained from the data presented here. See Section 9.2 for suggested future obsewat ions.

It would seem that we must continue to improve supernovae outflow models as

-' This galaxy was a non-detection in 20 cm continuum at A array (Irwin, Saikia St Engiïsh. 2000) which might ruie out an active jet in the recent past. X-rays are also a good tracer of nuclear activity and there has been a soft ,Y-ray upper limit reporteci (Burstein et aI., 1997) but no high resolution ,Y-ray data are available. 8. Discussion 100 rr.eH as magnetic and tidsl dismption models. Our outflow models ahneed to be more specific in predicting which components dlgo where (ie. Will the hot ionized gas always be on the inside of neutral gas in outflow features'? Where should the continuum emission lie'?). There remains the larger issue of the galactic circulation models. which have not received much attention since Bregman's 1980 work. Clearly. ive are now beginning to see that the interplay between the disk and halo and the dynamical exchange of gas is more cornplex and challenging a mystery than we t hought. 9. CONCLUSIONS AND FUTURE RESEARCH

9.1 Major Findings

This thesis has presented an analysis of the neutral hydrogen environment of NGC 2613.

The HI observations. through detailed velocity information. have allowed a better

understanding of the disk-halo connection in this galauy. These data are the first to

show that ES0 495- G 017 is indeed a companion galauy of NGC 2613 and that the

two galaxies will likely merge in the future. Consequences of the current interaction

between the galaxies may include a warping of the disk of NGC 2613 with two possible small tidal tails and an asymrnetry of gas in the companion. possibly caused by tidal stripping. This research has resulted in the first values of the dynamical mass of

NGC 2613 (found to be extremely high), the first published (Cliaves Sr Invin. 2001) channel maps and moment 1 map, as well as an improved moment O map. The Brst map and mass of the companion, ES0 495- G 017. have also been presented-

There are six high latitude features seen in NGC 2613, in three pairs apparently symmetric about the midplane. Two pairs reside roughly at either end of the optical disk. while the third pair lies close to the galaxy's centre. These features, which do not appear shell-like have an age on the order of 10' years if the largest radial wlocity range is taken to be representative of an expansion or 10' years if a smaller velocity range is used. The masses of these features range from 10' to 10' SIo. 9. Condusions and Future Research 102

Several important resutts of this work have corne fmm the apparent dectine of

the rotational velocity toward systemic of the discrete features with increasing height

above the plane observed in the velocity fields. This gradient rules out secular motion of the galavy t hrough an intergalactic medium since in that case the features would be al1 either redshifted or blueshifted with height. It is unlikely that the gradient is due to a Bregman-style galactic fountain since the features do not show any displacement parallel to the major auis. This result is the first to challenge Bregrnan's galactic fountain mode1 as an explanation of the lagging high latitude gas seen in this and some other galaxies. As an alternative explanation for this lag with height. it has been suggested that the neutral gas is being ejected into a preexisting lower density

(possibly hot) gaseous halo with slower rotation. A density of 0.002 cm-% a height of 6 kpc and a mass of 3 x 10' 11 have been calculated fur thb guruus lialu, aÿsu~iiirig that the gas is arranged in a spherical halo of radius 35 kpc and foilows the density distribution of the dark matter halo. This is an important result since only a few galaxies show evidence for a halo region with slower rotation than that of the disk. I postulate that this halo has formed along nith the disk, out of the dark matter halo.

Another interesting result of the velocity fields is the presence of spurs in the features with several distinct velocities. These different velocity components of the gas suggest that the structure of the features could be complex or unstable. There are also several srna11 components of gas at low intensity levels, mostly above the features. moving with both red and blueshifted velocities. These could be the results of a break up of the features once they have reached a certain height which would implp that the gas enters another velocity regime at z $ 10 kpc in which high velocity 9. Conciusions and Future Researcb f 03 dispersions exist (ie. a Population II regirne). This behaiiour is inconsistent mith any simple galactic circulation model.

CVe have also seen a stunning correlation between the central pair of HI features and the continuum emission. Here, there is a strong continuum source in the disk with two large featiires ertending from it and arranged along the outside of the HI features. These are the largest continuum extensions, smoothly connected to the disk of a spiral galaxy ever detected. The relative positions of different components of the ISM is not included in current outflotv models. however the simple conclusion that the relativistic electrons are leading the heavier hydrogen atoms in the outfiow is consistent with the data.

It has been argued t hat the features in NGC 2613 have most likely been formed by some mcchanisrn intcrnnl tc thc galaq due to the common gradient in velucily witii height observed for a11 the features. Multiple supernovae and magnetic instabilities rnay have formed the features but the high energies of the features and their large galactocentric positions present objections to this model. .\lternatively, the features may have been caused by a relativistic jet, travelling almost parallel to the plane through the ISbI. that flared up when it reached the density drop at the end of the optical disk and pushed the surrounding gas into the halo. This model requires a compact object in this galaxy as well as a small ejection angle for the proposed jet.

9.2 Possible Future Work on NGC 2613

There are many avenues for future research open to this galq In keeping with

HI observations, there are several things which could be done. One is to observe 9. Conclusions and Future Researcb 104

XGC 2613 again in D array at the VL4. Longer observation times (when the sun is not at maximum activity) would allow for a lower noise level and more low Ievel ernission to be seen. Observations of this nature could help to study broader structures and more of the disk gas. This would help determine whether the Iiigh latitude features we have seen are actually part of larger, more coherent structures and whet her we are seeing an arc or Stream of HI around the two galaxies. More continuum observations at different arrays could also be helpful in determining the reality of suspected large arcs from the disk as well.

Observations in other tvavebands could also be very helpful to the understanding of the circulation of interstellar material in this galaxy. Low resolution X-ray observations should definitelp be taken, as it would confirm the existence of a hot gaseous halo in this plau- .Uso. some highly ionixed spcrirs: siirh as 0TV;miilri provid~ velocity on such a hot ionized halo. High resolution 'Y-ray data may also point out the existence or lack thereof of an active compact source in the core of this galauy. t hus strengt hening or weakening the jet-disk interaction model for the formation of the high latitude features. Ha observations would be able to trace the regions of star formation in the disk. This would greatly help to determine if the high latitude features seen here were formed by correiated supernovae. If there was no correlation seen between star forming regions and the HI features. this would argue against the supernova format ion model in t his gai- Submillimetre observations would trace the molecdar gas and dust in this gaiaxy and help to understand the structure of the high latitude features. the nature of the outflow of material into the halo, and the structure of the ISXl on the whoie. Such correlations between HI features and 9. Coaclusiom and fiture ResearLh 105 rnolecular gas, dust. and cookiauum feetures are oow being seen in other edge-on spirals and are helpful in studying outflow physics.

Finally, modelling and simulations of the results found here would also be helpful.

In particular. the modelling routine of Irwin Sr Seaquist (1991) could be used to esplore the intrinsic properties of both the disk and any gaseous halo that might exist. This modelling would allow us to determine parameters such as the density. scale height and rotation velocity of such a halo. 1 would also like to see t hese results and those of other similar galaxies being compared to simulations of new rnodels of galactic circulation which are now being created. REFEREN'CES

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This section reviews some of the basic principles involved in radio telescopes. both single-dis h and interferomet ers' and synt hesis imaging. See Ro hlfs Sr Wilson (2000) for more detail.

A.1 Single Dish Antenna Theory

The Reciprocity Theorem states that a receiving and a transmitting antenna are indistinguishable. Thus. for any given antenna, its receiving and transmitting prop erties are equivalent. For illustrative purposes. we wi11 now consider a parabolic transrnitting antenna with respect to a distant point. This is equivalent to a one dimensional ffat antenna. radiating a plane wave. Consider also a point P. in the

Far-field. a distance R away from the centre of a Bat aperture (see Fig. .A. 1). Let the emitted electric field. E. at a point x in the aperture be

E (x)= Eo(x)e i(d-4(~)) (A.1) tvhere Eo(x)is the amplitude, w is the angular frequency, and d(x) is the phase of the elect romagnetic wave, where it is understood that the instantaneous electric field is giwn by the real part of this quantity. Then. by Huygens' principle. the response A. Radio Telescope Theory 113 at P due to mission from an etement dx in the aperture mit1 be

where r/X is the distance to P in units of A, the chosen transmitting (or observing) wavelengt h. However. r = R + x sin O. Letting sin t9 = S. r = R + xs. so

For any given point. P. R and 0 are fixed so the quantity e-'(2"R'A) is a constant and we can define a quantity F(s) which is proportional to the electric field at point P due to contributions from the entire aperture:

where now x is erpressed in units of wavelength. X. This is a 1 dimensional Fourier integral which gives the response at a point P to an ernitted field E as a function of the off-auis angle 0 and distance across the aperture, in units of wavelength. x/X.

It follows then that the field forms a Fourier Transform (FT) pair with the response and can be inverted to give

We would expect the field transmitted by the antenna (the aperture distribution) to be a "top hat" function. that is to be constant over the aperture and drop to zero at the edges. The FT of this function is a sinc function. Fig. 1.2 (top row) shows the two functions. A Nider top hat wiil produce a nanower sinc function. This is andogous to a wider aperture providing better resolution. The angular power A. Radio Teiescope Theory 114

Figure -4.1: A Bat 1 D aperture radiating into the far-field. pattern of the antenna is equal to the square of the far-field pattern (illustrated in

Fig. -4.2. bottom right). From Fourier theory. it is also equivalent to the FT of the autocorrelat ion function (XF.convoiut ion of a function wit h itself) of the aperture distribution (see Fig. -4.2).Svte that multiplicativn in une spaw eurrr+uiicls LU convolut ion in Fourier space. so that if

FT{f (41 = F(4

FT{9(x) = G(s) t hen

where * represents convolution. For a real receiving antenna. the top hat function

E(x)can be thought of as the actual evenly illuminated aperture (radio dish) on the ground and the anguiar power spectrum can be thought of as the response of that antenna to a point source in the sky as a function of angle.

The power received, CI.' (in ergs s- '), from incident radiation on a perfect , lossless. A. Radio Telescope Theory If5

Figure A.2: The Fourier Transform pairs of the aperture distribution and the angular spec- trurn.

2 D antenna is

where Sv is the flux density (in ergs s-l cmd2 HZ-l). ;Lp is the collecting area (in cm-*) and -IV is the bandwidth (in Hz). For a real antenna, however. some of the radiation dlheat the antenna and not be detected, while some of the incoming radiation will be scattered. The power expression then becomes

where the one half cornes in because only one polarization is detected at a time and q~ is the radiation efficiency. By the Yyquist Theorem

where k is the Boltzmann constant and T4is the antenna temperature. which is the temperature of a resistor which wouid deliver the same power output as is being A. Radio 'Ikiescope Theory II6 deteeteci by the antenna. For an antenna pointed dkctiy at the center (80, of a source of small angular size, the flux density is

where 1, is the specific intensity P,,is the normalized power pattern of the aperture. and Rs is the solid angle of the source. Yote t hat 0 is normally rneasured from the zenith down to the source tvhile d is an azimuthal angle measured from an arbi- tra. point. The specific intensity of the radiating source is related to its brightness t emperat ure TB by the Rayleigh-Jeans relation

(A.7)

Putting this al1 together gives

where nom R.4 is the beam solid angle. the angle through which al1 the power from a transmitting aritenna would stream if the power were constant over this angle. The antenna temperature is therefore a convolution of the source brightness temperature with the beam power pattern. The antenna temperature can be measured by comparing the noise power output from the antenna to the noise power output from a matched load of known temperature. Knowing the beam power pattern as well. one can then obtain the brightness temperature (or specific intensity. via Equation -4.7) of the source. The power response of a two element interferorneter (in 1 D) is

where TB is the brightness temperature of the source. D is the distance between the two antennas (the baseline), P, is the normalized response of a single antenna, 0 is the angular position somewhere on the source, and Bo is the angular position of the centre of the source being observed on the sky, at which both antennae are pointing.

We now introduce the est-west spatial frequency

(A.10) wliicli is tllr pruje~t.ec[biwaliiie (iii wavele~ighu~ii~sj as seeii frorri the direction of the source (see Fig. -4.3). Setting 61 = 0 - Bo and noting that the respoose of a baseline of length zero (a single antenna) is

we can rewrite the power response of our two element system as

The quantity in square brackets is a normalized FT of the 1 D brightness distribution

(modified by the single dish power pattern). This is called the fringe ç-isibility? C'(u). and it is a ccmplex quantity with an amplitude and phase. It is the vector amplitude of the oscillating hinge e-'"U "&. The output from the baseline consists of the sum A. Radio Teleseope Theory 118

over the source of the brightness distriburion f tirnes the power pattern of a single

dish) multiplied by the fringe pattern. If WC let

C'(u) = 1 VI eluv

then P simplifies to

where tbo represents the phase offset due to the hour angle of the source (a known

quantity) and is the visibility phase which contains information on the source and

its brightness distribution. This powver response is the signal which is output frorn

the correiator (part of the antenna hardware) and both its amplitude and phase can be rneztrured. The amplitude can be conserted to appropriate astrophysicnl units through calibration against a reference point source of known flux. The phase can also be calibrated to a nearby point source whose phase is defined to be zero.

These calibrations are done with the VLACALIB and VLA CLCAL tasks within the

Astronomical Image Processing System (AIPS).

In 2 D, a north-south spatial frequency can also be defined D v = - cos &, (A. 13) X and rve can generalize the calibrated. measured visibiiity to

where we are expressing the source brightness distribution in terms of I again (by

Eqn. -4.7) and 0 and t$ are angular positions on the sky, relative to the centre of the ,4. Radio Teiescope Theory 119

Figure A.3: Projected baseiine u. object, and both V(u9u) and I(B1,d) have units of Jy bearn-'. where a Jansky (Jy) is l~-~~erg s-l cm-? Hz-'. So. if one wants to recover the specific intensity as a lunction of angle in the sky. one sirnply takes the inverse FT of the visibility:

Since the response of a single dish, P., is well known, in principle it is a simple matter to divide it out to recover I(Br,4').

Sampling a single baseline recovers a single Fourier component from the source. We must then sample every baseline in the uv plane to recover dl the Fourier components

(i.e. al1 spatial variations) over the source. This is equivalent to observing the source with a single large fi11ed aperture within which phase information is retained

(hence the term aperture synthesis). Generdly, however, there dlbe gaps in the uu plane. If not every Fourier component of the source is sampled, the map obtained by A. Radio Telescope Theory 120 apptying the inverse FT ditt only be an appdrnation of the mesource structure.

The sampling function S(u.o) is a collection of ones and zeros and reflects the

*'holes" in the uu plane where data are not collected but must still be considered.

The "dirty image" is what we get immediately after the FT is performed and is given by

The "dirty beam" is the response of the array if al1 the antennas are pointing to a point source of unit magnitude and is given bv

Writing these as FTs gives VZ = FT{SLW}

1P,= FT{CV}

va = FTIS}. from Eqns -4.15. h.13. and A.16, respectively. 'lote that now 1 represents the intensity of the image with al1 measured Fourier components rather than a single

Fourier component as specified in Eqn. A.15. By Eqn. A.4. we can wnte the dirty image as

The di- image is what we get from the FT of the incoming signai. CVe know the dirty beam (as we know the positions of the antennas) so we just have to deconvolve the dirty beam from the dirty image to obtain the "clean" image and t hen divide A. Radio Teiescope Theory 121

y the primary bearn. P.. These concepts are ihstrated in Fig. A.4, mhich shows

the dirty beam (upper left), the dirty image (lower left), and the clean image before

(upper right) and aRer (lower right) the primary beam is divided out.

A.4 Cleaning and uu Weighting Within AIPS

The CL EAN algorithm used in AIPS performs the deconvolution task. The routine proceeds as follows. First. the brightest peak point on the dirty map is found. The dirty beam pattern centred at that position is divided out. This effectively removes al1 sidelobes due to this point and also rernoves the spike of real emission from the map. The task keeps track of what is being removed by placing the removed fluxes and corresponding positions in a clean components table. The next brightest point is chosen and the process repeats until al1 emission is removed and only noise remains in the rnap. .\II points (as recorded in the clean components table) are then restored to the rnap, reconvolving with a clean bearnl.

Within AIPS, different weightings can be given to the sampling function. equivalent to different parts of the uv plane. So called natural weighting giws al1 visibilities equai weight. Since there are naturally more visibilities toward the centre of the au plane (due to the earth's rotation), the sampling function is seemingly weighted toward the centre of the uv plane, giving lower spatial resolution but higher signal- to-noise. Lniform weighting gives equal weight per unit area in the UV plane. This effective- increases the weight of the outer parts of the UV plane. which results in higher angular resolution but lower signal-to-noise since some data is disregarded. The clean beam wiil be a true Gaussian wit h the same MI width at half maximum as the dirty beam but zithout any sidelobes- A. Radio Telescope Theory 122

Figure A.4: The dirty beam for the Cd3 may is shown at the upper left and has contours of 7. 20, 50. 100, 500 mJy beam-l. The dirty image (lower left), clean image (upper right). and prirnary beam correcteci clean image (lower right) are shown for one velocity charme1 (1964.7 km s-') of the CnB array data. Thesc three images have contour levels of 0.64, 1.5, 3, 5, 7, 10, 20 mJy beam-l. where 0.64 is the 20 level for the cleaned, uncorrected data cube. Note that the synthesized or clean beam (see 5 A.4) is shown at the lower Ieft of the latter t hree images. 4. Radio Telescope Theory 123

-411 data sets induded in this thesis have ken CLEANed with riaturd hgtrting to accent uate any broad scale structure.