Radio Continuum Imaging of the Massive Edge-On Galaxy NGC 2613

Radio Continuum Imaging of the Massive Edge-on Galaxy NGC 2613

Amanda Desouza

A thesis submitted to the Graduate Program in Physics,

Engineering Physics & Astronomy

in conformity with the requirements for the

Degree of Master of Science

Queen’s University

Kingston, Ontario, Canada

September 2017

We present the results of new, high sensitivity continuum imaging of NGC 2613, a massive edge-on galaxy 23.4 Mpc away. The high-resolution EVLA maps of NGC

2613 were observed at C-band (C-array and D-array) and L-band (B-array, C-array and D-array), as part of the Continuum Halos in Nearby Galaxies (CHANG-ES) survey. Using new techniques during the imaging process, we successfully generate in-band combination maps for all arrays at each band. We also create a combined array and frequency map (C-band and L-band); with an intermediate frequency of

4.13 GHz, it is one of the few known images for a galaxy generated using multiple data sets at diﬀerent frequency bands.

High resolution maps of NGC 2613 reveal the presence of a compact nuclear source at the centre of the galaxy. We ﬁnd evidence for the presence of an AGN, as predicted in Li et al.(2006), as the source of this feature. Gaussian ﬁts to the radio core allow us to constrain the position of the galactic centre, with RA =

08h33m22.776s±0.004s & DEC = −22◦58024.86”±0.16” and a core size of ∼ 200pc.

We detect the presence of several continuum features embedded in the disk of the

galaxy, including the detection of a complex spiral feature in the eastern edge of

the galaxy. We also uncover the existence of a broad continuum ring along which

we ﬁnd the presence of several continuum ”hotspots”, including the eastern spiral

feature. The broad ring width suggests an older ring formed by the collision of the ii galaxy with its companion, ESO 495-G017, with the eastern spiral feature being an artifact of the collision.

We also detect the presence of large extensions surrounding the disk of the galaxy, some of which reach heights of 12 kpc making them among the largest such extensions found. The extensions are believed to have formed through internal mechanisms, like supernovae, and appear to be connected to the continuum features detected in the galaxy. The presence of large loop-like extensions above and below the radio core hints at the existence of early AGN activity. Acknowledgements

I would like to take this opportunity to thank several people for the help I have received during my time at Queen’s University.

First I would like to thank my supervisor, Judith Irwin, for her advice and continuous support during my time at Queen’s and away. Her belief in me pushed me to give my best and will continue to push me forward in the future and I will remain forever grateful for her mentorship. I’d also like to extend my appreciation to the members of the CHANG-ES consortium, with special thanks to Theresa Wiegert, Amanda Kepley and Carlos Vargas.

I’d also like to thank the Astronomy department and faculty members, with a special shout out to my fellow graduate students. I would also like to thank the support staﬀ at the Physics and Astronomy department; Loanne, Tammi, Kyra and Peggy.

A high ﬁve for my old room-mate and drinking buddy, Alex, for his friendship and support with special thanks for completing levels in GTA:V that I would get stuck on. To my BFF and soul sister, Jen, who continues to be my cheerleader. Your continued faith in me and wise words can power me through any diﬃculty.

My parents, Judson and Angela, who pushed me to follow my dream, I will never be able to thank you enough. To my sisters, Averil and Abigayl, thank you for being there for me, no matter how hard it got. Also, pets and treats for my cats, Lsa and Pudding, for being cute and distracting me from my work. I’d also like to extend thanks to my new family, Peter, Mars and Dave. This thesis could not have been completed without your support during my last two semesters at Queen’s University.

Finally, I’d like to thank my partner in crime and in life, Richard. I couldn’t have made it this far without your love and support. This is for you.

iii Contents

Abstracti

Acknowledgements iii

Contents iv

List of Figures vii

List of Tables xi

Abbreviations xii

Symbols xiv

1 Introduction1 1.1 NGC 2613...... 2 1.2 Continuum HAlos in Nearby Galaxies: an EVLA Survey (CHANG- ES)...... 5

2 Data Acquisition and Reduction6 2.1 Observing...... 6 2.1.1 Jansky Very Large Array (EVLA)...... 6 2.1.1.1 Interferometers...... 8 2.1.1.2 CHANG-ES Observations of NGC 2613...... 11

3 Data Reduction 14 3.1 Flagging and Calibration...... 15 3.2 Total Intensity Imaging...... 20 3.3 Polarization Calibration and Imaging...... 26 3.4 Combined Array And Frequency Images...... 28 3.5 Table of parameters...... 29

iv Contents v

4 Results 31 4.1 Total Intensity Emission...... 32 4.1.1 Combined Data Sets...... 37 4.2 Spectral index maps...... 40 4.3 Continuum features - new results...... 43 4.3.1 Continuum ring...... 43 4.3.2 Central nuclear source (N2613-A)...... 44 4.3.2.1 Core size and astrometry...... 45 4.3.2.2 Flux Densities and Spectrum...... 47 4.3.3 Spiral feature - F1...... 50 4.3.4 North-western tail feature - F2...... 51 4.3.5 C-shaped feature - F3 and the F3 complex...... 51 4.3.6 Large scale continuum extensions...... 52 4.4 NGC 2613 at Other Wavebands...... 55

5 Separation of thermal and non-thermal emission 61 5.1 Introduction...... 61 5.2 Methodology...... 64 5.2.1 Preparing the data for thermal/non-thermal separation... 64 5.2.2 Extinction-correction for Hα emission...... 67 5.2.3 IR-only method...... 67 5.2.4 Mixture method...... 68 5.2.5 Obtaining the thermal ﬂux...... 69 5.2.6 Emission measure (EM) method...... 69 5.2.7 Star formation rate (SFR) method...... 70 5.2.7.1 Murphy Method...... 70 5.2.7.2 Jarrett Method...... 71 5.3 Results...... 72

6 Discussion 80 6.1 The radio disk...... 80 6.1.1 Origin of the continuum ring...... 83 6.1.2 Spiral feature - F1...... 84 6.1.3 Tidal feature - F2...... 86 6.2 Extra-planar features...... 86 6.3 Central nuclear source - NGC2613-A...... 88 6.3.1 AGN...... 89 6.3.2 Supernovae remnants...... 90 6.4 Thermal/Non-Thermal separation of the continuum emission.... 91

7 Conclusion 92 Contents vi

A Stokes Parameters 95

Bibliography 98 List of Figures

1.1 Optical image of NGC 2613 taken from the Las Campanas Obser- vatory in Chile at λ = 4050 A(˚ Sandage & Bedke, 1994)...... 2

2.1 Satellite view of the JVLA. The red line makes the longest baseline of the array, and yellow marks the shortest. (Google, 2016).....7 2.2 Block diagram for a baseline response. The plane wave approached dish 1 with a time delay τ. The measured voltages at each dish are multiplied and averaged at the correlator. (NRAO, 2016)...... 9

3.1 C-array L-band flux calibrator data. Different colors signify the contiguous spws. Top: Unflagged data before calibration. Middle: Flagged data before calibration (with initial corrections applied). Bottom: Flagged data after calibration...... 18 3.2 NGC 2613 C-array L-band data - Flagged and calibrated data for the galaxy...... 19 3.3 Top row: D-array C-band total intensity maps of NGC 2613 with contours (non-pbcorrected) at 20.2 µJy/beam, in white. The beam is represented by a white ellipse at the bottom-right corner. Left: Non-PB-corrected. Right: PB-corrected. Bottow row: D-array C-band spectral index maps of NGC 2613 with contours, in black, at 20.2 µJy/beam. The beam is represented by a white ellipse in the bottom-right corner.Left: Non-PB-corrected. Right: PB-corrected. 24

4.1 C-band total intensity maps of NGC 2613 with contours in magenta. The beam is represented by a white ellipse at the bottom-right corner and the lowest contour level is marked in thick white line. Top row: D-array: Left - Robust 0 (RMS = 10.1 µJy/beam) - with contours at 20.2, 50.5, 101, 202, 404 and 808 µJy/beam. Right - Robust 0 + 6 klambda uv-taper (RMS = 11.8 µJy/beam)with contours at 23.6, 59, 118, 236, 472 and 944 µJy/beam. Bottow row: C-array: Left - Robust 0 (RMS = 3.3 µJy/beam) with contours at 12, 40, 100, 200 and 300 µJy/beam. Right - Robust 0 + 16 klambda (RMS = 3.3 µJy/beam) uv-taper with contours at 12, 25, 50, 100 and 200 µJy/beam...... 32

vii List of Figures viii

4.2 L-band total intensity maps of NGC 2613 with contours in magenta. The beam is represented by a white ellipse at the bottom-right corner and the lowest contour level is marked in thick white line. Top row: D-array: Left - Robust 0 (RMS = 42 µJy/beam) with contours at 84, 150, 300, 1000, 2000, 4000 and 9000 µJy/beam. Right - Robust 0 + 2.5 klambda uv-taper (RMS = 45 µJy/beam) with contours at 90, 200, 500, 1200, 3000, 7000 and 12000 µJy/beam. Middle row: C-array: Left - Robust 0 (RMS = 27.6 µJy/beam) with contours at 90, 270, 500, 550, 700, 900, 1000, 1500, 3000, 7000 and 10000 µJy/beam. Right - Robust 0 + 6 uv-klambda uv-taper (RMS = 31.2 µJy/beam) with contours at 90, 270, 500, 550, 700, 900, 1000, 1200 and 1400 µJy/beam. Bottow row: B-array: Left - Robust 0 (RMS = 19.6 µJy/beam) with contours at 12, 50, 60, 80, 100, 150, 200 and 250µJy/beam. Right - Robust 0 + 16 klambda uv-taper (RMS = 19.4 µJy/beam) with contours at 40, 60, 100, 150, 220, 300 and 350 µJy/beam...... 35 4.3 Combined arrays (C+D) at C-band total intensity maps of NGC 2613 with contours in magenta. The beam is represented by a white ellipse at the bottom-right corner and the lowest contour level is marked in thick white line. Left - Robust 0 with contours at 12, 25, 50, 100 and 200 µJy/beam. Right - Robust 0 + 16 klambda uv-taper with contours at 12, 25, 50, 100 and 200 µJy/beam.... 37 4.4 Combined arrays (B+C+D) at L-band total intensity maps of NGC 2613 with contours in magenta. The beam is represented by a white ellipse at the bottom-right corner and the lowest contour level is marked in thick white line. Left - Robust 0 with contours at 50, 80, 150 and 250 µJy/beam. Right - Robust 0 + 16 klambda uv-taper with contours at 80, 150, 250, 500, 750 and 1000 µJy/beam..... 38 4.5 Combined B-array, C-array and D-array at L-band; and C-array and D-array at C-band total intensity maps of NGC 2613 with contours in magenta. The beam is represented by a white ellipse at the bottom-right corner and the lowest contour level is marked in thick white line. Left - Robust 0 with contours at 28, 35, 70, 100, 140 and 200 µJy/beam. Right - Robust 0 + 10 klambda uv-taper with contours at 28, 35, 70, 100, 140 and 200 µJy/beam...... 39 4.6 PB-corrected C-band spectral index maps of NGC 2613 with contours in magenta. The beam is represented by a white ellipse at the bottom-right corner. top row: D-array: Left - Robust 0 with contours at 20.2 and 50.5 µJy/beam. Right - Robust 0 + 6 klambda uv-taper with contours at 23.6 and 59 µJy/beam. Bottow row: C-array: Left - Robust 0 with contours at 12, 40 and 100 µJy/beam. Right - Robust 0 + 16 klambda uv-taper with contours at 12, 25, 50 and 100 µJy/beam...... 41 List of Figures ix

4.7 PB-corrected L-band spectral index maps of NGC 2613 with contours in magenta. The beam is represented by a white ellipse at the bottom-right corner. Top row: D-array: Left - Robust 0 with contours at 84 and 150 µJy/beam. Right - Robust 0 + 2.5 klambda uv-taper with contours at 90 and 200 µJy/beam. Middle row: C-array: Left - Robust 0 with contours at 90 and 270 µJy/beam. Right - Robust 0 + 6 klambda uv-taper with contours at 90 and 270 µJy/beam. Bottow row: B-array: Left - Robust 0 with contours at 50, 60 and 80µJy/beam. Right - Robust 0 + 16 klambda uv-taper with contours at 40, 60 and 100µJy/beam...... 42 4.8 Close-up of central region, host to N2613-A marked by a white square in both bands. The beam is represented by a transparent ellipse at the bottom-right corner. Left column: C-array C-band: top - Robust 0 with contours at 12, 40, 100, 200 and 300 µJy/beam. Middle Core with same parameters as above. Bottom - Spectral index map of core with errors contours at 0.08, 0.3 and 0.5. Right column B-array L-Band: top - Robust 0 with contours at 12, 50, 60, 80, 100, 150, 200 and 250µJy/beam.Middle Core with same parameters as above. Bottom - Spectral index map of core with errors contours at 0.08, 0.3 and 0.8...... 46 4.9 Spectrum from polynomial ﬁts. The red shaded zone is the upper and lowed bounds for the ﬁt. Blue points marks the B/L and C/C intergrated fulx densities at their respective frequencies...... 49 4.10 Close-up of F1 in L-band Robust 0 with B-array L-band contours at 50, 60, 80, 100, 150, 200 and 250µJy/beam. The beam is represented by a white ellipse at the bottom-right corner...... 50 4.11 Left - D array C-band Robust 0 + 6 klambda uv-taper with contours (in magenta) at 23.6 (in white), 59, 118, 236, 472 and 944 µJy/beam. µJy/beam Right - C-array L-band contours (in magenta) with robust 0 with contours at 90 (in white), 270, 500, 550, 700, 1200,and 1400 µJy/beam. The beam is represented by a white ellipse at the bottom-right corner and the lowest contour level is marked in thick white line. The extensions, E1-E4, are marked with white-dashed lines drawn from the mid-plane to the highest point of the extension (2 σ contour for D/C and 3 σ contour for C/L. 52 4.12 WISE (infrared) images of NGC 2613 with C/C robust 0 + 16 klambda uv-taper contours at 12, 25, 50, 100 and 200 µJy/beam. The lowest contour level is marked in thick white line...... 55 4.13 H-alpha images of NGC 2613 with C/C robust 0 + 16 klambda uv-taper contours at 12, 25, 50, 100 and 200 µJy/beam. The lowest contour level is marked in thick white line...... 57 4.14 DSS (optical) images of NGC 2613 with C/C robust 0 + 16 klambda uv-taper contours at 12, 25, 50, 100 and 200 µJy/beam...... 58 List of Figures x

4.15 Left - EPIC-PN-05-2-keV-intensity-contours-overlaid-on-the-digitized- sky-survey...... 59 4.16 Left - Zeroth moment map with HI continuum map contours. Con- tour levels are 0.23, 0.30, 0.5, 0.75, 1, 1.25, 1.5, 1.75, 2, 2.5, 3, and 3.5 mJy/beam. Right -Optical DSS image with Moment 0 map contours 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, and 29.3 ×20cm−2. The dashed line marks the feature connected to F1...... 60

5.1 Final prepared maps for the thermal/non-thermal separation: Top left - L-band radio maps (Map units : Jy/pix). Top right - C-band radio maps (Map units : Jy/pix). Bottom left - Infrared map (Map units : ergs/s/pix). Bottom right - H-alpha maps (Map units : ergs/s/pix)...... 66 5.2 IR-only/EM Method: Top left - C-band thermal fraction. Top right - L-band thermal fraction. Bottom left - Non-thermal spectral index. Bottom right - Total spectral index...... 73 5.3 Mixture/EM Method: Top left - C-band thermal fraction. Top right - L-band thermal fraction. Bottom left - Non-thermal spectral index. Bottom right - Total spectral index...... 74 5.4 IR-only/Murphy Method: Top left - C-band thermal fraction. Top right - L-band thermal frraction. Bottom left - Non-thermal spectral index. Bottom right - Total spectral index...... 75 5.5 Mixture/Murphy Method: Top left - C-band thermal fraction. Top right - L-band thermal fraction. Bottom left - Non-thermal spectral index. Bottom right - Total spectral index...... 76 5.6 Jarrett Method: Top left - C-band thermal fraction. Top right - L-band thermal fraction. Bottom left - Non-thermal spectral index. Bottom right - Total spectral index...... 77

6.1 Top : D-array total intensity maps at 1.46 GHz of M31, Beck et al. 1998. Bottom : Eﬀelsberg total intensity maps at 4.85 GHz, Berkhuijsen et al. 1982...... 82 List of Tables

1.1 Properties of NGC 2613. Properties taken from NASA/IPAC Ex- tragalactic Database, unless speciﬁed otherwise. Optical size is taken from Jarrett et al.(2003)...... 3

2.1 Jansky VLA conﬁgurations...... 7 2.2 NGC 2613 observation parameters for EVLA...... 13

3.1 NGC 2613 map parameters for individual data sets...... 30 3.2 NGC 2613 map parameters for combined data sets...... 30

4.1 Astrometry of the core...... 45 4.2 In-band spectral indices and ﬂux densities and N2613-A...... 49 4.3 Parameters of the polynomial ﬁt...... 49 4.4 Vertical heights of extraplanar extensions...... 53

5.1 Methods used to perform the thermal/non-thermal separation. Equa- tions used in the steps performed are stated...... 72 5.2 Integrated ﬂuxes for thermal/non-thermal separation methods, as described in table 5.1 for C-band radio. maps...... 78

xi Abbreviations

2MASS Two Micron All-Sky Survey AGN Active Galactic Nucleus APO Apache Point Observatory B/L B-array L-band C/C C-array C-band C/L C-array L-band CASA Common Astronomy Software Applications CHANG-ES Continuum HAlos in Nearby Galaxies: an Evla Survey D/C D-array C-band D/L D-array L-band DEC Declination EM Emission Measure EVLA Karl G. Jansky (Expanded) Very Large Array FT Fourier Transform FWHM Full width at Half Maximum GBT Green bank telescope HI Neutral Hydrogen IDL Interactive Data Language IR Infrared MS Measurement set NRAO National Radio Astronomical Observatory PB Primary beam xii Abbreviations xiii

PSF Point Spread Function RA Right Ascension RFI Radio Frequency Interference RMS Root Mean Square noise S/N Signal to noise ratio SFR Star Formation Rate SNe Supernovae SNR Supernova Remnant SPW Spectral window VLA Very Large Array WISE Wide-ﬁeld Infrared Survey Explorer Symbols

λ Wavelength Page 1

ze Scale height Page 4 A Amplitude Page 8 φ Phase Page 8 R Right circularly polarized light Page 8 L Left circularly polarized light Page 8 V (u, v) Continuous visibility function Page 9 S(u, v) Sampling function Page 9 I(l, m) True sky brightness Page 9

Bmax Projected baseline length Page 9 θ Maximum resolution Page 9 ν Frequency Page 10

D Iν (l, m) Dirty map sky brightness Page 10

ν◦ Reference frequency Page 22 α Spectral index Page 22 β Curvature Page 22

Iν Intensity at ν Page 22

Iν◦ Intensity at ν◦ Page 22 w line of sight distance Page 22 I Stokes I Page 26 Q Stokes Q Page 26

xiv Symbols xv

U Stokes U Page 26 V Stokes V Page 26

Pν Percentage polarization at ν Page 26

Qν Stokes Q at ν Page 26

Uν Stokes U at ν Page 26

Iν Stokes V at ν Page 26 χ Absolute polarization angle Page 26 σ RMS noise Page 26

αν in-band Spectral index at ν Page 47 α¯ Weighted mean of spectral index Page 47 ∆¯α Error in the Weighted mean of spectral index Page 47

N p Number of pixels per beam Page 47 b k Constant of proportionality Page 47

Snu Integrated ﬂux density at ν Page 48

Iν,th Thermal emission intensity Page 62

Iν,nth Non-thermal emission intensity Page 62

αnth Non-thermal spectral index Page 62

F24µm 24 micron ﬂux density Page 65

F22.1µm 22.1 micron ﬂux density Page 65 Hαcorr Corrected Hα map Page 67

corr Hαrel Corrected Hα using IR-only method map Page 67 L(24µm) Observed 24 micron luminosity Page 67

corr corr L(Hαrel ) Corrected Hα luminosity for Hαrel map Page 67 corr Hαmix Corrected Hα using mixture method map Page 68 Hαobs Observed Hα map Page 68

corr Hαmix Corrected Hα using the mixture method map Page 68 a Scaling factor Page 68

ne electron number density Page 69

Te Electron Temperature Page 69 Symbols xvi

4 Te4 Electron Temperature in units of 10 K Page 69

τe Optical depth of the thermal emission Page 69

Tb Brightness Temperature Page 70

Sν,th Integrated thermal ﬂux density at ν Page 70 corr SFRrel Star formation rate measured using for Hαrel Page 70 corr SFRmix Star formation rate measured using Hαmix Page 70

SFR22.1µm Star formation rate measured using 22.1µm emission Page 71 L(22.1µm) 22.1µm emission luminosity Page 71

Lν,th Thermal emission luminosity at ν Page 71 Chapter 1

Introduction

Studies of edge-on spiral galaxies can provide special insight on structures extending from the disk; revealing features such as extra-planar arcs and shells, to disentangling the disk and continuum halo components. The goal of this thesis is to study the morphology of the disk (and halo) of NGC 2613, an edge-on galaxy in the constellation Pyxis. For the purpose of this thesis, “halo” will be used to describe the continuum halo, which includes gas, dust and the magnetic ﬁeld, and not the stellar or dark matter halo.

Multi-wavelength observations of galaxies can provide different views of the same galaxy. Neutral hydrogen (HI) is detected in radio observations of galaxies using the hydrogen line. This is caused by the splitting of energy levels by the magnetic interaction between the electron and proton spins. The flip in the orientation of the spins from parallel to anti-parallel configuration results in the emission of a photon whose energy corresponds to a wavelength at λ = 21 cm. HI observations trace clouds of neutral hydrogen, which are precursors to collapse of matter into

1 Chapter 1. Introduction 2 stars. High abundances of these clouds indicate potentially active star forming galaxies. HI observations also provide kinematic information of neutral gas.

1.1 NGC 2613

Figure 1.1: Optical image of NGC 2613 taken from the Las Campanas Ob- servatory in Chile at λ = 4050 A(˚ Sandage & Bedke, 1994)

NGC 2613 is a massive spiral galaxy (Fig. [1.1]), part of a group of four galaxies in the constellation Pyxis (table [1.1]) It also possesses a companion galaxy ESO 495- G017, north-west of NGC 2613 . This companion appears to have a prograde orbit around its host galaxy (Chaves & Irwin, 2001). High resolution observations of NGC 2613 shows the presence of a tidal tail, formed as result of a tidal interaction between the galaxy and its companion. Chapter 1. Introduction 3

Parameter Galaxy

Morphology SA (s) b Galactic centre coordinates (J2000) : RA 08h 33m 22.84s DEC -22d 58m 25.2s Optical size 7.2’ × 1.8’ Distance (Mpc) 25.9 Minimum Inclination (deg) 75 Position angle (deg) 112.8 ± 1.2

Table 1.1: Properties of NGC 2613. Properties taken from NASA/IPAC Extragalactic Database, unless speciﬁed otherwise. Optical size is taken from Jarrett et al.(2003).

Chaves & Irwin(2001) used HI observations from the VLA to study the structure of NGC 2613. With these observations Chaves & Irwin(2001) were the ﬁrst to discover high-latitude features along the plane of the galaxy, extending out to the halo. They showed the presence of six high latitude HI features, two pairs at opposite edges of the galaxy and one pair residing close to the centre with each pair symmetric about the mid-plane of the disk (Chaves & Irwin, 2001). They also detected the presence of a bright radio source at the centre of one of these pairs with a luminosity of 1.1 × 1021 W Hz−1. This suggests that this pair is physically connected to each other, possibly formed from the same disturbance to the disk. The velocity gradient observed in these features were similar to features formed by internal mechanisms like supernovae or magnetic instabilities within the disk. These continuum features extend about 20 kpc from the mid-plane, larger than the features detected in starburst galaxy NGC 5775 (Duric, Irwin, & Bloemen 1998). They concluded that the internal mechanism that gave rise to the features could Chapter 1. Introduction 4 not explain their high energies and large galiocentric positions. They suggested that a ﬂare up of a relativistic jet travelling parallel to the plane of the disk could explain the large galiocentric positions but would require the presence of a compact source in the galactic disk.

Irwin & Chaves(2003) further expanded on the observations presented in Chaves & Irwin(2001) using new VLA data. Prior to the Irwin & Chaves(2003) paper, there was no obvious tidal disruptions observed in the radio data. However, with the new VLA data, Irwin & Chaves(2003) discovered the presence of a tidal tail on the eastern side of NGC 2613. This was the ﬁrst detection of interactions between NGC 2613 and its companion galaxy. They also discovered more discrete disk-halo features along the disk. These features, like the ones found in Chaves & Irwin(2001) showed remarkably high z-heights which would require high-energy inputs.

Discrete extra-planar features, similar to those found in the radio observations, were detected in the XMM Newton observations of the galaxy (Li et al., 2006). These diﬀuse X-ray features were caused by the presence of hot gas. This emission closely follows the IR emission. The ratio of the X-ray luminosity and IR luminosity is similar to the ratio observed in low mass X-ray binaries. This indicates that the unresolved x-ray emission was produced by an old stellar population.

The extra-planar radio features are extensions of these X-ray sources. The study concluded that these features were most likely formed by outflows from the galactic disk A three dimensional model for the HI distribution shows the absence of a global HI halo around NGC 2613, which is better fit by a thin disk model with an exponential scale height of ze = 188 pc. A thin disk model for NGC 2613 could also explain the presence of a significant number of discrete HI features. Chapter 1. Introduction 5

Recent X-ray observations of NGC 2613 detected the presence of a deeply embedded AGN (Li et al., 2006). Observations of the X-ray spectrum of this AGN predicted ﬂattening of the spectral index at lower frequencies.

1.2 Continuum HAlos in Nearby Galaxies: an

EVLA Survey (CHANG-ES)

The overall aim of CHANG-ES is to study the presence of large scale halos around galaxies and examining the disk-halo interface. With this, we can study the connection between a galaxy’s radio halo, disk and environment, while studying the magnetic fields present in both disk and halo. The survey uses C band (6 GHz) and L band (1.4 GHz) observations of 35 nearby edge-on galaxies with three different array configurations. The high inclination of these galaxies allows us to examine the disk-halo interface directly while also studying the structure and physics of the disk-halo outflows. CHANG-ES was awarded 405 hours of observing time on the EVLA in the B, C and D array configurations and, as of August 2012, all data have been collected.

In this thesis we will describe the process of acquiring and reducing the continuum data from the EVLA in chapter2 and chapter3. Results from the above process, along with other multiwavelength data, are presented in detail in chapter4. We explain and perform the process of thermal/non-thermal separation, a method that allows us the study the sources of the continuum emission we observe using the VLA in greater detail, in chapter5. We discuss the results of the observations presented in chapter4 and separation performed in chapter5 in chapter6. Chapter 2

Data Acquisition and Reduction

2.1 Observing

This thesis makes use of data obtained from the Jansky Very Large Array as part of the CHANG-ES survey.

2.1.1 Jansky Very Large Array (EVLA)

The Karl G. Jansky Very Large Array (EVLA) 1 is located 50 miles west of So- corro, New Mexico, and is operated by the National Radio Astronomy Observatory (NRAO). It is a large multi-dish interferometer which consists of 27 antennae, each of which are 25 m in diameter. The galaxy data were acquired as part of the Con- tinuum Halos in Nearby Galaxies (CHANG-ES) survey, an EVLA survey which has targeted 35 edge-on galaxies in the C-band (6 GHz) and L-band (1.4 GHz).

1The Expanded Very Large Array was the original name for the Karl G. Jansky Very Large Array after the upgrade of the VLA completed in 2012

6 Chapter 2. Data acquisition and reduction 7

Figure 2.1: Satellite view of the JVLA. The red line makes the longest baseline of the array, and yellow marks the shortest. (Google, 2016)

The array is arranged in a Y shape (Fig. [2.1]) configuration with the antennae placed on railway tracks with each arm, consisting of 9 antennae each, running in the north, south-west, and south-east, equiangular to each other. The required resolution can be achieved by adjusting the size of this array with the standard configurations running from A (largest) to D (most compact) as shown in table [2.1]. When the array is in its A configuration, the finest resolution can be achieved, whereas the coarsest resolution is achieved using the D configuration.

Conﬁguration Maximum baseline (km) Minimum baseline (km)

A 36.40 0.680 B 11.10 0.210 C 03.40 0.035 D 01.03 0.035

Table 2.1: Jansky VLA conﬁgurations. Chapter 2. Data acquisition and reduction 8

2.1.1.1 Interferometers

A two-element interferometer, with two identical dishes on a baseline separated by distance ‘b’, measures a plane wave from the source with some time delay (fig. [2.2]) between the two dishes. The output voltages from these antennae are correlated to each other by multiplying the time-varying voltage and averaging the values over some integration time (10 s for CHANG-ES data). The correlated voltages from each baseline allow us to measure a complex visibility, V(u,v), each represented by an amplitude ‘A’ and phase φ. These visibilities are measured at a projected plane as seen from the source called the uv-plane, where ‘u’ and ‘v’ as defined are points on this plane and measured in units of wavelengths. The baselines, however, are three-dimensional with an additional term, w-term, that is perpendicular to the sky plane. This w-term is ignored in small fields but can be an issue in wide-field imaging. The correction for this term is applied in the imaging process discussed in section 3.2

A second correlator is used which alters the wave by 90◦, allowing the antennae to measure sine and cosine terms simultaneously. This allows us to multiply the sine and cosine terms which is used to measure the right and left circularized polarized radiation, denoted by R and L respectively. Signals with the same polarization (co-polarization) are multiplied with each other (RR and LL) and are used to produce Stokes total intensity maps, discussed in appendixA. Cross correlation by multiplying orthogonal signals (RL and LR) produce cross polar visibilities that can be used to produce polarization maps.

Interferometers allow us to study extended sources, like NGC 2613, at high resolution compared to single dishes, such as the GBT, thanks to the use of large Chapter 2. Data acquisition and reduction 9

Figure 2.2: Block diagram for a baseline response. The plane wave approached dish 1 with a time delay τ. The measured voltages at each dish are multiplied and averaged at the correlator. (NRAO, 2016)

baselines. The maximum resolution of the interferometer at a particular wavelength λ is set by the longest projected baseline length Bmax and is given by the ‘Rayleigh Criterion’ for a circular aperture,

1.22 λ θ (rad) = (2.1) Bmax

The visibility function contains an amplitude A and phase φ which is related to the sky brightness I(l,m), via a Fourier transform, where l and m are the sky coordinates. The array can measure certain values in the continuous visibility function V(u,v), which gives us the uv sampled function V’(u,v) = S(u,v)V(u,v) where S(u,v) is the sampling function which is represented by a series of delta functions. The sampling function corresponds to the location of the antennae on Chapter 2. Data acquisition and reduction 10

the ground in the uv-plane. The Fourier transform for this function is the point spread function (PSF), also known as the synthesized (dirty) beam, B(l,m). New points are sampled as the earth rotates creating a more complete sample of the

D uv-plane. The dirty image (Iν ) is obtained by performing a Fourier transform with just the sampled visibilities(eq. [2.2]).

ZZ D 2πi(ul+vm) Iν (l, m) = Vν(u, v)S(u, v)e dudv (2.2)

D Iν (l, m) = FT (Vν(u, v)S(u, v))

= FT (Vν(u, v)) ∗ FT (S(u, v))

= I(l, m) ∗ B(l, m)) (2.3)

In order to recreate the true sky brightness I(l,m), a continuous visibility function is needed. Since the longest distance between antennae in baselines is limited by the array, while the shortest distance is limited by the physical separation of the dishes, a continuous sampling function cannot be achieved. Eq. [2.3] shows that the dirty image can be written as the convolution (’* ’ denotes the convolution operator) of the Fourier transform of the visibility function with the PSF. Therefore, the true

−1 sky brightness can be extracted by deconvolving the dirty image FT (Vν(u, v)) with the dirty beam B(l,m). Details of the deconvolution process, in this case the multi-scale multi-frequency deconvolution CLEAN used in the data reduction for this thesis, is discussed in further detail in section 3.2. Chapter 2. Data acquisition and reduction 11

2.1.1.2 CHANG-ES Observations of NGC 2613

NGC 2613 data were obtained in the D-array (L-band and C-band), C-array (L- band and C-band) and B-array (L-band) as part of CHANG-ES (table [3.1]). C- band reciever is a single band (4.979 GHz to 7.021 GHz) with 16 spectral windows (spw), each containing 64 channels, giving a total of 1024 channels centred at 6.0 GHz and bandpass of 2.048 GHz. The L-band has 2 sub-bands (1.247 to 1.503 GHz and 1.647 to 1.903 GHz respectively) each with 16 contiguous spectral windows (spw) (32 in total) and 64 channels for each spw, giving a total of 1024 channels for each sub-band centered at 1.5 GHz and a total bandpass of 512 MHz. The two sub-bands in the L-band have a gap between them in order to avoid radio frequency interference (RFI), present between 1.52 - 1.64 GHz.

With each galaxy scan there is a primary gain and phase calibrator (henceforth called the flux calibrator), a secondary gain and phase calibrator which is close to the source (henceforth called the phase calibrator), as well as a polarization leakage calibrator. Details regarding the calibrators are discussed in greater detail in section 3.1. The flux calibrator is a bright source whose flux density is well modelled which sets the amplitude scale for the source. 3C286 was used as the flux calibrator for both C and L bands. Flux calibrators can also be used as bandpass calibrators and are also used in polarization calibrations to calculate the absolute polarization angle for the sky. Imperfections in the instrumentation can lead to polarization “leakage”. A zero polarization calibrator is a source with no intrinsic polarizations and is used to determine the instrumental polarization and correct the leakage terms.

In table [3.1], the D-array (L-band and C-band) and C-array (C-band) were calibrated and imaged by Dr. Judith Irwin and Dr. Theresa Weigert. C-array L-band, Chapter 2. Data acquisition and reduction 12

B-array L-band and combinations of the all VLA data sets (in various array and frequency combinations) were performed by the author. Thermal/Non-thermal separation of the radio data is performed by the author with Hα images captured by CHANG-ES consortium member, Carlos Vargas. Chapter 2. Data acquisition and reduction 13 a 3C286 B-array L-band 512 MHz J0846-2610 100 minutes J1407+2827 21-Mar-2011 1.247 to 1.503 GHz 1.647 to 1.903 GHz a 3C286 L-band 512 MHz J0853-2047 J1407+2827 02-Apr-2012 25-Mar-2012 1.247 to 1.503 GHz 1.647 to 1.903 GHz 15 minutes 40 seconds 27 minutes 50 seconds C-array 3C286 C band 2.048 GHz J0853-2047 17-Feb-2012 J1407+2827 178 minutes 40 seconds 4.979 GHz to 7.021 GHz 3C286 L-band NGC 2613 observation parameters for EVLA. 512 MHz J0853-2047 J1407+2827 Dec. 21, 2011 Mar. 17, 2013 1.247 to 1.503 GHz 1.647 to 1.903 GHz 9 minutes 20 seconds 9 minutes 30 seconds D-array Table 2.2: 3C286 C band 2.048 GHz J0853-2047 J1407+2827 13-Dec-2011 38 minutes 10 seconds 4.979 GHz to 7.021 GHz b b b • Receiver Data reduced by author Before ﬂagging Bandwidth Flux calibrator b a Phase calibrator Time on galaxy Frequency range Zero-pol calibrator Date of Observation Chapter 3

Data Reduction

Calibrations for the EVLA data were performed using the Common Astronomy Software Applications (CASA) package ver 4.2.21 which is used to calibrate, image and analyse the data. The data reduction process can be simpliﬁed into 5 steps:

• Flagging

• Calibrating

• Imaging

• Polarization calibration

• Polarization imaging

The data are downloaded from the archive as a measurement set (MS) ﬁle which consists of tables with visibilities of sources and calibrators along with information about antenna positions. The main table has three columns: DATA (uncalibrated

1For imaging (as described in section 3.2) we used CASA 4.7.0. There are no changes between these versions that aﬀect our results.

14 Chapter 3. Data Reduction 15

uv data), CORRECTED DATA (calibrated uv data) and MODEL. The downloaded MS usually contains other galaxies observed in the same observing run along with their corresponding calibrators. Hence, the source of interest along with its calibrators are split out from the original MS using the CASA routine split. The ﬂux calibrator MS is also split out into a separate ﬁle to speed up the process of calibrating the dataset by creating initial calibration tables on a smaller data set.

3.1 Flagging and Calibration

Calibration of the data takes place in the following steps:

1. Initial calibrations using the flux calibrator: The flux calibrator can be used to do first-order corrections (like adjustments to the antenna positions) for the whole measurement set. The data from the flux calibrator are first examined (Fig. [3.1]: Top) and bad data, like major Radio Frequency Interference (RFI) or bad antennae (from observation logs), are flagged from it. RFI is the unwanted radio signals that are generated locally from ground sources that transmit at frequencies that are observed by the antennae. These signals can be stronger than the weaker source signal which can corrupt the observations and must be removed from the continuum data. As the bandpass response at the edges of each spectral window declines sharply, the first and last 5 channels for each spw are also flagged. Flagging is done manually by visually inspecting the data set for each spw, using the routine casaviewer. Initial tables are created using CASA routines like gencal. gencal creates tables for antenna-based calibrations. First a table for antenna-position corrections is created using the antpos option in gencal. This routine automatically checks for known adjustments to the antennae from the NRAO site. A gaincurve Chapter 3. Data Reduction 16

table is also created to correct for differing sensitivities at different elevations caused by the atmosphere. However, these corrections are minor at the frequencies of our observations. A delay calibration table is also created to remove antenna- based delays which induce phase slopes in frequency for each spw, using only a one minute scan in the flux calibrator as to minimize the effects of time-based gains and phase variations. These calibrations are applied for all sources within the dataset using applycal. The flags used on the flux calibrator are extended to all sources and the corrected data column is then split into a new MS. The new MS now has the corrected column from the old MS as the data column in the new MS which will be used for the second stage of calibrations (Figure 3.1: Middle).

2. Set the flux density of the flux calibrator: The new MS is hanning smoothed to remove ringing across the channels due to the Gibbs phenomenon. Fourier transformations of the data at jump discontinuities (like the data at the edges of the channel or RFI) generates oscillations near the jump, hence is strongly pronounced around channels with high RFI. This effectively lowers the spectral resolution of the dataset by a factor of 2. The flux calibrator is used to set the absolute flux density of the data using the setjy function which applies the Perley- Butler 2010 (Perley & Butler, 2013) flux density scale for Stokes I. The calibrator’s spatial structure is a function of the frequency, so a known frequency-dependent model is applied over the entire frequency band. Channel by channel, with this model, the source flux can later be converted from instrumental units to absolute flux density, in units of Jy, by comparing it to the measured flux density of the flux calibrator.

3. Phase and amplitude calibrations: An initial gain calibration table for phase and amplitude as a function of time is created using the central channels in each spectral window of the flux calibrator. These calibrations correct for Chapter 3. Data Reduction 17 changes in amplitude and phase induced in the signal due to the instruments and the atmosphere. Systematic errors due to instruments are different for different antennae and are corrected in this step.

A bandpass calibration is applied to correct for the varying sensitivity of the channels in the spectrometer for each spw and baseline. This is done ‘on the fly2’ for the flux calibrator correction using the previous phase calibration. The bandpass calibration is applied on the phase calibration tables for each source on the fly. The phase solutions are measured at each integration time (10 s) for the flux calibrator and for each scan (∼5 min) for the other two calibrators. The phase calibration is then applied on the fly along with the bandpass calibration when forming the amplitude calibrations whose solution is measured for each scan for all calibrators. The flux from the flux calibrator is then bootstrapped to the other calibrators using fluxscale. With this the solution for flux densities of the flux calibrator is used to determine the true flux densities of the other calibrators. After deriving all the calibration tables, they are applied to the calibrators and source, using applycal.

2‘on the ﬂy’ means that, in each calibration step, the calibration tables made in the previous steps are applied while the new calibration tables are being formed. Chapter 3. Data Reduction 18

Figure 3.1: C-array L-band flux calibrator data. Different colors signify the contiguous spws. Top: Unflagged data before calibration. Middle: Flagged data before calibration (with initial corrections applied). Bottom: Flagged data after calibration. Chapter 3. Data Reduction 19

We now examine the corrected calibrated data which reveals low level RFI in the calibrators, which could not be seen in uncalibrated data. These are then ﬂagged and steps 2-3 of the calibration process are repeated until the calibrators are free of RFI (Fig. [3.1]: Bottom). The source galaxy, which has now been calibrated, is then ﬂagged and split from the MS (henceforth called galaxy MS) and used for total intensity (Stokes I) imaging (Fig. [3.2]).

Figure 3.2: NGC 2613 C-array L-band data - Flagged and calibrated data for the galaxy. Chapter 3. Data Reduction 20

3.2 Total Intensity Imaging

Stokes I maps for the galaxy are created using the calibrated source as described in section 3.1. The Fourier transform of these calibrated data is used to generate

D a dirty image Iν (l, m). From eq. [2.3], we know that the true sky brightness can be created by deconvolving the dirty beam from the dirty image. CASA uses the Clark CLEAN algorithm (Clark, 1980) for the deconvolution process using the clean task. The CLEAN algorithm can be summarized in the following steps:

D 1. The dirty map Iν (l, m), is used as the initial map (henceforth called the residual map) for the CLEAN algorithm. The CLEAN algorithm creates an empty map (henceforth called the source model) with values for the initial map at all positions set to zero.

2. In its ﬁrst cycle, CLEAN searches for the brightest points in the image and adds them to the source model. The convolution of the pixel value of the point and dirty beam is subtracted from the residual map. This process is repeated several times for the residual map until the peak brightness of the residual map reaches a user-deﬁned threshold.

3. The total intensity image is ﬁnally created by convolving the source model with an ideal beam which has the same FWHM as the dirty beam but without the presence of side lobes and adding in the remaining residuals which fell below the threshold.

The EVLA offers large bandwidths as part of its expanded capabilities, which allows us to reconstruct the spectral structure of the radio continuum with greater accuracy. However, the wide bandwidth adds a layer of complexity to the imaging Chapter 3. Data Reduction 21 process. To address these concerns and take advantage of wide-band wide-field imaging, we use CASA’s multi-scale multi-frequency synthesis CLEAN algorithm as prescribed in Rau & Cornwell [2011]. Details regarding wideband imaging are discussed briefly below:

Multi-frequency synthesis (mfs)

As discussed in section 2.1.1.1, locations on the uv-plane are measured in dimen- sions of wavelength. Hence, the location of the antennae on the ground in the uv-plane (the sampling function) will vary as the frequency changes along a wide frequency band. This allows us to ﬁll in gaps in the uv-plane giving a “cleaner” dirty beam i.e. lower sidelobes. In order to take advantage of the greater uv- coverage, mfs grids each spectral channel instead of averaging the channels (Con- way et al., 1990).

Multi-scale clean

Traditional deconvolution algorithms model all sky emission as a collection of point sources (point #2 in the CLEAN process), resulting in less accurate reconstruction of extended emission. Multi-scale CLEAN sets the scales for the deconvolution so emission can be represented over a speciﬁed spatial scale as opposed to a point source alone, allowing extended emission to not be removed as noise. Step 2 in the CLEAN process is adjusted to account for the multi-scale clean. Beginning at the lowest speciﬁed scale, which is always a point source, the location and strength of peaks in the image are subtracted and added to a source model. The residual map is smoothed to the next scale size. The peak corresponding to the new scale is located within the residual map and then subtracted and added to the source model again. The largest scale used is set by increasing the scales until the residual only consists of noise. Chapter 3. Data Reduction 22

Fitting the spectral index

In-band spectral indices for the source can be measured due to the wide bands used by the EVLA. CASA’s clean task allows us to ﬁt the spectral index of some emission Iν at some position by assuming that,

α + β log ( ν ) ν νo Iν = Iνo , (3.1) νo

where Iνo is the specific intensity at a reference frequency νo (in this case the central band frequency). α and β are the spectral index and spectral curvature respectively. The spectral index of a source can be defined as the slope of its power spectrum. Changes in the slope are studied by observers as they may rep- resent changes in the source and/or provide insight into acceleration mechanisms experienced by the particles. The fit of the power spectra, calculated over a given frequency band for each pixel, gives us the spectral index and error in the spectral index at each point for the given map.

The function is Taylor expanded about νo resulting in the polynomial ﬁt for the spectrum (Irwin et al., 2012a). The number of terms used in the ﬁt is set using the nterms parameter in clean. Tests on CHANG-ES data found that the best results were achieved using nterms = 2 rather than 3. That is, a spectral index, α, is measured while ignoring the curvature, β, which requires higher S/N to be measured accurately.

Wide-ﬁeld imaging

Wide-ﬁeld images could possibly be distorted by non-coplanar baselines. These errors increase with distance from the pointing center and pose a problem for extended sources. The W-projection algorithm (Cornwell et al., 2008) is used by Chapter 3. Data Reduction 23

CASA’s clean task to make corrections. W-projection takes the uvw visibility, w being the line of sight distance to the source, and projects it onto a 2D plane with w = 0 and the phase shift proportional to distance from the pointing center.

Bandwidth smearing

Bandwidth smearing is caused by averaging visibilities over a ﬁnite bandwidth producing a radial smearing in the image plane which worsens with distance from the pointing center. The CHANG-ES data as designed use narrow band channels which essentially eliminate this eﬀect. Chapter 3. Data Reduction 24

Figure 3.3: Top row: D-array C-band total intensity maps of NGC 2613 with contours (non-pbcorrected) at 20.2 µJy/beam, in white. The beam is represented by a white ellipse at the bottom-right corner. Left: Non-PB-corrected. Right: PB-corrected. Bottow row: D-array C-band spectral index maps of NGC 2613 with contours, in black, at 20.2 µJy/beam. The beam is represented by a white ellipse in the bottom-right corner.Left: Non-PB-corrected. Right: PB-corrected.

Primary beam correction

Once the clean map (I(l,m) in eq. [2.3] has been made, it is modulated by the response of a single dish called the primary beam (PB). Correction for the PB is needed to correct the ﬂux densities and α. The varying primary beam size Chapter 3. Data Reduction 25

over the frequency band (inverse dependence) imposes a frequency dependence in the emission which has a strong effect on the spectral index. Also the varying size of the primary beam results in averaging out “nulls” in wide band images that would be present in monochromatic beams. The frequency-averaged primary beam is corrected for in the final map. Fig. [3.3] illustrates how the primary beam correction affects the total intensity and spectral index maps. The correction takes into account the decreasing sensitivity at the edge of the primary beam. As shown in the intensity maps, the flux density in the PB-corrected image is higher than the non-PB-corrected maps, which is especially noticeable at the edge of the primary beam. The imposed frequency dependence on the emission has a much stronger effect on the spectral index.

Taking all these points into consideration, NGC 2613 (at both C-band and L-band) was mapped as just described. First a wide field was mapped in order to determine the final map size needed to include all sources of significant flux. The images were cleaned down to at least 5 σ. Briggs weighting is applied to the visibilities at the imaging step. This weighting system is derived from uniform weighting, that minimizes sidelobes, and natural weighting, that reduces noise level. This weighting system is flexible between the two weighting systems with the robust parameter setting which system the final weighting will closely follow. The ‘robust’ parameter for all of the maps in the thesis is set to ‘0.0’ (henceforth will be referred to a robust 0), where ’robust’ can take a value between ‘-2.0’ (uniform weighting) to ‘2.0’ (natural weighting). Visibilites from poorly sampled regions near and beyond the maximum spatial frequency are not well-estimated which can give rise to artifacts during the deconvolution process. To correct for this, a uv-taper is applied to the observation sets. A uv-taper is similar to the weighting applied in the CLEAN process, however, it focuses on adding further weights to the data Chapter 3. Data Reduction 26

obtained by the largest baselines that tend to be poorly sampleds. The uv-taper adds a multiplicative Gaussian taper to the spatial frequency grid in the uv-plane, which is used to suppress strong artefacts from poorly sampled regions by weighing down these high-spatial frequency measurements. The uv-taper used is chosen by a combination of previous measurements performed by the CHANG-ES group for similar datasets and trial-and-error.

3.3 Polarization Calibration and Imaging

In order to obtain the linear polarization intensity map from Stokes Q and U maps, the cross correlation terms (RL and LR) have to be correctly calibrated. Before the polarization data can be calibrated, the individual cross correlation data, LR and RL, are flagged for the galaxy and calibrators. The flux calibrator is used to set both the flux and the absolute polarization angle on the sky. A known model for Stokes Q and U for the flux calibrator is used to correct for the frequency dependant variation of the percentage polarization. The absolute polarization angle on the sky for 3C286 is initially set to χ = 33◦. The percentage

polarization at frequency ν (MHz), Pν, can be represented by (Irwin et al., 2012a):

−6 2 −3 Pν = −1.43 × 10 · ν + 6.026 × 10 · ν + 3.858 (L-band) (3.2)

−4 Pν = 1.583 × 10 · ν + 10.57 (C-band) (3.3)

Using Pν and Iν, previously set from a known model, the Qν and Uν at a given frequency are given by: Chapter 3. Data Reduction 27

P Q = ν × I × cos(2χ) (3.4) ν 100 ν

P U = ν × I × sin(2χ) (3.5) ν 100 ν

Delay calibration tables are created for the cross correlations on the flux calibrator, similar to the table created in total intensity calibration. The instrumental leakage terms are calibrated out using the zero polarization calibrator while correcting for gains, bandpass, and delays on the fly. The absolute polarization angle on the sky is calibrated from the flux calibrator, corrected for amplitude and phase gains, delays, and leakage terms on the fly. These calibration tables are applied to the galaxy and calibrators, creating a new corrected column which is split off for use in polarization imaging.

Stokes Q and U maps are created using the same methods described in section 3.2 to make the total intensity maps. Imaging the polarization data is similar to the process used in imaging the total intensity map with the largest multi-scale dropped as the polarization emission is weaker than the total intensity and the largest scale is not required. Linear polarization intensity maps are created using the Q and U stokes images using the formula P = pQ2 + U 2, which is achieved

1 U using the immath task. The polarization angle of the electric ﬁeld χ = 2 arctan Q where the linear polarization is greater than 4σ. Primary beam corrections are applied for the polarization intensity maps using the same frequency averaged primary beam that was created for total intensity maps. Chapter 3. Data Reduction 28

3.4 Combined Array And Frequency Images

With the help of CASA, the calibrated uv data created from the methods described in section 3.1, can be used to create images from combined arrays at specific bands (B, C, and D arrays at L-band and C and D arrays at C-band). These combination techniques can be used to achieve in principle higher S/N images, but also more thorough uv coverage, than the individual data sets. Each data set was combined in the uv plane during the clean process. A uv-taper was also applied to the observation sets in order to suppress the larger contributions from the high-spatial frequency measurements that are over-sampled in some arrays (eg. B-array) while poorly sampled in the others (eg. D-array). A combination of observations in all arrays and frequency bands (C and D arrays at L-band and C and D arrays at C-band) was also created resulting in increased sensitivities to different spatial scales. The clean algorithm applies a point-by-point fit to the spectral index of the individual maps which generates a combined map corresponding to an intermediate frequency of ν = 4.13 GHz. Aside from Irwin et al.(2012b), this map is the only known image of a galaxy for which 5 data sets at 2 different frequencies have been successfully combined to form a map at the intermediate frequency. Chapter 3. Data Reduction 29

3.5 Table of parameters

The parameters for the total intensity (Stokes I) maps created using the methods described above are listed in table [3.1] and table [3.2]. The ﬁnal maps are shown in chapter 4. For each map created, the beam size and position angle is noted, as described in section 2.1.1.1, which provides us with information about the resolution of the image. The parameters for the multiscale clean are also noted in units of pixels. These parameters along with information about the pixel size (in arcsec), reveal the scales of the diﬀuse emission that were added to source model used in the clean process (section 3.2 : Multi-scale clean).

The RMS values for the map are calculated by averaging the RMS noise in regions without any detectable continuum sources, usually away from the centre of the map where the main source is usually located. Consequently, residual artifacts from the clean process will also be prevalent near the centre of the map giving a higher RMS value at the location. In section 3.2 (Primary beam correction), we see that a PB-correction was applied to the maps. As stated before, a multiplicative factor is used to correct for the decreasing sensitivity of the primary beam toward the edge. Before these corrections were applied the RMS noise was uniform over the map. Hence, all values of RMS noise recorded in table [3.1] and table [3.2] are measured using non-PB corrected images. Flux density measurements are measures using the PB-corrected maps. The map peak gives the peak intensity of the galaxy. The map peak over the RMS noise gives us the dynamic range of the image. Chapter 3. Data Reduction 30 0.5 19.4 1015 -3.24 Robust 0 0,10,20,40 7.03 by 0.23 + 16 klambda 7.3 1340 B-array -32.11 0.5 21.89 by 15.71 905 19.6 -179.7 Robust 0 0,10,20,40 5.29 by 2.99 Robust 0 + 10 klambda BCD CL 2 31.2 3060 -5.23 Robust 0 7.1 1196 0,10,20,40,60 + 6 klambda -29.89 21.89 by 15.71 Robust 0 18.62 by 9.31 C-array L-band 2 27.6 1990 -4.79 Robust 0 0,10,20,40,60 18.62 by 9.31 412 3.50 -12.52 5 45 1340 -32.11 0,10,20 7.11 by 5.93 Robust 0 76.55 by 51.18 + 2.5 klambda CD C Robust 0 + 16 klambda D-array 5 42 1196 -29.89 Robust 0 0,10,20,40,60 401 3.17 73.43 by 41.97 -6.25 Robust 0 5.19 by 2.50 0.5 3.3 406.6 -10.88 Robust 0 6.79 by 5.75 0,10,20,50,100 + 16 klambda C-array 1028 -2.24 16.05 0.5 3.3 -6.19 398.6 NGC 2613 map parameters for combined data sets NGC 2613 map parameters for individual data sets Robust 0 0,10,20,40 8.48 by 7.03 4.95 by 2.37 BCD L C-band Robust 0 + 16 klambda 2 11.8 1090 0,9,18 -11.44 Robust 0 Table 3.2: Table 3.1: + 6 klambda 19.94 by 15.81 951 0.48 16.9 D-array Robust 0 6.15 by 3.71 2 643 10.1 -11.05 0,5,10,25 Robust 0 15.42 by 8.30 Jy) µ beam) / Jy) µ Jy beam) / µ Jy µ Weighting Receiver Conﬁguration Weighting Map peak ( RMS ( Cell size (arcsec) Position angle (deg) RMS ( c Map peak ( Array conﬁguration Position angle (deg) CLEAN scales (pixel) Synthesized beam size (arcsec) Synthesized beam size (arcsec) Chapter 4

Results

A variety of maps are created from the radio-continuum data acquired (3 observations in L-band, at B-array, C-array, and D-array conﬁgurations, and 2 observations in C-band, at C-array and D-array conﬁgurations) from the EVLA and produced using the methods described in chapter3.

These images reveal the presence of several new features in NGC 2613, along with higher resolution images of features observed in previous studies. In this chapter, the results of the CHANG-ES observations of NGC 2613 are presented and new ﬁndings are discussed in the context of other multi-wavelength observations of this galaxy.

31 Chapter 4. Results 32

4.1 Total Intensity Emission

Figure 4.1: C-band total intensity maps of NGC 2613 with contours in magenta. The beam is represented by a white ellipse at the bottom-right corner and the lowest contour level is marked in thick white line. Top row: D-array: Left - Robust 0 (RMS = 10.1 µJy/beam) - with contours at 20.2, 50.5, 101, 202, 404 and 808 µJy/beam. Right - Robust 0 + 6 klambda uv-taper (RMS = 11.8 µJy/beam)with contours at 23.6, 59, 118, 236, 472 and 944 µJy/beam. Bottow row: C-array: Left - Robust 0 (RMS = 3.3 µJy/beam) with contours at 12, 40, 100, 200 and 300 µJy/beam. Right - Robust 0 + 16 klambda (RMS = 3.3 µJy/beam) uv-taper with contours at 12, 25, 50, 100 and 200 µJy/beam.

Total intensity images (henceforth called Stokes I) for NGC 2613 for individual data sets (i.e. single array) at C-band and L-band are shown in fig. [4.1] and fig. [4.2] respectively. The maps are created at two weightings; Robust 0 and rob0 + Chapter 4. Results 33 uv-tapered. Each map will be referred to in this text with the format ‘array/band’. For example, a C-array L-band map will be referred to as a C/L map. Stokes I contours for the continuum emission are also shown in magenta in fig. [4.1] and fig. [4.2], along with the beam size in the bottom right corner. Table 3.1 contains the parameters for fig. [4.1] and fig. [4.2].

In this section we identify the features that will be discussed in further detail in section 4.3. In D-array C-band (fig. [4.1] - top row left), within the disk itself are “hotspots”, also called continuum knots, embedded in the continuum features. One of the more striking features is a spiral like feature in the eastern part of the disk (labelled F1 in fig. [4.1]). Also visible is a tidal-like tail feature in the north-western corner of the disc (labelled F2 in fig. [4.1]). These connections are described in section 4.3.6 and discussed in greater detail in chapter 5 along with the features found in other data sets mentioned in this section.

At C-band, the low resolution D-array data reveal the presence of extra-planar emission extending beyond the disk. These extensions are found above and below the disk of the galaxy. The uv-tapered map helps further illustrate the extent of these features and the structures within them (ﬁg. [4.1] - top row right).

The C-array C-band map (fig. [4.1] - bottom row) reveals the presence of a continuum ring, marked within the red dashed lines in the bottom left map in fig. [4.1], which is the first detection of such a structure for this galaxy. In the robust 0 map, there is a clear hole in the continuum at the centre of the galaxy out to the inner edge of the ring. Within this hole is the first detection of a point source at the centre of the galaxy (referred to as N2613-A and marked in fig. [4.1]. The nature of this core is discussed in section 4.3.2. The “hotspots”, mentioned previously, become well resolved but the extra-planar features have been resolved out. The spiral feature, F1, and other hotspots appear to be embedded along the continuum Chapter 4. Results 34 ring within the disk. The tidal-like feature is more apparent in the uv-tapered C/C map with its strongest point of emission at the tip of this tail (referred to as F2 and marked in fig. [4.1]). In the uv-tapered C/C map is a large extended wave-like feature to the north-west of the nuclear point source. This feature isn’t visible in the robust 0 map, however there is a corresponding C-shaped feature (referred to as F3 and marked in fig. [4.1]). In the corresponding uv-tapered map, this feature appears to be embedded in a thick extended continuum feature. These features will be further discussed in fig. [4.3].

At D-array L-band (fig. [4.2] - top row), we have the lowest resolution VLA data set taken for this galaxy as part of CHANG-ES. Other extra-planar features include two large extensions just north-east and south-west of the galactic centre and a faint extra-planar extension extending below the center of the disk is visible in this map (Labelled in fig. [4.2]: top row with asterisks). The north-eastern extension appears to form a loop. The source of the south-western source could be the background source seen in the C/L uv-tapered map (fig. [4.2] - top left, marked with an asterisk).

In the C/L maps (ﬁg. [4.2] - middle row), we detect extensions similar to the features found in the D/C maps. Due to the similar beam size between the D/C and C/L maps, the images look very similar to each other. Upon closer inspection we can see that the extra-planar extensions in the C/L uv-tapered map extend further out and show larger scale structure than its D/C counterpart. There are loops in the western half of the galaxy, both above and below the disk, along with other extra-planar features which will be discussed in section 4.3.6. These loops have only been detected in the uvtapered C/L map. The spiral feature and tidal tail are also clearly visible in the C/L uv-tapered map. Chapter 4. Results 35

Figure 4.2: L-band total intensity maps of NGC 2613 with contours in magenta. The beam is represented by a white ellipse at the bottom-right corner and the lowest contour level is marked in thick white line. Top row: D-array: Left - Robust 0 (RMS = 42 µJy/beam) with contours at 84, 150, 300, 1000, 2000, 4000 and 9000 µJy/beam. Right - Robust 0 + 2.5 klambda uv-taper (RMS = 45 µJy/beam) with contours at 90, 200, 500, 1200, 3000, 7000 and 12000 µJy/beam. Middle row: C-array: Left - Robust 0 (RMS = 27.6 µJy/beam) with contours at 90, 270, 500, 550, 700, 900, 1000, 1500, 3000, 7000 and 10000 µJy/beam. Right - Robust 0 + 6 uv-klambda uv-taper (RMS = 31.2 µJy/beam) with contours at 90, 270, 500, 550, 700, 900, 1000, 1200 and 1400 µJy/beam. Bottow row: B-array: Left - Robust 0 (RMS = 19.6 µJy/beam) with contours at 12, 50, 60, 80, 100, 150, 200 and 250µJy/beam. Right - Robust 0 + 16 klambda uv-taper (RMS = 19.4 µJy/beam) with contours at 40, 60, 100, 150, 220, 300 and 350 µJy/beam. Chapter 4. Results 36

The B/L maps (ﬁg. [4.2] - bottom row) have a resolution similar to the C/C maps. However, the higher RMS value of the B-array L-band suppresses the detection of large scale emissions that get cleaned out as noise. This is clearly visible when comparing the B/L and C/C maps in (ﬁg. [4.1] - bottom row) respectively. With its small beam size, we can resolve the features we see in the C/C maps e.g. the spiral feature and the ring structure. The core is also visible in both, the robust 0 and uv-tapered maps but the absence of large scale emissions reveals the continuum hole at the centre of the disc more clearly than the C/C uv-tapered map where the hole contains some more emission.

In summary the features we have identiﬁed and will later discuss, in 4.3, are:

1. The continuum ring.

2. Spiral feature, F1.

3. Tidal tail, F2.

4. C-shaped feature, F3, and its counterparts, F3-complex.

5. Extra-planar features.

6. Nucleus, N2613-A. Chapter 4. Results 37

4.1.1 Combined Data Sets

Combination techniques described in section 3.4 are used to make in-band combination maps for arrays at individual frequency bands (C-band (fig. [4.3]) and L-band (fig. [4.4])). The resulting images have a higher S/N ratio than the individual sets, as shown in table [3.1] and table [3.2]. A total intensity map is also created by combining all available arrays and frequency bands. Table 3.2 contains the parameters for fig. [4.1] and fig. [4.2].

Figure 4.3: Combined arrays (C+D) at C-band total intensity maps of NGC 2613 with contours in magenta. The beam is represented by a white ellipse at the bottom-right corner and the lowest contour level is marked in thick white line. Left - Robust 0 with contours at 12, 25, 50, 100 and 200 µJy/beam. Right - Robust 0 + 16 klambda uv-taper with contours at 12, 25, 50, 100 and 200 µJy/beam

The knot-like disk features found in the C/C maps, discussed above, are visible in the combined array C-band maps. The combination maps have a comparable resolution to the highest resolution source maps (like B/L and C/C), while simultaneously having lower RMS values than said maps. The C-band images reveal the presence of several knots embedded throughout the galactic disk. In the robust 0 map (ﬁg. [4.3] - Left), the ring found in the C/C maps shows more large Chapter 4. Results 38

scale emission while retaining the distinct ring structure. Features found in the C/C single array maps are also visible in the combined map. Otherwise, no new features have been found as a result of combining arrays.

Figure 4.4: Combined arrays (B+C+D) at L-band total intensity maps of NGC 2613 with contours in magenta. The beam is represented by a white ellipse at the bottom-right corner and the lowest contour level is marked in thick white line. Left - Robust 0 with contours at 50, 80, 150 and 250 µJy/beam. Right - Robust 0 + 16 klambda uv-taper with contours at 80, 150, 250, 500, 750 and 1000 µJy/beam

From ﬁg. [4.4], the L-band maps (in both robust 0 and uv-tapered) reveal the presence of a spur, south of the central nuclear source. In the uv-tapered L-band map, two similar features are seen south of the bright eastern spiral-like feature. This feature is at the 4 σ level. There are also narrow extensions along the north of the galactic disk in the L-band maps. This spur could be connected to the larger extraplanar emissions found in the in D/C and C/L uvtapered map.

The ring structure visible in the L band robust 0 map shows asymmetry in its projected structure. The F1 feature shows more detailed structure than the single array maps and the C-band combined maps. Chapter 4. Results 39

Figure 4.5: Combined B-array, C-array and D-array at L-band; and C-array and D-array at C-band total intensity maps of NGC 2613 with contours in magenta. The beam is represented by a white ellipse at the bottom-right corner and the lowest contour level is marked in thick white line. Left - Robust 0 with contours at 28, 35, 70, 100, 140 and 200 µJy/beam. Right - Robust 0 + 10 klambda uv-taper with contours at 28, 35, 70, 100, 140 and 200 µJy/beam.

Using the method described in section 3.4, a combined array and frequency map is generated using the C-band (C-array and D-array) and L-band (B-array, C-array and D-array) data. The resulting maps have an intermediate frequency of ν = 4.13 GHz. The RMS values for the robust 0 and uv-tapered map are, on average, lower that the values found in most of previous maps, particularly in the L-band data.

In the robust 0 maps in ﬁg. [4.5] (Left), the distinct structure of the F1 spiral feature is clearly visible. Also visible is the central nuclear source and ring found in the previous datasets. No new features were found in the map. Chapter 4. Results 40

4.2 Spectral index maps

The in-band spectral index maps were created using widebandpbcor as described in section 3.2, having corrected for the frequency dependence of the primary beam. The in-band spectral index maps for C-band and L-band are shown in fig. [4.6] and fig. [4.7], respectively. The sharp edge of the map represents the 5-sigma cutoff applied to all the maps during the primary beam correction. Highly discordant values at the edges are likely spurious (Irwin et al., 2015).

The C-band spectral maps have a mottled appearance (fig. [4.6]) and any variations smaller than the beam size should be ignored (Irwin et al., 2015). In the D-array maps (fig. [4.6] -top), the overall disk has large contributions from the non-thermal emission as seen by the negative spectral index. Points in the disk, however, show a flatter spectral index. In both the robust 0 and uv-tapered map, the spectral index flattens out (goes to 0) from the surrounding disk. The 5-sigma cutoff applied for PB-correction eliminates most of the extended sources around the disk. However, the spectral index appears to flatten out at the disk-outflow. The spectral indices have higher errors at the edge of the map. In the D/C uv- tapered map, disk emission close to the edge of the disk is also has a flatter spectral index than the overall disk.

The uniform negative spectral indices in the low resolution (D/L) spectral index maps indicate that, similarly to the C-band spectral index, most of the emission from the disk is non-thermal in nature (ﬁg. [4.7]). Similar to the D/C map discussed above, the C/L spectral index maps show a negative global spectral index as the emission is dominated by non-thermal emission within the disk. Chapter 4. Results 41

Figure 4.6: PB-corrected C-band spectral index maps of NGC 2613 with contours in magenta. The beam is represented by a white ellipse at the bottom- right corner. top row: D-array: Left - Robust 0 with contours at 20.2 and 50.5 µJy/beam. Right - Robust 0 + 6 klambda uv-taper with contours at 23.6 and 59 µJy/beam. Bottow row: C-array: Left - Robust 0 with contours at 12, 40 and 100 µJy/beam. Right - Robust 0 + 16 klambda uv-taper with contours at 12, 25, 50 and 100 µJy/beam.

In the uv-tapered C/L map, the region around F1 has a ﬂatter spectral index than the global value. At similar resolutions, the D/C maps show more ﬂattening at these features than in the C/L maps. These values are typical for most galactic disks with the thermal emission contributing a higher fraction at C-band than at L-band. Chapter 4. Results 42

Figure 4.7: PB-corrected L-band spectral index maps of NGC 2613 with contours in magenta. The beam is represented by a white ellipse at the bottom- right corner. Top row: D-array: Left - Robust 0 with contours at 84 and 150 µJy/beam. Right - Robust 0 + 2.5 klambda uv-taper with contours at 90 and 200 µJy/beam. Middle row: C-array: Left - Robust 0 with contours at 90 and 270 µJy/beam. Right - Robust 0 + 6 klambda uv-taper with contours at 90 and 270 µJy/beam. Bottow row: B-array: Left - Robust 0 with contours at 50, 60 and 80µJy/beam. Right - Robust 0 + 16 klambda uv-taper with contours at 40, 60 and 100µJy/beam. Chapter 4. Results 43

4.3 Continuum features - new results

In fig. [4.1] we detected the presence of several features, such as large-scale extra- planar extensions. Within the disk itself we also find the presence of several distinct features. Previous observations of the radio contresolvinuum were of lower resolution, which blended most of these features into the disk. The change in resolution in both C-band (fig. [4.1]) and L-band (fig. [4.2]) from the lower resolution D-array configuration to the higher resolution B-array (or C-array for C-band) illustrates the benefit of the new higher resolution data used in this work. The largest of these features are labelled in fig. [4.1] on the C/C map. Some of these features are described in detail below.

4.3.1 Continuum ring

In the C-array C-band map (fig. [4.1] - bottom row), a hole in the continuum emission at the central region around the central nuclear source, giving the galaxy a distinct ring-like structure. The continuum ring is also clearly visible in the B-array L-band maps. The inner edge of the ring is approximately 5.3 kpc from the core and extends out to 9 ∼ 10 kpc. Within this ring lies the presence of several continuum hotspots and the spiral feature, F1. This ring feature was fit using the imfit on the C/C robust 0 map where the ring is clearly visible and has a low RMS.

Using the ring-like structure found in the C/C robust 0 map, a new measurement for the inclination of NGC 2613 can be made. This is the ﬁrst such measurement made for this galaxy using the radio continuum. NED measurements of inclination, Chapter 4. Results 44

stated in table [1.1] use 3σ intensity isophotes to solve for the shape and orientation for the galaxy.

To measure the inclination of the ring, we fit the inner and outer edge of the ring through visual inspection. To do this we use CASA viewer and elliptical region tool. With this, we find an inclination of i = 77◦ ± 1◦. The error takes into account the variations in axis lengths due to errors in the visual fit. This inclination in higher than the inclination (75◦) taken from NED, which uses the optical disk from the measurement with the errors measured by 2MASS are much smaller than the errors we get for our measurements. The discrepancy between the two measurements is negligible compared to the errors we find. However, our value for inclination is a better measurement as the ring is well defined and radio emission does not suffer from dust absorption as the optical map does.

4.3.2 Central nuclear source (N2613-A)

The presence of an unresolved point source at the core of the galaxy is uncovered in the highest resolution data sets (i.e. C/C and B/L). X-ray observations of NGC 2613 by Li et al. (2006) suggest the presence of an AGN deeply embedded in core of the galaxy. While this feature is unresolved in both C/C and B/L maps, the connection between the X-ray and radio data shows strong support for the presence of an AGN. The discussion of the nature of this core is discussed in further detail in section 6.3. Chapter 4. Results 45

4.3.2.1 Core size and astrometry

To determine the position and size of the core, we fit a Gaussian to the point source on the highest resolution data available in the set (i.e. C-band and L- band). The results from the fit are presented in table 4.1, from which we find the average fitted position for the core to RA = 0833m22.776s ± 0.004s & DEC = −22◦58024.86” ± 0.16”, with the highest of the uncertainties (between C/C and B/L) chosen. The NED right ascension and declination used in table [1.1] had been found by an isophotal fit of 2MASS IR images to extract the position of the galaxy. The position of the IR core from such a measurement can be affected by the stars surrounding it. The radio core of NGC 2613 has never been resolved to this extent. This new measurement of the position of the radio core is the most precise measurement for the centre of the core as the radio core is distinct and not confused by the stars surrounding it. The measured value for RA is in line with the measurements of the core position by 2MASS. However, the DEC deviates from the 2MASS positions, and are not within the given 2MASS uncertainty of 0.5” (0.03 s). For the size of the core, we use the deconvolved core size.

Observation Position Core size (arcsec) RA Dec Major Minor 2MASS 08h33m22.84s ± 0.03s −22◦58025.2” ± 0.5” - - C-array C-band 08h33m22.779s ± 0.003s −22◦58025.00” ± 0.08” 2.1 ± 0.6 1.5 ± 0.4 B-array L-band 08h33m22.773s ± 0.004s −22◦58024.71” ± 0.16” 2.0 ± 0.7 0.7 ± 0.5

Table 4.1: Astrometry of the core Chapter 4. Results 46

Figure 4.8: Close-up of central region, host to N2613-A marked by a white square in both bands. The beam is represented by a transparent ellipse at the bottom-right corner. Left column: C-array C-band: top - Robust 0 with contours at 12, 40, 100, 200 and 300 µJy/beam. Middle Core with same parameters as above. Bottom - Spectral index map of core with errors contours at 0.08, 0.3 and 0.5. Right column B-array L-Band: top - Robust 0 with contours at 12, 50, 60, 80, 100, 150, 200 and 250µJy/beam.Middle Core with same parameters as above. Bottom - Spectral index map of core with errors contours at 0.08, 0.3 and 0.8. Chapter 4. Results 47

4.3.2.2 Flux Densities and Spectrum

The flux densities at the core are measured using the same method used to measure the core size for the PB-corrected B/L and C/C robust 0 maps. A major source of error for these values are the changes in the box size that affect the flux densities measured using imfit. The values for the flux density and the associated errors are reported in table [4.2]. A close-up of the core at B/L and C/C is shown in fig. [4.8]. This is the first radio measurement ever to be performed of the flux density at the core. While these images are of high resolution, the core isn’t completely resolved i.e. the emission measured at the core cannot be disentangled from the emission at the disk. This could be due to the high inclination of the galactic disk. Flux measured by interferometers can be affected by the ”missing spacing problem”. Measurements of flux by interferometers are ”missing” flux that would be measured by a single dish of the same aperture. However, this is largely applicable to large scale emissions rather than the point-like emission measured at the core.

To measure the inband spectral index α, a weighted mean is calculated over a single beam at the core. Within the beam, a weighted mean is calculated with pixels with highest S/N ratio having greater weight. The number of pixels within a beam, Np/b, must be taken into account since pixel values are not independent of each other (Irwin et al., 2015) . The weighted mean spectral index and its error are calculated as,

 P αi   i α2  α¯ = i (4.1) P 1  i 2  ∆αi Chapter 4. Results 48

,  1/2  N  ∆¯α = p/b (4.2) P 1  i 2  ∆αi .

The spectrum of the central nuclear source can be derived from the high-resolution data sets, B/L and C/C. Four data points (C-band and L-band in-band spectral indices and ﬂux densities) are used to constrain this spectrum. The spectrum can be ﬁt to the polynomial

2 3 Sν = a0 + a1ν + a2ν + a3ν (4.3)

where Sν is the integrated ﬂux density at frequency ν. The in-band spectral index,

dSν αν, is the tangent to the spectrum at ν. The derivative of eq. [4.3], dν , at ν gives us the spectral index eq. [4.4]. Similarly, the derivative of the spectral index power

α law, Sν ∝ ν is calculated and equated to the previous derivative (eq. [4.5]).

dS ν = a + 2a ν + 3a ν2 (4.4) dν 1 2 3

dS ν = kανα−1 (4.5) dν where ‘k’ is the constant of proportionality. With the values for C-band and L- band spectral index and flux density, eq. [4.2] is solved for the spectrum and presented in fig. [4.9]. The parameters of the fit are listed in table [4.3]. Chapter 4. Results 49

Observation SN2613−A(µJy) α B-Array L-band 267 ± 20 −0.11 ± 0.12 C-Array C-band 189 ± 7 −0.33 ± 0.06

Table 4.2: In-band spectral indices and ﬂux densities and N2613-A

Parameter a1 a2 a3 a4 Parameter 292.22 -12.59 -2.67 0.32

Table 4.3: Parameters of the polynomial ﬁt

Figure 4.9: Spectrum from polynomial ﬁts. The red shaded zone is the upper and lowed bounds for the ﬁt. Blue points marks the B/L and C/C intergrated fulx densities at their respective frequencies. Chapter 4. Results 50

4.3.3 Spiral feature - F1

The most obvious of these features, F1, located around RA = 08h33m27s and DEC = −22◦58060” is visible in most data sets. The S-shape of this spiral-like feature (fig. [4.10]) is well resolved at higher resolutions. The complexity of this feature’s shape suggests a non-trivial formation mechanism. At the north end of the feature is a westward tail (F1-B) that appears to have split off from the main feature that curves eastward (F1-A). These features are labelled in fig. [4.10]. Continuum extensions appear to emerge along the edge of the feature, giving us another probe into the formation of this feature.

Figure 4.10: Close-up of F1 in L-band Robust 0 with B-array L-band contours at 50, 60, 80, 100, 150, 200 and 250µJy/beam. The beam is represented by a white ellipse at the bottom-right corner. Chapter 4. Results 51

4.3.4 North-western tail feature - F2

F2 is a narrow extension along the plane of the disk that appears in the north- west corner of the disk, possibly formed by the interaction between NGC 2613 and its companion ESO-G017. The connection between this feature and the companion is inferred visually, but velocity information could allow us to confirm this relationship. This is the first time this feature has been seen in this galaxy. A north-eastern tidal tail was found in HI maps and confirmed using velocity data by Irwin et al. (2003), but this is not seen in the radio continuum. Within these features are a few continuum knots, especially noticeable in the C-band uv-tapered images (fig. [4.3]).

4.3.5 C-shaped feature - F3 and the F3 complex

The northern feature, F3, appears to be a small bubble of continuum emission with an arc-like shape uncovered with the well-resolved C/C data. The outgoing bubble-like shape could be formed from outﬂows due to internal mechanisms. However, the highly irregular shape of the bubble and the outﬂows around the vicinity of the bubble (F3-complex) could also have been caused by an interaction between a small satellite (or an impacting cloud) and the galactic disk. In the uv-tapered C/C image, this arc-like feature appears to part of a greater complex of extended emission. Chapter 4. Results 52

4.3.6 Large scale continuum extensions

Figure 4.11: Left - D array C-band Robust 0 + 6 klambda uv-taper with contours (in magenta) at 23.6 (in white), 59, 118, 236, 472 and 944 µJy/beam. µJy/beam Right - C-array L-band contours (in magenta) with robust 0 with contours at 90 (in white), 270, 500, 550, 700, 1200,and 1400 µJy/beam. The beam is represented by a white ellipse at the bottom-right corner and the lowest contour level is marked in thick white line. The extensions, E1-E4, are marked with white-dashed lines drawn from the mid-plane to the highest point of the extension (2 σ contour for D/C and 3 σ contour for C/L.

Large extensions emerging both above and below the disk are present in the D/C uv-tapered image (ﬁg. [4.1] - top-right) and C/L image (ﬁg. [4.2] - middle row). These features are above the 3 σ level in the L-band data and a little above the 2 σ level in the C-band data. Some of these extensions appear to be associated with the in-disk knots discussed in the previous subsections. All heights are given as the maximum vertical distance, measured from the mid-plane of the galaxy to 2 σ for the uv-tapered C/L map. The measurements are de-projected using the value of inclination obtained in section 4.3.1. Chapter 4. Results 53

Data-set Extension Vertical height (arcsec) De-projected height (kpc)

C-Array L-band E1 67 9.2 E2 58 7.9 E5 77 10.5

D-Array C-band E3 88 12.0 E4 77 10.5

Table 4.4: Vertical heights of extraplanar extensions

In the D/C uv-tapered image (ﬁg. [4.1] - top-right), extensions in the east of the disk appear to be associated with the unresolved F1 feature. The contours for this map is overlaid on the C/C uv-tapered map in ﬁg. [4.11 - Left]. These northern extensions extend to a de-projected height of about 9.2 kpc (henceforth called E1) from the mid-plane of the galaxy. Similarly, the southern extension reaches a de-projected height of 7.9 kpc (henceforth called E2).

In the C/L map (fig. [4.2] - middle row), the galaxy appears to have a small bubble emerging from the centre of the core. There are also large-scale extension, both above and below the plane, that could also be connected to the central nuclear source. In fig. [4.11 - Right], the C/L uv-tapered contour map is overlaid on the C/C uv-tapered map. The southern extension (referred to as E3) extends to about 12.0 kpc, at which point the gas is interrupted and runs parallel to the disk. The behaviour appears similar to a fountain, with the gas from the western extension falling back into the disk, making a loop of diameter 7.5 kpc (referred to as L2). Just east of the core is a northern extension, which will be referred to as E4 that extends to 10.4 kpc. In the uv-tapered D/C image (fig. [4.11] - Left) the extension appears to be pointing eastward (E5). In C/L image (fig. [4.11]- Right), the feature emerges at the same point as E4, but information on the shape Chapter 4. Results 54 and extent of the outflow is lost at it blends into other extensions in the region. This feature reaches a height of 8 kpc and while it may be hard to separate from other sources of emission, there is a clear dip in the continuum emission between E1 and E4, separating the two features. The E4 extension appears to form part of a loop of diameter 7.4 kpc to the west of the core (referred to as L1). The western most part of the loop coincides with the F3 complex, as smaller outflows trace the shape of this feature. Chapter 4. Results 55

4.4 NGC 2613 at Other Wavebands

The Wide-ﬁeld Infrared Survey Explorer (WISE) all-sky survey captures NGC 2613 in all four WISE bands, and these images are presented in ﬁg. [4.12]. The feature F3 appears to brighten at longer wavelengths. Following the stellar tracer at 3.4 µm and 4.6 µm, the feature appears to follow the spiral arm, but deviates sharply at the southern tip. At 22 µm, which is sensitive to dust in star-forming regions, F3 is brighter than the rest of the galaxy. The extended feature at F2 follows the dust emission at 12 µm and 22 µm.

Figure 4.12: WISE (infrared) images of NGC 2613 with C/C robust 0 + 16 klambda uv-taper contours at 12, 25, 50, 100 and 200 µJy/beam. The lowest contour level is marked in thick white line. Chapter 4. Results 56

The H-alpha maps for NGC 2613 were taken at the the 3.5-m telescope at Apache Point Observatory (APO) using the the ARC Telescope Imaging Camera (ARC- TIC) visible-wavelength CCD camera. The results are presented in ﬁg. [4.13] with contours for C-array C-band uv-tapered map. These images were acquired and reduced by Carlos Vargas for CHANG-ES.

In the continuum subtracted H-alpha map we find the presence of several star- forming regions that closely follow the spiral arms. The F3-spiral feature (fig. [4.11]), detected in the radio continuum, appears to be populated by regions of strong star-formation activity. While this star-forming region in the area closely trace out the spiral arm connected to F3, the southern region of the feature shows a deviation from the regular spiral arm. This can be seen as two distinct star- forming regions within the F3 feature, one of which is connected to the spiral arm. The second component of the star-forming region could be connected to the formation of the F3 spiral feature as discussed in section 6.1.2. Also regions of strong star-formation are seen at the tip of the F2 tidal tail (fig. [4.3.4]) and in the F3 complex (fig. [4.3.5]). The nature and origin of the star-forming regions in these features are discussed in section 6.1.3. Chapter 4. Results 57

Figure 4.13: H-alpha images of NGC 2613 with C/C robust 0 + 16 klambda uv-taper contours at 12, 25, 50, 100 and 200 µJy/beam. The lowest contour level is marked in thick white line.

The DSS optical maps (ﬁrst-generation) of NGC 2613 show several stellar groups populating the F1 feature, as shown in ﬁg. 4.14. The F2 feature appears to have an optical counterpart. Following this feature, to the north-west, is the companion ESO495-G017. This feature and the companion could be tidally linked, but the absence of velocity information from the continuum map leaves just a visual inspection of the feature. Chapter 4. Results 58

Figure 4.14: DSS (optical) images of NGC 2613 with C/C robust 0 + 16 klambda uv-taper contours at 12, 25, 50, 100 and 200 µJy/beam.

X-ray images of NGC 2613 by Li et al.(2006) using XMM-Newton reveal the presence of a few extra-planar emission from the disk of the galaxy. Through visual comparison of the X-ray and radio continuum maps, a clear relationship between the X-ray emission and radio halo is apparent. The northern bubble, south-western extension, and southern extension (as marked in fig. [4.15] - left) have radio continuum counterparts which are clearly seen in the D/C uv-tapered map in fig. [4.1] (discussed in section 4.3.6). The southern and south-western extensions have radio extensions that are connected to each other via the southern Chapter 4. Results 59 loop, as seen in the C/L uv-tapered map in fig. [4.2]. The radio feature connected to the northern bubble appears to be more diffuse and extended than its X-ray counterpart. In the C/L uv-tapered map the northern bubble appears to connect to an extension that splits off to form the north-western loop.

Figure 4.15: Left - EPIC-PN-05-2-keV-intensity-contours-overlaid-on-the- digitized-sky-survey

(Li et al., 2006).

Previous radio-continuum images of NGG 2613 (Chaves & Irwin, 2001; Irwin & Chaves, 2003) do not show the presence of the southern and north-western loop. Similarly, the loops shown in the HI continuum map (Contours: fig. [4.16] - Left) are not seen in the new data sets; however, the F1 feature is clearly visible. The central pair of high latitude features marked in the HI zeroth moment (Contours: fig. [4.16] - Right) are found in the new maps, clearly visible in fig. [4.11]. Chapter 4. Results 60

Figure 4.16: Left - Zeroth moment map with HI continuum map contours. Contour levels are 0.23, 0.30, 0.5, 0.75, 1, 1.25, 1.5, 1.75, 2, 2.5, 3, and 3.5 mJy/beam. Right -Optical DSS image with Moment 0 map contours 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, and 29.3 ×20cm−2. The dashed line marks the feature connected to F1 Chapter 5

Separation of thermal and non-thermal emission

5.1 Introduction

Continuum emission can be separated into two components - thermal emission and non-thermal emission, characterized by their sources. The separation of the thermal and non-thermal components in the radio continuum allows us to study the properties of their sources in greater detail. The radio continuum emission is produced by a combination of the thermal bremsstrahlung and synchrotron emission.

Thermal bremsstrahlung emission (free-free emission), caused by the emission of a photon due to the deceleration of an electron by a positive nucleus, typically a proton, and is the dominant source of thermal emission in the radio continuum. The velocities of the particles in the gas follow a Maxwellian distribution with the

61 Chapter 5. Separation of thermal and non-thermal emission 62 shape of the spectrum being deﬁned by a characteristic temperature. Thermal

−0.1 bremsstrahlung emission follows a power law, Iν,th ∝ ν .

In the radio continuum, non-thermal emission is formed by synchrotron emission which is a result of photons emitted by electrons trapped in a magnetic ﬁeld.

αnth Synchrotron emission follows a power law, Iν,nth ∝ ν . However, αnth can vary with position. Globally, αnth is typically ∼ −0.7 but varies with position, for instance, the non-thermal spectral index at sources of acceleration is flat but steepens as it moves further away. As electrons diffuse as they move further from their source and diffuse, higher energy electrons particles lose energy more rapidly, causing the spectra to steepen with position. Also, cosmic rays, thought to be formed in supernovae (SNe) or shocks from supernova remnants (SNR), suffer energy losses as they propagate causing a steepening of the synchrotron spectral index that varies over time. Hence, a flatter spectral index could correspond to younger The EM method is applied for both Hαrelcorr map sources of non-thermal emission.

The measured spectral index of the radio continuum emission has contributions from both the thermal and non-thermal component, as described in eq. [5.1].

−0.1 αnth Iν = k1ν + k2ν . (5.1)

In order to study the sources of radio continuum emission, the two components mentioned above must be disentangled from each other. Several methods have been used to perform this separation. The relationship between the infrared (IR) emission and thermal radio emission is used to estimate the thermal radio emission which is subtracted out from the total radio emission leaving behind the non- thermal radio component. Chapter 5. Separation of thermal and non-thermal emission 63

However, there are a lot of caveats with using IR emission to predict the thermal radio emission. For one, the thermal component can be overestimated due to emission from typically non-thermal sources, like super-massive black-holes, that can simulate IR emission, polluting the thermal predictions. The synchrotron spectral index is shown to aﬀect the relationship between the IR emission and thermal radio emission (Niklas & Beck, 1997). As mentioned earlier, the non- thermal spectral index varies with position, making the separation non-trivial. This leads to over- or under-prediction in the thermal emission as the non-thermal spectral index varies across the galaxy.

Another tracer for thermal emission is recombination lines, especially the Hα line which is the strongest Balmer line and is easily detected, making it a candidate for the separation (Vargas et al., in prep.). The separation works similar to the IR-emission method, however, the strong extinction of the Hα line by dust can result in the underestimation of the thermal component. This can be a problem for CHANG-ES data that consist of edge on galaxies that are heavily obscured by dust lanes. Other recombination lines (e.g., the IR Brackett or Paschen lines of Hydrogen) could be used that are not heavily obscured but those are too weak to currently be easily observed by the current generation of telescopes.

As part of the CHANG-ES project, Vargas et al.(in prep.) propose several methods, including a new method that uses a combination of both IR and Hα emission, to achieve thermal/non-thermal separation that could yield measurements from both obscured and unobscured regions from edge-on galaxies. In this thesis we will discuss and attempt to use some of the methods to perform the thermal/non- thermal separation on the continuum emission from NGC 2613. Chapter 5. Separation of thermal and non-thermal emission 64

5.2 Methodology

5.2.1 Preparing the data for thermal/non-thermal separa-

tion

To perform the thermal/non-thermal separation, a wide breadth of multi-wavelength maps of NGC 2613 is utilized. This includes:

• C-array L-band robust 0 radio map (C/L radio maps).

• D-array C-band robust 0 radio map (D/C radio maps).

• 22.1 µm WISE infrared map.

• H-alpha (Hα) map.

Before performing the separation the images must be prepared to make corrections and streamline the process. In order to keep a uniform shape for all the maps used in the separation all maps are required to have the same resolution and pixel size. A mother grid is chosen, in this case the C/L robust 0 map, to which all maps will be re-gridded so the coordinate structure, i.e. the pixel (or cell) size and shape of the map, will be the same for all maps. All maps are also converted to units per pixel so the algorithm can perform pixel-to-pixel calculations. Chapter 5. Separation of thermal and non-thermal emission 65

Radio maps

The C/L and D/C radio maps are ﬁrst smoothed to a beam of 20” in CASA. The C/L radio map was chosen to be the mother grid as it had the largest pixel size (2”) in order to prevent spatial oversampling. The D/C map is re-gridded to the C/L map and both maps are then converted to units of Jy/pix.

Infrared map

Some of the methods used to perform the thermal/non-thermal separation requires 24 µm IR images. Hence, the 22.1 µm maps must be scaled to create a 24 µm using a scaling factor of 1.03 (Wiegert et al., 2015). Before we can smooth and regrid the map, the units were converted to Jy/pix so the image header can be read by CASA. Before the conversion can be made, a corrective factor was also applied to account for aperture and color correction. The ﬁnal conversion formula, with the necessary correction, is given by;

−5 F24µm [Jy/pix] = 4.68 × 10 × 1.03 × F22.1µm [counts/pix]. (5.2)

The IR map is then convolved with a gaussian beam of 20” using Aniano kernels (Aniano et al., 2011) in IDL and then regridded in CASA to the C/L template map. The map is then converted to ergs/s/pixel.

Hα map

The Hα map is converted to Jy/beam so the map header can be read by CASA. The map is ﬁrst re-gridded to the C/L map in CASA and then smoothed to a resolution Chapter 5. Separation of thermal and non-thermal emission 66 of 20” in CASA. The map is then ﬁnally converted to units of ergs/s/pixel.

The final prepared maps used in the thermal/non-thermal separation are shown in fig. [5.1]. The final IR and Hα map is in units of ergs/s/pixel and the radio maps are in units of Jy/pix.

Relano/Tabatabaei Method

Figure 5.1: Final prepared maps for the thermal/non-thermal separation: Top left - L-band radio maps (Map units : Jy/pix). Top right - C-band radio maps (Map units : Jy/pix). Bottom left - Infrared map (Map units : ergs/s/pix). Bottom right - H-alpha maps (Map units : ergs/s/pix). Chapter 5. Separation of thermal and non-thermal emission 67

5.2.2 Extinction-correction for Hα emission

Two diﬀerent methods were utilized to measure the Hα extinction. One method, used in Rela˜noet al.(2007), uses just the 24 µm IR-emission while a new method, proposed in Vargas et al.(in prep.), that uses a mixture of the H α emission and 24 µm IR emission.

5.2.3 IR-only method

In Rela˜noet al.(2007), the relationship between the 24 µm IR-emission and extinction-corrected Hα emission, Hαcorr, was calculated using several HII regions in high metallicity galaxies (eq. [5.3]).

log[L(24 µm)] + (−7.28 ± 0.52) log [L(Hαcorr)] = . (5.3) rel 1.21 ± 0.01

Eq. 5.3 is normally applied to the integrated values of ﬂux but the calculations for thermal/nont-thermal separation are applied on a pixel-to-pixel basis. There- fore, the sum of the pixels in a map generated by pixel-to-pixel calculation for Hαcorr prediction map will be diﬀerent from the prediction map formed using the integrated values of the IR map. To correct for this, the pixel-to-pixel map is multiplied by a corrective factor to force the sum to be equal to that of sum of the integrated values. Chapter 5. Separation of thermal and non-thermal emission 68

5.2.4 Mixture method

The other method for preparing the Hαcorr prediction map used in Vargas et al. (in prep.) uses a linear combination of the observed Hα,Hαobs, and 24 µm IR- emission (Kennicutt et al., 2007). H-alpha maps are heavily obscured by dust, but the 24 µm emission is linked to warm dust so regions with more IR emission are connected to dustier regions. Hence, the IR-emission can be used to correct the H-alpha map as A scaling factor, a, is multiplied to the IR-emission, which depends on the nature of the IR-emission. The relationship is given by,

corr obs µ log [L(Hαmix )] = log[L(Hα )] + a log[L(24 m)]. (5.4)

Using integrated measurements of galaxies, Kennicutt et al.(2009), the value for

log[L(24µm)] the scaling factor, a = 0.021 ± 0.005 is derived from the ratio Log[L(Hαobs)] . Chapter 5. Separation of thermal and non-thermal emission 69

5.2.5 Obtaining the thermal ﬂux

corr Two methods were tested to create the thermal prediction maps for both, Hαrel

corr and Hαmix . The ﬁrst is the method used in Tabatabaei et al.(2007) to calculate the thermal emission using the corrected Hαcorr maps. The second approach uses the star formation rate (SFR) to estimate the thermal emission from calibrations found in Murphy et al.(2011) and Jarrett et al.(2013).

5.2.6 Emission measure (EM) method

The Tabatabaei et al.(2007) method uses the relationship between the H αcorr emission and the emission measure, EM. Emission measure is deﬁned as the volume integral of the square of the electron number density ne, along the line of sight, as shown in eq.[5.6]:

Z 2 EM = nedV. (5.5)

The EM is proportional to the intensity of both the thermal radio emission and corrected H-alpha emission. The relationship between the extinction corrected H-alpha emission is shown in eq. [5.6](Valls-Gabaud, 1998). The EM method is applied for both Hαrelcorr map (eq. [5.3]) and Hαmixcorr (eq. [5.4]).

0.029 corr 2 1.017 T e Hα [erg/cm s sr] × T e4 × 10 4 EM[pc cm−6] = rel/mix . (5.6) 9.41 × 10−8

4 Where, T e4 is the electron temperature (Te = 10,000 K) given in units of 10

K. Once the EM is calculated, the optical depth of the thermal emission, τe, is Chapter 5. Separation of thermal and non-thermal emission 70

calculated using eq. [5.7] for each radio frequency, ν, and then converted to a brightness temperature (eq. [5.8]).

−2 −1.35 τe = 8.235 × 10 × a × T e4 × ν [GHz] × 1.08 × EM (5.7)

−τe Tb[K] = Te(1 − e ). (5.8)

The brightness temperature is then converted to thermal ﬂux density, Sν,th, with units of Jy/beam using eq. [5.9] (Basu et al., 2012).

−7 2 2 2 2 Sν,th = 8.18 × 10 × Tb × ν [GHz ] × FWHM [arcsec ] (5.9)

5.2.7 Star formation rate (SFR) method

5.2.7.1 Murphy Method

Murphy et al.(2011) calibrated the relationship between the SFR and the H αcorr emission using star forming regions in the nearby galaxy, NGC 6946. Thermal emission measured in the Ka band in these regions is used to test the SFR diag- nostics at various wavelengths and ﬁnd that the calibrations calculated for both

corr corr Hαrel and Hαmix are a good match to the thermal emission measured in the Ka band. The calibrated relationship for the Rela˜noet al.(2007) method is given by,

−36 corr SFRrel = 5.58 × 10 × Hαrel . (5.10) Chapter 5. Separation of thermal and non-thermal emission 71

Similarly, the SFRmix calibrated by Murphy et al.(2011) for the mixture method is given by,

−36 corr SFRmix = 5.37 × 10 × Hαmix . (5.11)

5.2.7.2 Jarrett Method

Jarrett et al.(2013) calibrated the relationship between the SFR and WISE maps (in each waveband) using a sample of 17 well-studied galaxies. The calibrated relationship for the 22.1 µm IR-emission is given by,

−10 SFR22.1µm = 7.50 × 10 × L(22.1µm). (5.12)

Murphy et al.(2011) derives the relationship between the thermal radio emission

Lν,th and the SFR which is given by,

−28 −0.45 0.1 SFR [M /yr] = 4.46 × 10 × T e4 × (ν [GHz]) × Lν,th [erg/s/Hz]. (5.13)

The ﬁnal step involves solving for Lν,th giving us the thermal radio emission. Dividing the thermal emission by the radio maps (C-band or L-band) gives us the thermal fraction which allows us to generate thermal and non-thermal maps for C-band and L-band maps. The results of the separations are shown in section 5.3. Chapter 5. Separation of thermal and non-thermal emission 72

5.3 Results

The results for the different combination of method used to perform thermal/non- thermal separation for NGC 2613, as discussed in section 5.2, are presented below. A total of five possible methods are used for the separation. Each method is performed for both B-band and L-band radio maps. The thermal fraction and non-thermal spectral index are plotted for each method, shown in figures 5.1-5.5.

The diﬀerent methods are outlined in table 5.1.

Name Extinction correction Thermal Flux

IR-only/EM Rela˜noet al.(2007) Tabatabaei et al.(2007) eq. [5.3] eq. [5.8]

Mixture/EM Kennicutt et al.(2007) Tabatabaei et al.(2007) eq. [5.4 eq. [5.8

IR-only/Murphy - Murphy et al.(2011) eq. [5.10]

Mixture/Murphy Kennicutt et al.(2007) Murphy et al.(2011) eq. [5.4] eq. [5.11]

Jarrett (22.1 µm) - Murphy et al.(2011) eq. [5.13]

Table 5.1: Methods used to perform the thermal/non-thermal separation. Equations used in the steps performed are stated. Chapter 5. Separation of thermal and non-thermal emission 73

IR-only/EM Method

Figure 5.2: IR-only/EM Method: Top left - C-band thermal fraction. Top right - L-band thermal fraction. Bottom left - Non-thermal spectral index. Bottom right - Total spectral index. Chapter 5. Separation of thermal and non-thermal emission 74

Mixture/EM Method

Figure 5.3: Mixture/EM Method: Top left - C-band thermal fraction. Top right - L-band thermal fraction. Bottom left - Non-thermal spectral index. Bottom right - Total spectral index. Chapter 5. Separation of thermal and non-thermal emission 75

IR-only/Murphy Method

Figure 5.4: IR-only/Murphy Method: Top left - C-band thermal fraction. Top right - L-band thermal frraction. Bottom left - Non-thermal spectral index. Bottom right - Total spectral index. Chapter 5. Separation of thermal and non-thermal emission 76

Mixture/Murphy Method

Figure 5.5: Mixture/Murphy Method: Top left - C-band thermal fraction. Top right - L-band thermal fraction. Bottom left - Non-thermal spectral index. Bottom right - Total spectral index. Chapter 5. Separation of thermal and non-thermal emission 77

Jarrett Method

Figure 5.6: Jarrett Method: Top left - C-band thermal fraction. Top right - L-band thermal fraction. Bottom left - Non-thermal spectral index. Bottom right - Total spectral index. Chapter 5. Separation of thermal and non-thermal emission 78

Map Total Integrated Flux (mJy)

C-array C-band rob0 12.6

Map Integrated Thermal Flux (mJy)

IR-only/EM 4.6

Mixture/EM 2.6

IR-only/Murphy (24 µm) 4.2

Mixture/Murphy 2.4

Jarrett (22.1 µm) 4.4

Table 5.2: Integrated ﬂuxes for thermal/non-thermal separation methods, as described in table 5.1 for C-band radio. maps

At first glance we notice that for IR-only maps, i.e. IR-only/EM Method (fig. [5.2]), IR-only/Murphy (fig. [5.4]) and Jarrett Method (fig. [5.6]), the thermal fraction is much higher that the methods that use the mixture method. This means that the mixture methods have lower predictions for the thermal fraction of the radio continuum, when compared to the IR-only methods.

The maps were smoothed out to a much lower resolution. This smears out details, like the core in the centre of the galaxy. So details regarding the thermal fraction at the centre of the galaxy, specifically the central nuclear source, are lost. Above the core, however, is a source that has a higher thermal fraction that the surrounding region. This results in the flattening of the total spectral index as evidenced by the comparison between the non-thermal spectral and total spectral index at the core, seen in all methods. The difference between the non-thermal spectral and total spectral index at sources with high thermal fractions is higher than the surrounding regions. This is evidence for spectral index flattening by a strong thermal source. Chapter 5. Separation of thermal and non-thermal emission 79

The ﬂattening of the spectral index is also noticeable in the F1 - spiral feature (ﬁg. [4.3.3]), along with a strong thermal fraction compared to the surrounding region. As a result of higher thermal predictions, the non-thermal spectral index for the IR-only maps is much steeper than the mixture methods.

In the IR-only maps, regions of high thermal fraction are much higher than the surrounding disk when compared to the same regions in the mixture methods. The underestimation of the thermal components could be a result of the extinction of the Hα emission in edge-on galaxies. Li et al.(2016a) showed that there could be dust absorption in the IR emission. Hence, the H-alpha extinction correction applied could be too low for edge-on galaxies. Chapter 6

Discussion

Drawing from the results and analyses performed in chapters 4 and 5, we uncover new information on the structure of NGC 2613. We discuss the structure of NGC 2613; speciﬁcally the radio core, extra-planar features, and the spiral feature F1. In this thesis we present the ﬁrst detection of a compact radio source, NGC 2613- A, at the centre of this galaxy. Using the results from preceding chapters, we describe the source of this compact core.

6.1 The radio disk

The structure of the disk component of NGC 2613 shows several features, including the presence of a continuum ring that extends out to 10 kpc. Comparisons of the optical map to the radio map, as seen in ﬁg. 4.14, show that the extent of the radio disk is similar to that of the optical disk. In section 4.3, several continuum features were detected in the disk, with most of these features found within the continuum ring. In the H-alpha images, these features appear to correspond with

80 Chapter 6. Discussion 81 regions of heavy star formation. We also detected several extra-planar features that surround the disk, which are discussed in greater detail in section 6.2.

Our high-resolution maps of NGC 2613 in the C-band and L-band have allowed us to study these discrete features in greater detail than ever before. This thesis presents the ﬁrst detection of the F3 feature and the complex that surrounds it. Other studies of this galaxy have detected the presence of the F1 spiral feature (Irwin & Chaves, 2003), but the shape of this feature has hitherto been undeﬁned.

In section 4.3.1, we found an inclination of 77◦ ± 1◦, which is similar to the inclination of our well-studied galactic neighbour, the Andromeda Galaxy (M31). Li

12 et al.(2006) found a stellar mass of 1 .2 × 10 M , which is comparable to the

11 M31 stellar mass of 10.3 × 10 M (Sick et al., 2014). Most of the star formation for M31 is concentrated in a 10 kpc ring that can be seen in the 1.46 GHz VLA observations done by Beck et al.(1998) and 1.46 GHz VLA observations done by Berkhuijsen et al.(2003), along with a central nuclear source which is also seen in the high-resolution radio images of NGC 2613 (ﬁg. [6.1]). A similar continuum ring was found in section 4.3.1 that extended out to 9 ∼ 10 kpc. These similarities suggest that NGC 2613 could be a possible analogue to M31. As such, under- standing the origin of the continuum features in M31 will provide greater insight into their counterparts in NGC 2613. Chapter 6. Discussion 82

Figure 6.1: Top : D-array total intensity maps at 1.46 GHz of M31, Beck et al. 1998. Bottom : Eﬀelsberg total intensity maps at 4.85 GHz, Berkhuijsen et al. 1982. Chapter 6. Discussion 83

6.1.1 Origin of the continuum ring

The continuum ring found in NGC 2613 is similar to ones found in several other spiral galaxies, the most notable example being the 10 kpc ring in M31. Studies for the origin of this ring suggest two possibilities, as stated in Lewis et al.(2015):

1. A resonance ring formed due to the presence of a bar

2. A ring formed by the collision of a satellite galaxy

Lewis et al.(2015) used N-body simulations to show that the presence of a 4-5 kpc bar in M31 could lead to the formation of a 10 kpc ring. In the case of NGC 2613, no evidence of such a bar has been detected. Using HI observations, Chaves & Irwin(2001) suggested the possible existence of an oval distortion in the disk, which could be explained by the existence of a bar. However, the hole we detect within the ring would be diﬃcult to explain in the case of a resonance ring.

Lynds & Toomre(1976) and Theys & Spiegel(1977) hypothesized that a head-on collision between a satellite galaxy and the larger disk could result in the ring structure we observe. Models created to study the possibility of a collisional ring in M31 use its neighbour M32 as the driver for the formation of this ring. Dierickx et al.(2014) created N-body simulations for the M31-M32 interaction and found that an oﬀ-centre collision with M32 could explain the ring structure. Their model requires that the current position of M32 (∼85 kpc as projected in front of M31) is near the furthest point of its orbit. The projected barycentric distance (the minimum distance) between NGC 2613 and its companion is ∼50 kpc. The current position of the companion ESO 495-G017 in its orbit isn’t known. However, considering the large minimum distance between NGC 2613 and Chapter 6. Discussion 84 its companion, it is possible that the separation between the two galaxies is larger than the M31-M32 separation.

Observation of the dispersion (or width) of the ring in NGC 2613 suggests that it was formed much earlier in that galaxy’s history than the ring in M31. Lewis et al.(2015) suggested that a large ring dispersion indicates an older ring, with star formation occurring primarily along the centre of the ring, with broadening of the ring as stars disperse. The broadening of the disk over time could also explain the dispersion of features within the disk. Hence, there could be two possible explanations for the existence of an older collisional ring: (1) A collision with another satellite galaxy that has been completely cannibalized by NGC 2613, or (2) ESO 495-G017 passed through the disc at a much earlier time, which is possible if it has a large orbital radius. In the latter case, the repeated passing of ESO 495-G017 could generate a long-lived disk. Future N-body simulations for the formation of the NGC 2613 continuum ring could uncover its formation mechanism.

6.1.2 Spiral feature - F1

The F1 spiral feature (fig. 4.10) is a very bright source of radio emission that is clearly detected in the radio maps and WISE infrared maps. This source also appears to be closely related to star forming regions, given its proximity to bright H-alpha emission in the same area. It is unclear if this feature is along the plane of the disk or a projected view of a feature that lies above the plane. The feature appears to follow the spiral arm, but in fig. [4.10] we see that the structure of this feature shows more complexity than the arm itself. As such, we suggest that this feature may have formed separately from the spiral arm. Continuum extensions Chapter 6. Discussion 85 surrounding this feature (fig. [4.10]) point to several distinct star forming regions (identified by the H-alpha images) that could result in the outflows we observed in fig. [4.10].

The origin of this source has several possible mechanisms. The F1 feature could be an unusual spiral structure smeared out by galactic rotation. It could also have formed from stellar winds or supernovae in the star forming region that lead to outﬂows in the disk, which then became distorted by disk rotation. From section 5.3, we can see that the non-thermal spectral index in the region is much ﬂatter than the surrounding region, suggesting that CRs are younger than those in the surrounding regions. This shows that strong sources of acceleration exist within this region, possibly with strong star formation activity or supernovae remnants.

Another possible source for the F1 feature is infalling material, such as from a satellite galaxy that is slowly being cannibalized by NGC 2613. An infalling satellite galaxy could also explain the formation of the continuum ring as described in section 6.1.1. The broadening of the continuum ring with age, as mentioned in section 6.1.1, could also explain the broad nature of the spiral feature. In this case, the F1 spiral feature could be related to the continuum ring, where the spiral feature is an artifact from the collision between NGC 2613 and its companion at the point of impact. Infalling molecular clouds could also produce these results, though they would not explain the existence of the ring.

The complex spiral structure of the F1 feature suggests that this feature could have been formed by a combination of the above mechanisms. One such possibility is that as the satellite passes though the host galaxy, it may distort the material in the given region and simultaneously trigger star formation. This could lead to outﬂows that become further distorted as the galaxy rotates. Chapter 6. Discussion 86

6.1.3 Tidal feature - F2

Irwin & Chaves(2003) ﬁrst detected the presence of the F2 feature (section 4.3.4). Using the velocity ﬁeld information for the galaxy, they found that the feature was blue-shifted compared to the underlying gas. They concluded that this feature could be linked to ESO 495-G017, which is also blue shifted relative to the galaxy. While we don’t have velocity information for the new VLA data, the observations of the shape and warp of the F2 tidal feature does appear to match with the conclusions made by Irwin & Chaves(2003).

The H-alpha map (ﬁg. [4.13]) reveals star forming regions at the tip of the tidal tail, closely related to continuum knots we detected in the radio continuum maps. While most of the disk appears to be symmetric about the minor axis of the galaxy, the tidal tail appears to have no counterpart at the eastern edge of the disk.

From Chaves & Irwin(2001), we know that ESO 495-G017 shows motion around NGC 2613, and that the motion of this satellite is connected to blue-shifted tidal tail This indicates that the tidal tail could be tidally formed by the passage of ESO 495-G017 after it passed by the western edge of the disk, giving the tidal arm the appearance of a debris path. As the satellite swung by this region, outer disk material within the region begin to clump at the tip of the tidal arm giving rise to the star forming region we observe in the H-alpha maps.

6.2 Extra-planar features

Extraplanar features, discussed in section 4.3.6 and labelled in ﬁg. [4.11], could be formed from internal or external mechanisms. Stellar winds from multiple supernovae within a small region could be a possible external mechanism for the Chapter 6. Discussion 87

formation of some of these extraplanar features. However, some of the larger extraplanar features extend out to heights of ∼10 kpc. If these features were the result of supernovae, many such supernovae would be required to produce structures of that scale. However, some of these large extensions (E3-E5) do not appear to be connected to strong star forming regions.

An active core is another internal mechanism for the formation of the extraplanar features, speciﬁcally E3 and L1. E3 appears to be part of a larger fountain-like feature which includes the L2 loop. E4 appears to be a combination of at least two extraplanar features, E5 and L1. They could be artifacts of AGN activity which produced a pair radio lobes that have long since disappeared. The radio jets could explain the energy required to create such large extraplanar features around the core. It also explains the symmetry of these features about the major axis. The L1 and L2 loops could be explained by outﬂows that move vertically upwards before the decrease in the gravitational force caused the gas to move radially, and slow down tangentially. This is consistent with drag due to a low velocity gaseous halo environment (Irwin & Chaves, 2003). The plausibility of this mechanism would require knowledge of the orientation and energy of the radio lobes needed to form the features we observe today. We would also need knowledge of the decay time for these features, which must be longer than the age of the features themselves.

The F3-complex appears to be associated with the L1 feature. The F3 complex shows the presence of several star forming regions in the western region of the complex. This could be caused by the L1 loop depositing material onto the disk, triggering star formation in the area.

E1 and E2 may be physically connected to the F1 spiral feature, although velocity information for this region is needed for conﬁrmation. If true, this feature could be related to the starburst activity within the region. In Chaves & Irwin(2001), they Chapter 6. Discussion 88

ﬁnd that for supernovae to form these and similar extensions, input energies on the order of 1055 − 1056 ergs are needed, which would require close to 104 spatially correlated SNe.

6.3 Central nuclear source - NGC2613-A

Using high resolution data from the EVLA, we uncovered the presence of a compact nuclear source, the ﬁrst such detection for NGC 2613. The details regarding the size and nature of the core are limited by the size of the beam (that is, features within the beam cannot be resolved any ﬁner). Taking these limitations into account, measurements of the size of the compact source can be used as be upper limits to the actual values of core size. The high resolution data along with an unobstructed view of the core allows us estimate fairly well the position of the centre of the galaxy. The position of this core is found to be RA = 08h33m22.776s± 0.004s & DEC = −22◦58024.86” ± 0.16”, and the size of this core is found to be ∼ 2” (table 4.1). This corresponds to a physical size of ∼ 200 pc. The AGN itself is likely smaller than the measured size, with the observed nuclear source encompassing an accretion disk.

The new detection of a compact nuclear source may point to the existence of an AGN at the centre of the galaxy. The core could also be explained by the presence of several supernova remnants populating the central region of the galaxy that could not be resolved. In order to characterize this source, we look other indicators for AGN activity such as X-ray observations of the core, and the spectral index. Chapter 6. Discussion 89

6.3.1 AGN

As an extinction-free tracer, radio continuum emission allows us to detect weak AGN activity in nearby galaxies. Ho(2008) states that AGNs appear in the radio continuum as nuclear point sources at optical or UV wavelengths. While the near edge-on inclination of NGC 2613 obscures the core from sight, Li et al.(2006) predicted the presence of an AGN using the X-ray luminosity of the unresolved galactic centre.

The ﬁrst diagnostic tool to look for AGNs is a study of the morphology of the disk. The presence of jets or lobes are a usual signature for AGN activity. While there is an absence of these features, extended features close to the core that resemble outﬂows are present that could be indicative of an AGN, star formation, or a combination of both.

We find that the core is unresolved, i.e. the emission measured within the beam could have contributions from the core and the disk, in the C-array C-band and B-array L-band maps. The radio spectral index at the core can be used to find the source of the emission. A strong AGN would be expected to have a flat spectral index α > −0.5 compared to the steeper spectral index associated with star forming regions. Measuring the spectral index at the core (table 4.2), we find a relatively flat in-band spectral index for both C-band (α ∼ −0.33 ± 0.06) and L-band (α ∼ −0.11 ± 0.12). The spectral index between the frequency bands, as measured in section 4.3.2.2, shows a flat spectral index as well.

In chapter5, the thermal/non-thermal separation could not be achieved for the core alone, because the radio maps were smoothed to a FWHM of 20” (the core is only about 5”). However, the region surrounding the core shows a ﬂatter total spectral index compared to the disk. As cosmic rays propagate out from the core Chapter 6. Discussion 90 through the disk, higher energy particles preferentially lose their energy faster, causing a steepening of the spectral index. Methods that included observed H- alpha emission in their calculation to correct for extinction show this steepening, demonstrating that there is indeed a loss of energy in the cosmic rays leaving the core.

Ho(2008) studied the existence of low-energy, low-luminosity AGNs. Most AGNs may exist in a low state of activity, due to the quiescent nature of black-holes (Greene & Ho, 2007), resulting in low luminosities for this population of AGNs.

6.3.2 Supernovae remnants

Another possible origin of the compact nuclear source are several unresolved supernova remnants. Since these SNR are much smaller than the beam size of the highest resolution radio data, we cannot resolve them. Supernovae and SNR can be radio-bright as their expanding shell emits synchrotron emission. The average flux density for a supernova in M82 in the C-band is around 2.45 mJy, which translates to a spectral power of 3 × 1018 W/Hz for a distance of 3.2 Mpc to M82. The flux density of the C-array C-band core is 1.89 mJy. For a distance of 23.4 Mpc to NGC 2613, the spectral power of the core is found to be 1.27×1019 W/Hz. Comparing the two values, we can see that 4-5 supernovae remnants within the core could result in a similar flux density in the C-band.

The presence of SNR could explain the presence of the compact nuclear source and the absence of jets or lobes. The typical X-ray luminosity for an average SNR in M82 is 1 × 1040 ergs/s (Stevens et al., 1999). 3-4 SNR could also explain the 3.3 × 1040 ergs/s X-ray luminosity for the core of NGC 2613 (Li et al., 2006). Chapter 6. Discussion 91

Considering the energy output alone, the central region could be produced by several SNR or AGNs. However, spectral line fitting performed by Li et al.(2006) shows that an AGN could be the best fit for the X-ray data. The flatter radio spectral index also supports the AGN interpretation.

6.4 Thermal/Non-Thermal separation of the con-

tinuum emission

The final results for the thermal/non-thermal separation, as seen in section 5.3, show very little variation between different methods. The largest difference occurs between the methods used to perform the H-alpha extinction correction. Methods that use the observed H-alpha emission (i.e. the mixture method) show lower estimates for the thermal fraction when compared to the IR-only method.

In the case of the mixture method, it is possible that the correction applied is not enough to successfully create an accurate extinction-corrected H-alpha map. The existing relationship used for the mixture method eq. [(5.4)] is calibrated using face-on galaxies, introducing a diﬃculty in precisely performing the necessary corrections for edge-on galaxies.

The ideal correction could exist between the IR-only method and the mixture method, but observations from Li et al.(2016b) show that both methods could be lower limits for the thermal prediction. Li et al.(2016b) showed that strong absorption by dust grains in the case of edge-on galaxies aﬀect the IR emission, resulting in a lower thermal prediction. Chapter 7

Conclusion

We have presented new results from VLA observations of the edge-on galaxy NGC 2613. These radio observations, along with other multiwavength observations, allow for a careful study of its morphology. The observations provide new insight and improve upon previous studies of NGC 2613. The radio continuum is mapped in the C-band and L-band with high-resolution data available at both bands. The overall morphology of the radio continuum resembles “normal” edge on galaxies. However, with these maps we detected the presence of several continuum features. The characteristics of the detected radio features can be summarized as follows:

• We find evidence supporting the existence of an AGN at the core of the galaxy, as predicted by Li et al.(2006). The high resolution maps uncovered the first detection of a resolved nuclear core. The core also has a flatter spectral index, −0.11 ± 0.12 for L-band and −0.33 ± 0.06 at C-band, than the surrounding region. We attribute this flattening to the presence of an AGN at the core.

92 Chapter 7. Conclusion 93

• We estimated the size and position of the core. The location of the centre of the galactic core was found to be RA = 08h33m22.776s ± 0.004s & DEC = −22◦58024.86” ± 0.16”.

• This study also found evidence for the presence of a continuum ring that extends out to 10 kpc, similar to the continuum rings found in other spiral galaxies like M31. Using this ring, we ﬁnd NGC 2613 to have an inclination of 77◦ ± 1◦.

• A strong spiral continuum feature is detected on the eastern corner of the continuum ring, and the morphology of this feature is uncovered using the highest resolution radio maps.

• We ﬁnd evidence of high-latitude extraplanar features around the galaxy, some of which appear to be connected to continuum features also detected in the galaxy. These extraplanar radio continuum features are some of the largest detected, and the largest of these features extends out to ∼ 12 kpc (making it the largest such detected in the CHANG-ES sample of galaxies).

• We separated the thermal and non-thermal contribution to the continuum emission. While there is not a large diﬀerence between the models used to predict the thermal emission, the source of the largest variations in the models occur in the formation of the corrected H-alpha maps.

NGC 2613 is part of a large and diverse sample of edge-on galaxies, as part of the CHANG-ES survey that can uncover the nature of galactic disks and halos. Study of the outflows between the disk and halo could help us understand the formation history of the galaxy. The results from the observations and reductions performed on the CHANG-ES VLA data for NGC 2613 can be used in conjunc- tion with other findings from the CHANG-ES survey to uncover the nature and Chapter 7. Conclusion 94 morphology of edge-on galaxies. The large extra-planar features along the plane of the galaxies could uncover internal and external mechanisms for the formations of these features. They could also uncover the formation mechanism of radio halos observed around galaxies as they flow between the disk-halo interface.

Follow up observations will be required to further understand the morphology and dynamic nature of the galaxy. Examples include:

• HI line observations to provide velocity ﬁeld information for newly discovered extraplanar features. This allows us to study the dynamic nature of these features and constrain formation models for these extensions as discussed in chapter6.

• Higher resolution X-ray maps will allow us to conﬁrm the existence of the AGN by evaluating the X-ray luminosity at the core. This will allow us to examine whether the measured AGN luminosity measurements from Li et al. (2006) have any contribution from star formation.

• Higher resolution radio imaging of the core (for example A-array at the EVLA) will allow us to study it in greater detail, and could provide further evidence for the existence of the AGN. It also allows us to study the continuum features, especially the F1 spiral feature, is greater detail.

• Optical spectra for the galaxy can uncover information about the galaxy core. Some AGNs, like Narrow Line Seyfert 1 galaxies and LINERS, have distinct spectra that could be used to classify the AGN. Appendix A

Stokes Parameters

Stokes parameters can be deﬁned as the quantitative representations of the polar-

ization. Consider the orthogonal components of the electric ﬁeld, Ex and Ey that propagate along the z-axis, with a phase diﬀerence φ. For a monochromatic wave, the components are,

Ey = E◦y cos(kz − ωt + φ) (A.1)

As these components propagate along the z-axis, the electric ﬁeld appears to trace an ellipse over time. The total power in each direction (X and Y) is given by

2 2 Ex and Ey respectively. Radio telescopes detect radio emission with the use of antennae (e.g., dipoles) that respond to polarization by generating a voltage along a cable in response to the incoming electric ﬁeld. This results in antennae (called feed) that measure the total power along their respective axes. Depending on the design of the receiver, these feeds can detect linear and circular polarization.

2 2 Linear feeds will detect Ex = I0◦ and Ey = I90◦ . On the other hand, circular feeds

95 Appendix A. Stokes Parameters 96

2 2 will detect E◦l = IL and E◦r = IR, where E◦l and E◦r are the left-handed and right-handed circularized polarization.

2 2 Stokes I is the sum of the power, Ex and Ey , in the the X and Y direction that gives us the total power of the incoming radiation i.e. the total intensity of the emission.

2 2 I(linear) =< Ex > + < Ey >= I0◦ + I90◦ (A.2)

2 2 I(circular) =< E◦l > + < E◦r >= IL + IR (A.3)

2 2 The diﬀerence between the power, Ex and Ey , gives us the Stoke Q parameter which provides us with information about the measure of linear polarization of the emission.

2 2 Q(linear) =< Ex − Ey >= I0◦ − I90◦ (A.4)

Q(circular) = 2 < E◦lE◦r cosφ > (A.5)

Stokes U also gives us information on measure of linear polarization for the emission and is measured at diagonals to the orthogonal X and Y axis.

U(linear) = 2 < ExEy cosφ > (A.6)

U(circular) = 2 < E◦lE◦r sinφ > (A.7) Appendix A. Stokes Parameters 97

The Stokes V measures the circularly polarized intensity and is given by,

V (linear) = 2 < ExEy sinφ > (A.8)

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