couleurs density of principal see the a Smith come cmin Mon photographs gart that galaxy NGC ROSAT WN starburstSeyfert Accepted julescarmenphyscolumbiaedu astro-ph/9604162 26 Apr 1996 Institute Columbia member arcmin Athanassoula arcsec centroid INTRODUCTION Sandage is Not and quite Basic partially of Peterson enhancements Brandt of outer from Weedman vigorous type R of Astrophysics in PKS Astronomy faint the Astron the of size facts February SBsb the arms The covered Bedke the Dorado PSPC arms Sandage outside star centre Lo cal So c radial and and Laboratory can asso ciated formation It Madingley as by JP cloud is ab out of these references Received is Group velocity the hereafter b e the consistent NGC Bedke Its complex seen Large is Halp ern and principal contain that source HRI sup erbubble little sion source gas The are source galaxies and at Key b oard at tive in The ABSTRACT bar a Road is Columbia is with  arcmin of the the V to addition seen also emission OSW Magellanic therein Prop ortional has sup erbubble sp ectra nearby from can on NGC words evidence extend see nucleus the with Cambridge bars ends is is December app ears many at HRI although a ab out bright galaxy erg b e thermal quite km Seyfert galaxy from length its the g On University ROSAT calculations comfortably see barred of to s of and out barred ends interpretation Conspicuous s H deep While galaxies putative It Cloud the Printed the for the and in the in soft relative its of to i i section of CB are to has they Osmer mo dels a de Baum Counter emission H observations regions optical intrinsic b e scale other strong bar K centre Xrays Xray spiral the in ab out group

spiral clearly having Vau the HA four and original and b e dominated ar to of Seyfert other is Iwasawa NGC individual of West accommo dated April prop erties two for also satellite galaxy Internet nearinfrared starburst PSPC the from galaxies identied absorption a its them form Xray two th RaymondSmith probably and gested shap e deceleration Baumgart one ble dust a The corresp onds mensions ble p eculiar recently nucleus large distance gas Pastoriza NGC lie kp c not constant ow show third that have nucleus Street wnbastcamacuk and are lanes sources of near The NGC that of activity that density with long by Octob er the velocity velocity this separated NGC High of identied as that although the images by resolved The are to New Xray the nuclear NGC is  nuclear of to massive and parameter of NGC interaction such are Peterson photoionized NGC same is NGC of three ab out present H Resolution ends ROSAT and York thought of Observations the  harder plasma binary At mo dels by The NGC an the strengths nuclear Pastoriza region emission source The of as galaxies Xray HRIs NY this The may arcsec along alternative the kiastcamacuk of NGC nuclear may km arcmin than band was p c The of Xray to emission temp erature distance q centres have Imager prominent We sources spatial are s by comp osite have ring have the noted Elmegreen with measure strongest that and is Xray do a individual with generally Mp c USA source interacted sp ectrum and small hidden Mp c leading led a of and throughout not the resolution HRI its mo del weaknesses to We of but with weak NGC Internet to stated NGC the b e ux the longest bar detect bar assuming relative one shall and is stretch the of rather Xray et p eculiar Seyfert luminosities edges resolved Position Seyfert supp ortive instruments nuclear for  with of of of o cular of lo cations al a that NGC adopt NGC cosmological direct axis the the the the :  to source by NGC of the This is and it keV by nucleus that its nucleus nuclear nuclear nuclear sources roughly lying the source a diuse arcsec has by central Sensi limits Sersic emis NGC Hub Hub gives that sug of and lies the bar the di on in is a

2 WN Brandt JP Halpern and K Iwasawa

Lindblad Jorsater in preparation recently used the the northeast direction An isophotal map of the nuclear

Australia Telescope National Facility to make synthesis region can b e found in Sersic The nuclear sp ectrum

maps of the nuclear region and they nd that it is com has a p olarization of  p er cent in the B R and H bands

p osed of a small nucleus surrounded by an almost cir Brindle et al The relative strengths of the optical

cular ring with a radius of ab out arcsec cf section emission lines in the sp ectrum taken through a arcsec

of Sandqvist Jorsater Lindblad This ring is ap erture are similar to those found in ordinary H i i regions

also seen in H but the correlation b etween radio and OSW although sources of gas ionization in addition to

H substructure is low Tovmassian presented normal OB stars cannot b e ruled out StorchiBergmann

evidence that the cm ux of NGC increased by Wilson Baldwin hereafter SWB classied the

at least a factor of b etween and there are nuclear sp ectrum as a LINER although the details of spa

no further data we know of that examine the claimed tially resolved emissionline proles and ratios in the inner

variability in more detail The radio sp ectral index of arcsec have also b een interpreted as evidence of a comp osite

Harnett is  typical of optically thin sp ectrum of a Seyfert nucleus and H i i regions as describ ed

synchrotron emission b elow NGC do es not have a warm IRAS colour in

the sense of Sanders et al

The manifestations of Seyfert activity in the centre of

Previous Xray observations

NGC are the following

NGC was rst detected as an Xray source by the Ein

Optical sp ectra of the nucleus in a  arcsec ap er

stein Observatory Griths Feigelson van Sp eybro eck

ture show that its O i i i lines have FWHM  km

Fabbiano Feigelson Zamorani Fabbiano Kim

s while its H line has a FWHM of only km s

Trinchieri The ks Einstein observation yielded

Veron Veron Zuiderwijk Veron et al argued

 counts after background subtraction The Xray

that the presence of O i i i broader than H is the sig

emission was clearly extended but details of the shap e were

nature of a comp osite Seyfert H i i region sp ectrum

unclear Crude sp ectral tting to a p owerlaw mo del was

in which the O i i i emission comes primarily from a

p erformed by Krup er Urry Canizares The ob

Seyfert nucleus whose O i i iH ratio is large The

served keV ux was   erg cm s corre

H ux then comes primarily from a region of circum

sp onding to an unabsorb ed isotropic luminosity of  

nuclear starburst activity where the O i i iH ratio

erg s

is small Daz and GarcaVargas et al

The Ginga Xray satellite made b oth scanning and

presented spatially resolved sp ectroscopy of NGC

p ointed observations of NGC on August

which shows a strong increase in the O i i i line strength

with the Large Area Counter LAC instrument Awaki

near the nucleus In the very central region 

Koyama A hard Xray source was seen during the

arcsec O i i iH  while away from this region O

 

scanning observations in the  error b ox After back

i i iH is signicantly less than unity Their sp ectra re

ground subtraction the Ginga LAC count rate during the

veal that in the very central region b oth O i i i and H

p ointed observation was count s the p ointed obser

have ab out the same FWHM of  km s They

 

vation had an entrance ap erture of  FWHM and no

also show broad wings of H in absorption the sig

imaging capability within this ap erture The observed

nature of earlytype stellar photospheres All of these

keV ux was   erg cm s corresp onding to a

features are present in the nuclear sp ectra of SWB as

luminosity of   erg s The keV photon index

well although they classied NGC as a LINER

was measured to b e  p er cent condence level

We favour slightly the interpretation of Veron et al

errors and the cold column was constrained to b e less than

who concluded that the sp ectrum is a comp os

 cm We shall compare the Ginga and ROSAT

ite Seyfert H i i region The LINER classication may

data b elow

simply b e an artefact of applying lineratio diagnostic

diagrams to a twocomponent sp ectrum which is spa

tially unresolved

OBSERVATIONS DATA REDUCTION AND

Kawara Nishida Gregory claimed to detect

ANALYSIS

a broad Brackett line of atomic hydrogen from NGC

ROSAT PSPC Trumper Pfeermann et al ob with a width of  km s In addition

servations were made of NGC starting on Novem they found that the K band and molecular hydrogen

b er RP total raw exp osure of ks ROSAT emission of NGC suggest that it has an active nu

HRI observations were made of NGC starting on cleus see their section I I Ic However Mo orwoo d

June RH total raw exp osure of ks NGC Oliva did not detect the Brackett line and set

was in the centres of the elds of view for b oth ob an upp er limit on it that is a factor of ab out two b elow

servations The ROSAT observations were p erformed in the detection of Kawara et al

the standard wobble mo de to avoid accidental shadow NGC has a compact radio source lo cated at its

ing of sources by the coarse wire grid which forms part of optical nucleus cf plate of Harnett The sp ec

the PSPC entrance window supp ort structure ROSAT p er tral luminosity of NGC at MHz cm is

forms a slow dithering motion diagonal to the detector axes  W Hz Comparison with g of Ulves

with a p erio d of  s and an amplitude of arcmin tad Wilson shows that this is a fairly typical

Reduction and analysis of the PSPC and HRI data were MHz sp ectral luminosity for a Seyfert nucleus al

p erformed with the Starlink ASTERIX Xray data pro cessing though comparison with g of Davies shows

system that it is not entirely out of the range of normal spirals

ROSAT PSPC and HRI observations of NGC 3

X-6

12 X-5

13

14

X-3 15 X-1 X-2 Declination (2000)

X-4 16

17

-59° 18′ 10s 46m 00s 50s 40s 30s 04h 45m 20s

Right Ascension (2000)

Figure Contours of the adaptively smo othed HRI image overlaid on the image of NGC from the UK Schmidt southern sky

survey J plate Contours are at and p er cent of the maximum pixel value see the text for absolute source

uxes Note the strong central Xray source and the lo cations of Xray sources near b oth ends of the bar

tons lie within it and we use a circular tophat smo othing Spatial analysis

function Fig shows the adaptively smo othed PSPC im

Xray sources and naming convention

age using PSPC channels We have set the area that

we smo oth over by requiring that photons lie within it Fig shows contours of the adaptively smo othed HRI

In Table we list the p ositions of Xray sources detected image overlaid on the image from the UK Schmidt southern

near NGC and give their statistical signicances and sky survey J plate see section I Ib of Lasker et al

numbers of counts for more information on the optical image The adap

NGC also lies in the PSPC observation It is tive smo othing algorithm is describ ed in Rangara jan et al

arcmin from the centre of the eld of view As weak and Eb eling White Rangara jan We have

sources can b e dicult to see when they are far o axis set the area that we smo oth over by requiring that pho

4 WN Brandt JP Halpern and K Iwasawa

10

X-6 12 X-5

14

X-4

16 X-2X-1 X-3 Declination (2000)

18

-59° 20′

15s 46m 00s 45s 30s 15s 04h 45m 0s

Right Ascension (2000)

Figure The adaptively smo othed PSPC image of NGC using PSPC channels The shading is logarithmic and contours

are at and p er cent of the maximum pixel value see the text for absolute source uxes

are not enough counts in this source for further analysis we have used the ASTERIX p oint source searching program

PSS Allan to search the region near NGC for

Xray sources We detect an  Xray source for PSPC

Association of the Xray sources with sources at

channels with a most probable p osition of

other wavelengths

h m s  0 00

and a p ositional error of

The compact radio core of NGC has a centroid p osition arcmin This p osition is consistent with that of NGC

h m s  0 00

of Lindblad although we cannot rule out the p ossibility that this

Jorsater in preparation and this agrees with the Xray emission arises from an unrelated background source After

centroid p osition of X to within its error The centroids corrections for vignetting and wire shadowing we derive a

of X and X are separated by arcmin and lie near count rate in PSPC channels of  

the ends of the bar We also note that in the PSPC image count s the error bar is for p er cent condence There

ROSAT PSPC and HRI observations of NGC 5

Table Xray sources near to NGC

Source Source Source Separation HRI HRI PSPC PSPC

name from X counts signicance counts signicance

X

X

X

X

X

X

Source and Source give the J Xray centroid p ositions as determined with the ASTERIX

p oint source searching program PSS Allan We quote HRI p ositions whenever p ossible but when

sources are detected only by the PSPC we quote PSPC p ositions HRI p ositions have errors of arcsec

and PSPC p ositions have errors of arcsec taking into account b oresight and other p ositional errors

The fourth column is the separation in arcmin b etween the centroid of the source and the centroid of source

X which lies in the centre of NGC

HRI and PSPC signicances are determined using PSS When determining signicances we use the full

HRI band and channels corresp onding to roughly keV of the PSPC band These signicances

are strictly valid only for p oint sources for X and X the two sources that show evidence for spatial extent

in the HRI image there is no question of their reality see the text We list all sources near to NGC

that are detected with greater than signicance by either the HRI or the PSPC

HRI counts and PSPC counts are the raw numbers of counts after background subtraction When

determining the numbers of counts we use the full HRI band and channels of the PSPC band Count

errors are for p er cent condence For sources other than X X and X we state the numbers of

counts determined with PSS see Allan For X X and X we have determined the numbers of

counts manually For the HRI we use circular source cells of radii and arcsec for X X and X

resp ectively We carefully choose these source cells so as to include essentially all of the source counts

retain a reasonably low background contribution and minimize p otential crosssource confusion We

use lo cal circular sourcefree cells with radii of arcsec for HRI background subtraction For the PSPC we

use source and background cells as p er Table When sources are not detected by one of the two detectors

at greater than we quote p er cent condence upp er limits on their numbers of counts derived using

the uplim mo de of PSS

Xray spatial extents there app ears to b e some weak emission from a separate

p ointlike source just to the northwest of X This emission

coincides with an optically bright region along one of the

arms compare Figs and

In Fig we plot HRI radial brightness proles of X X

X is of interest b ecause it lies reasonably close to the

and X In making this gure we have conservatively

centre of NGC and b ecause it is fairly rmly detected

excluded HRI channels to avoid any contamination by

by the HRI yet not by the PSPC despite the fact that

UV light cf section of David et al Source X

the PSPC observation is deep er The second fact suggests

and p erhaps source X app ear to b e extended when com

variability of this source It lies ab out arcmin o the

pared with the HRI p oint spread function PSF we obtain

main part of one of the arms and there is no matching source

the PSF from section of David et al and con

on the UK Schmidt image or in the NASA Extragalactic

sider the empirical range of PSFs discussed there We are

Database NED

aware of the eects that asp ect solution errors can have on

source extensions cf Morse but we are in the fortu

X is lo cated near a bright foreground star which

h m s

nate p osition of having three sources to compare The fact

has an optical p osition of

 0 00

that X is signicantly more extended than X strongly

The separation b etween the optical star and

suggests that its apparent extent is real and not an artefact

the Xray centroid is arcsec and this is the most probable

of asp ect solution errors Similarly the extent of X is also

identication for the Xray source The star is not listed in

probably real Examination of the raw HRI image suggests

SIMBAD and we have not b een able to correct its optical

that the extent of X may arise via patchy gas emission or

p osition for any prop er motion Similarly X also app ears

via emission from  individual sources but these data

to b e asso ciated with a foreground star

do not allow this p ossibility to b e examined with condence

None of the sources in the HRI eld is bright enough to al The other unnamed sources listed in Table do not

low us to p erform asp ect error correction using the Morse have any bright optical counterparts At least some of them

co de HRIASPCOR in FTOOLS are probably background sources

6 WN Brandt JP Halpern and K Iwasawa

Xray sp ectral energy distributions Fig Counts from

the corrected PSPC source cells were binned so that one

data p oint in Fig corresp onds to PSPC channels and

we have ignored channels b elow to minimize crosssource

contamination Note that X is the softest of the three

sources while X is the hardest

Spectral tting preliminaries and the neutral

hydrogen column

For our sp ectral tting counts from the corrected PSPC

source cells were binned into channel pulseinvariant

sp ectra We ignored channels and rebinned the ex

tracted sp ectra so that at least source photons were

present in each bin Systematic errors of p er cent were

added in quadrature to the data p oint rms errors to account

for residual uncertainties in the sp ectral calibration of the

PSPC We have used the January resp onse matrix

We mo del our Xray sp ectra using the sp ectral mo dels

in XSPEC Shafer et al The errors for t parameters

will b e quoted for p er cent condence unless explicitly

Figure HRI radial proles of NGC X circles X

stated otherwise taking all free parameters to b e of inter

squares and X stars Each sources rst data p oint is nor

est other than absolute normalization Lampton Margon

malized to unity The exp ected range of the HRI PSF from David

Bowyer

et al is graphed as the three solid curves shifted so that

We have used the measurements of Heiles Cleary

they asymptotically match the background level

to estimate a Galactic neutral hydrogen column in

the direction of NGC We adopt N  cm

H

and estimate that the error on this value is ab out 

Source count extractions for light curves and

cm

spectra

The nuclear region of NGC has a large apparent

Balmer decrement of HH  which OSW argue is

When preparing the light curves and sp ectra presented b e

caused by dust reddening in NGC with E B V 

low we have extracted the source counts from carefully cho

If we assume a Galactic dusttocoldgas ratio the corre

sen circular source cells that include as many source counts

sp onding neutral hydrogen column density is N  

H

as p ossible while minimizing crosssource contamination at

cm see section VI of Burstein Heiles However

low energies b etween X X and X This is dicult

SWB argue that the Balmer emission lines need to b e cor

in the regions b etween the sources due to the electronic

rected for the underlying absorption lines of the early stellar

ghost imaging which widens the p oint spread function b e

p opulation which aects H more strongly than H Af

low ab out keV Hasinger et al We discuss this

ter subtracting an appropriate stellar template sp ectrum

issue in more detail b elow

the resulting Balmer decrement at the nucleus corresp onds

Background counts were subtracted from the source

to E B V  or N   cm However

H

cells using large nearby circular sourcefree background

E B V values of are seen within the spatial re

cells Corrections were included for detector dead time vi

gion corresp onding to X

gnetting and shadowing by the coarse mesh window supp ort

Spectral tting to X

Temporal analysis

The ROSAT PSPC sp ectrum of X is shown in Fig To

Count rates should b e averaged over an integer multiple of

avoid confusion by crosssource contamination we t only

the s ROSAT wobble p erio d when used for source ux

the data ab ove PSPC channel where X X and X

determination cf Brinkmann et al We do not

are fairly cleanly separated A p owerlaw mo del as might b e

detect any highly statistically signicant variability of X

exp ected if electronscattered Xray emission from a Seyfert

X or X within either the PSPC or the HRI data

nucleus dominates the sp ectrum is statistically unaccept

When we compare count rates for X X and X using the

able and gives an unphysically steep photon in



PIMMS software and the sp ectral mo dels describ ed b elow

dex This is understandable due to the very steep

there is no strong evidence for variability b etween the PSPC

dropo in Xray ux ab ove  keV If we t only the data

and HRI observations

ab ove PSPC channel a simple p owerlaw mo del can still

b e ruled out with greater than p er cent condence unless

Sp ectral analysis

its photon index is greater than Such a steep soft Xray

sp ectrum is never seen in Seyfert galaxies and thus a simple

Relative Xray energy distributions

p owerlaw mo del for X is unphysical The residuals sug

gest that a p owerlaw t is p o or b ecause of the robust overall To gain mo delindep endent insight into the sp ectral dier

shap e of the sp ectrum and not b ecause of just a few stray ences b etween X X and X we compared their relative

ROSAT PSPC and HRI observations of NGC 7

A RaymondSmith thermal plasma mo del with cold ab

sorption gives a go o d t see Table The quality of the

thermal plasma t the arguments ab ove regarding scattered

Xray emission and the signicant spatial extent of X sug

gest that the starburst activity in NGC dominates its

soft Xray emission along our line of sight although we can

not rigorously rule out a substantial contribution of scat

tered Xrays from a Seyfert nucleus The derived metal

abundance by number relative to the cosmic one Anders

:

Grevesse is Such a low derived abundance

:

in the centre of a spiral galaxy app ears surprising and such

a low abundance is not suggested by observations at other

wavelengths eg SWB It must b e remembered how

ever that due to our limited number of counts we only t

ted a singletemp erature plasma mo del to what is probably

emission from a multipletemperature plasma This simpli

cation can confuse abundance determinations Fits with

two solarabundance RaymondSmith plasmas are statis

tically acceptable and physically reasonable alb eit p o orly

constrained by these data In addition scattered Xrays

from the Seyfert nucleus could further complicate the sp ec

Figure PSPC sp ectra of NGC X circles X

trum although as explained ab ove we susp ect they do not

squares and X stars These sp ectra have b een normalized so

dominate it Fluxes and the isotropic luminosity of X are

that their maxima corresp ond to unity Note the relative numbers

given in Table

of hard counts from each source

The tted cold hydrogen column is consistent with

the Galactic column and we do not see evidence for ab

data p oints Deleting sets of p oints conrms this conclusion

sorption by gas asso ciated with the nuclear E B V of

>

Electron scattering mirrors in which the atoms in the

SWB For reasons discussed in Section we



mirror are not fully stripp ed of their electrons can also im

adopt the reddening analysis of SWB rather than that of

print Xray emission lines on a scattered Xray sp ectrum

OSW The column asso ciated with this E B V can b e

The strongest such emission lines in the ROSAT band are

statistically accommo dated by multicomponent mo del ts

from iron L We consider an electron scattering mirror that

although the b esttting multicomponent mo dels generally

imprints iron L lines by tting our data to an absorb ed

have columns of  cm

p owerlaw mo del with a Gaussian emission line The cen

If we include the data p oints b elow keV in our t

troid energy of the line is chosen to lie in the range

ting our results are not signicantly changed from those

keV and the line width is chosen to lie in the range

ab ove

keV this is a reasonable mo del for the iron L

complex given the limited sp ectral resolution of ROSAT and

Spectral tting to X and X

the small number of counts in our sp ectrum While some

choices of line parameters in these ranges give statistically

We shall again use only the data ab ove PSPC channel to

acceptable ts the underlying photon indices derived from

reduce crosssource contamination Due to the small num

such ts are always signicantly larger than are seen

b ers of counts from X and X our sp ectral mo dels are

in Seyfert galaxies the soft Xray sp ectra of Seyfert s are

not tightly constrained and we shall quote p er cent

attened in scattering and have photon indices in the range

condence errors throughout this section

In addition the iron L complex equivalent widths

A simple absorb ed p owerlaw mo del for Xs sp ec

:

derived from our tting are always larger than eV and

trum gives is consistent with the Galactic col

:

atter sp ectra tend to require larger iron L complex equiva

umn and cannot b e ruled out on statistical grounds it has

lent widths Such equivalent widths while not imp ossible

However given Xs lo cation and probable star



are large compared with what is seen in for example Mrk

burst nature we susp ect that an absorb ed RaymondSmith

Iwasawa et al

thermal plasma mo del may b e more appropriate Using this

:

Simple absorb ed bremsstrahlung and blackbo dy mo dels

 cm k T mo del we obtain N

H

: :

are p o or ts to the data and can b e rejected with and

keV and the asso ciated abundance is not well con



p er cent condence resp ectively They b oth leave large

strained In the ab ove tting the parameters are lo osely

systematic residuals in the keV range

constrained due to the fact that the neutral hydrogen col

umn can b ecome very large If we assume that there is no Massive or lowmass Xray binary sources similar to

signicant intrinsic hydrogen column and x the column at those in the Milky Way are not likely to make ma jor contri

N  cm this is at least plausible given the butions to Xs ROSAT band ux due to its soft sp ectrum

H

inclination of NGC and our results for X we obtain and fairly steep Xray dropo ab ove  keV cf section

the results shown in Table I I Ib of Fabbiano section of David Jones For

A simple absorb ed p owerlaw mo del for Xs sp ectrum man They may of course contribute signicantly at

:

is consistent with the Galactic column gives higher energies Some contribution from sup ersoft Xray

:

and cannot b e ruled out on statistical grounds it has binaries to the ROSAT ux may b e p ossible 

8 WN Brandt JP Halpern and K Iwasawa

Table RaymondSmith thermal plasma mo dels of Xray sources in NGC

Source Source Source

Quantity X X X

of centre of source cell

of centre of source cell

Radius of source cell arcmin

of centre of background cell

of centre of background cell

Radius of background cell arcmin

Raw number of source counts in channels



: : :

k T keV

: : :

:

Abundance

:

:

y y N cm

H

:

keV Absorb ed F erg cm s

X

keV Unabsorb ed F erg cm s

X

keV L erg s

X

EM cm

n cm

M M

E erg

P dyne cm

We quote p er cent errors for X and p er cent errors for X and X We take all free parameters to

b e of interest other than absolute normalization EM is the emission measure calculated as describ ed in the

Raymond mo del description of Shafer et al n is the mean emitting gas density averaged over the

source M is the mass of the emitting gas E is the thermal energy content of the emitting gas and P is the

pressure of the emitting gas Our calculations of n M E and P parallel those given in section of Armus

et al and we have assumed homogeneous spherical emission regions We take the radial extent of X

to b e less than arcsec which corresp onds to p c the radial extent of X to b e less than arcsec which

corresp onds to p c and the radial extent of X to b e less than arcsec which corresp onds to p c see

Fig

Note that the ROSAT tted abundances are probably not physically meaningful see the text

y Column xed at this value see the text

nucleus decreased b etween the Ginga August and As p er X however we susp ect that an absorb ed

ROSAT November observations Obvious addi RaymondSmith thermal plasma mo del may b e more ap

tional contributors to the Ginga sp ectrum could b e Xray  propriate Using this mo del we obtain N

H

:

:

binaries in X and X If we create a ROSAT sp ectrum cm k T keV and the asso ciated abun



:

that is the sum of X X and X its mismatch with the dance is not well constrained If as p er the previous para

lowenergy end of the Ginga sp ectral mo del is only a factor graph we x the column at N  cm we obtain

H

of ab out two even though X and X are weaker overall the results shown in Table

than X note from Fig that they are harder If as our

analysis suggests X and X contribute to the Ginga ux

Comparison of ROSAT and Ginga spectral results

then the true column density to the nuclear source is prob

ably larger than the upp er limit that Ginga measured Of Using the Ginga sp ectral parameters given in Section and

course sources external to NGC could also contribute the ROSAT sp ectral parameters and data describ ed ab ove

to the Ginga ux Our ROSAT eld do es not have any we have compared the sp ectra from these two satellites If

sources that are much stronger than NGC in it but the Ginga sp ectral data were dominated by a central active

there are four sources of roughly comparable soft Xray ux nucleus in NGC and the ux from this nucleus did not

that could contribute hard ux vary with time then one would exp ect the lowenergy end

of the Ginga sp ectral mo del to join fairly smo othly on to

the highenergy end of the ROSAT PSPC X sp ectrum

DISCUSSION

This is not observed Instead the lowenergy end of the

Ginga sp ectral mo del is a factor of  times higher than

Source X

the highenergy end of the ROSAT PSPC sp ectrum of X

The soft Xray isotropic luminosity that we derive for X even when the upp er limit on the column given by Awaki

is only a small fraction of the total isotropic luminosity of Koyama is used This suggests that either the

the central region In the nearinfrared the K band nuclear Ginga sp ectral data were not dominated by a central ac

isotropic luminosity in a arcsec diameter ap erture is  tive nucleus in NGC at least at the lowenergy end

erg s Forbes et al and signicant extended of the Ginga band or the hard Xray ux from the central

ROSAT PSPC and HRI observations of NGC 9

number of ionizing stars in the central region of NGC

will b e  times larger than that in the R nebula It is

unlikely that O stars directly pro duce most of Xs Xrays

For a typical O star Xray luminosity of  erg s

cf Harnden et al ab out four million O stars would

b e required However most of the Xrays asso ciated with

O stars do not come directly from the stars themselves but

come instead from the energy that their winds and sup er

novae dep osit in the interstellar medium If we scale the

soft Xray luminosity of the R nebula  erg

s Wang Helfand by the factor of mentioned

ab ove we derive a characteristic soft Xray luminosity of

only  erg s This is to b e compared with X

s isotropic luminosity in soft Xrays of  erg s

Hence the nuclear activity app ears to b e signicantly more

ecient in generating Xrays p er Lyman continuum photon

than the activity in R p erhaps due to a larger fractional

contribution from nonstellar pro cesses such as sup ernovae

or heating by a hidden Seyfert nucleus

In Table we present emission measures for our ther

mal plasma ts as well as quantities that we estimate from

Figure PSPC sp ectrum of NGC X A RaymondSmith

these emission measures The internal pressure of X cor

thermal plasma mo del with cold absorption is also shown with the

resp onds to times that of the nearby hot interstellar

corresp onding datatomo del ratio

medium Bowyer et al The thermal plasma mo d

els for X X and X imply surprisingly large densities

emission is seen as well In the farinfrared NGC has a

and thermal energy contents Because the apparent radius

m isotropic luminosity of  erg s Maia

of each source is relatively small we are forced to consider

et al corrected to H km s Mp c

particle densities of order cm and larger X X

The MHz cm radio luminosity of NGC

and X contain energies corresp onding to ab out

is  erg s Comparison of our spatially resolved

sup ernovae Although the sizes and temp eratures of these

Xray maps and plate of Harnett is revealing It

regions are comparable to those exp ected for sup erbubbles

shows that despite the fact that X is stronger than X or

blown by this many sup ernovae over a yr p erio d their

X by a factor of only  the central region is at least

inferred internal densities are  times greater even when

times more p owerful than the regions near X and X at

the evaporation of the cold dense shell to the interior is in

>

MHz cm The fact that the central region is

cluded equation of MacLow McCray Armus et al



times more ecient at pro ducing MHz radio ux p er

Heckman et al Note that the densities we de

unit Xray ux suggests that either the energy generation

rive for our hot sup erbubble gas are comparable to those

mechanism in the central region is dierent from those near

seen in the cold interstellar medium in the regions b etween

X and X or there is an additional pro ducer of radio

diuse and molecular clouds see table of Spitzer

ux asso ciated with the central region that do es not generate

What asp ects of this scenario could b e altered to retain a

large amounts of observable Xray ux As noted b efore the

plausible thermal plasma mo del First the derived emission

MHz sp ectral luminosity of NGC is fairly typical

measures might b e overestimates if the tted abundances are

for that of a Seyfert nucleus

in error It is dicult if not imp ossible to measure star

burst abundances reliably using ROSAT data If the actual Using a arcsec ap erture OSW measured an uncor

rected H isotropic luminosity from NGC of 

metal abundances were the solar ones for example then we

erg s we correct to the distance of Section for the

estimate with XSPEC that the derived emission measure of

X could b e lower by a factor of ab out SWB derived OSW data here and hereafter If we adopt an aver

age E B V of  from the starlightsubtracted near

abundances that are slightly higher than solar in the nuclear

nuclear measurements of SWB then the intrinsic H lu

region of NGC and approximately solar along the bar

However since the average number density scales only as minosity is increased to  erg s This luminosity

requires  Lyman continuum photons s corre

the square ro ot of the emission measure it is hard to imag

sp onding to ab out  O stars Ward used the

ine that the density could b e reduced by more than a factor

of four with such a correction Secondly if the hot medium Brackett data of Kawara et al to derive an ioniz

ing photon number of  photons s we correct to

were clump ed as a result of the sup ernova explosions tak

the distance of Section The latter estimate is derived

ing place among a large number of giant molecular clouds

which have b een completely sho cked but have not nished using a smaller ap erture size of  arcsec

expanding then the required mass and energy content would These estimates for the Lyman continuum photon num

b e reduced since these quantities scale as the lling factor to b er ab ove are  times larger than that from the R

>

the onehalf p ower see section of Armus et al the nebula of Doradus photons s Wang Helfand



lling factor is less than unity Together these two eects the most luminous H i i region in the Lo cal Group

may render plausible a mo del in which one or more of the If we assume identical stellartype distribution functions for

main Xray sources in NGC are extremely luminous ex the central region of NGC and the R nebula the

10 WN Brandt JP Halpern and K Iwasawa

Sources X and X amples of sup erbubbles of the type envisioned by MacLow

McCray and thought to b e seen by ROSAT in

Sources X and X have not b een the sub jects of detailed

NGC Fabian Ward NGC Armus et

studies in the past Baumgart Peterson com

al and NGC Heckman et al A su

mented up on vigorous star formation at the ends of the

p erbubble may also account for the small Xray absorption

bar seen in nearinfrared photographic plates but did not

measured for X since it could dep osit hot gas ab ove the

present quantitative information Similarly H hotsp ots at

disc The spatial extents of X and X app ear to b e larger

the ends of the bar as well as along two of the four arms

than the  p c scale height of a typical galactic inter

can b e seen in g of Sersic Calderon and g

stellar medium and this fact also suggests that a signicant

c of SWB The abundances seem to b e ab out solar in

amount of Xray emitting gas may lie ab ove the starburst

these regions and the extinction is E B V  for

region which is probably conned to ab out the height of

b oth SWB X and X are each over times more

the molecular cloud layer

luminous in soft Xrays than the R nebula of Doradus

Their emission can b e mo delled by thermal gas emission al

though due to their faintness we cannot rigorously rule out

Alternatively for the nuclear source X we may con

a substantial contribution from Xray binaries An interpre

sider a contribution due to photoionization heating by a

tation in terms of sup erbubbles follows the same analysis as

hidden Seyfert nucleus Adopting the tted thermal plasma

for the nuclear source and encounters similarly extreme den

mo del as a rst approximation the emission measure is con

sities A signicant Xray binary contribution would help to

siderably smaller than that of extended Xray sources in

reduce these densities

other Seyfert galaxies For example the Xray emission ex

tended over the inner kp c radius in NGC requires

an emission measure   cm and an electron den

ACKNOWLEDGMENTS

sity n  at an assumed temp erature of K Halp ern

This medium would b e in rough pressure balance

We acknowledge discussions with D Calzetti H Eb eling

with the diuse ionized medium seen in N i i and H

AC Fabian S Sigurdsson E Terlevich R Terlevich M

BlandHawthorn Sokolowski Cecil Similarly Wil

Ward and the members of the Institute of Astronomy X

son et al argued that  M of hot gas would

ray group We thank PO Lindblad and S Jorsater for

b e required to account for the extended nuclear Xrays in

kindly sharing some of their results prior to publication

NGC In NGC Morse et al the extended

We thank an anonymous referee for helpful comments We

Xray emission has a luminosity of   erg s

acknowledge nancial supp ort from the United States Na

which would require a medium of T  K and n 

tional Science Foundation and the British Overseas Research

if it were to b e in pressure equilibrium with the extended

Studentship Programme WNB NASA grant NAG

narrowlineregion clouds Both NGC and NGC

JPH and the JSPC and the British Council KI The

have luminous enough ionizing continua to heat their gas to

ROSAT pro ject is supp orted by the Bundesministerium f ur

K and render it optically thin to keV Xrays The re

Forschung und Technologie BMFT and the MaxPlanck

quired ionization parameter Lnr is  for a

so ciety Much of our analysis has relied on the ASTERIX

p ower law For the case of NGC taking n and

and FTOOLS Xray data pro cessing systems and the XSPEC

r p c requires an ionizing luminosity L  

Xray sp ectral tting software and we thank the p eople who

erg s This would b e a fairly strong Seyfert and its ion

have created and maintain this software This research has

izing luminosity would b e comparable to its farinfrared lu

made use of the SIMBAD database op erated at CDS Stras

minosity although its IRAS colours show no evidence for a

b ourg France and the NASAIPAC extragalactic database

luminous Seyfert comp onent

Helou et al which is op erated by the Jet Propulsion

Lab oratory Caltech

The required intrinsic Xray luminosity in the Seyfert

scenario is ab out times larger than the keV lumi

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