Download ROSAT Ian R. Stevens* and David K. Strickland*

Mon. Not. R. Astron. Soc. 301, 215–230 (1998)
A ROSAT survey of Wolf–Rayet galaxies – II. The extended sample
Ian R. Stevens* and David K. Strickland*
School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT
Accepted 1998 July 29. Received 1998 July 1; in original form 1997 December 22
A B S T R AC T
Key words: surveys – stars: Wolf–Rayet – ISM: jets and outflows – galaxies: starburst –
galaxies: stellar content – X-rays: galaxies.
1
INTRODUCTION
Wolf–Rayet (WR) galaxies are a class of objects defined by the
presence of a significant number of WR stars, as revealed primarily
˚
by the presence of a broad emission feature at around 4650 A
characteristic of WR stars. This WR ‘bump’ is usually due to a
blend of several lines (He II l4686, C III/C IV l4650, N III l4640).
Other lines can also be important signatures of WR activity, such as
C IV l5808. WR galaxies are an important class of objects, both in
terms of understanding massive star evolution (Schaerer & Vacca
1998) and in understanding starburst evolution. Because the WR
phase in massive stars is short, this means that we are possibly
seeing a roughly coeval sample of starburst galaxies, with the
resulting opportunities for studying the dynamic evolution of
starbursts.
Conti (1991) compiled the first catalogue of WR galaxies. In an
earlier paper (Stevens & Strickland 1998, hereafter SS98) we
*E-mail: [email protected] (IRS); [email protected] (DKS)
q 1998 RAS
presented the first X-ray survey of WR galaxies. The main findings
of SS98 were that WR galaxies were substantially X-ray overluminous for their blue luminosity (LB ) compared with a nearby
sample of galaxies, and that their X-ray spectra were typically well
fitted with a single-temperature Raymond–Smith model with
kT ¼ 0:4–1 keV. The phenomological picture that SS98 developed
to explain their observations was of a young starbursting region,
with an age of 3¹6 Myr, blowing an X-ray-emitting superbubble in
the central regions of the galaxy. A necessary feature was that the
period of star formation was short, and this ties in with the general
picture of the stellar populations in WR galaxies (Vacca & Conti
1992). This model was able to explain a variety of features of WR
galaxies, such as the soft thermal X-ray spectrum and the LX :LB
relation.
The original X-ray survey was based solely on WR galaxies in
the catalogue of Conti (1991), and since then a large number of
additional WR galaxies have been identified. In this paper we
extend the original X-ray sample to include those additional WR
galaxies that have been observed (directly or serendipitously) by the
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We present results from an ongoing X-ray survey of Wolf–Rayet (WR) galaxies, a class of
objects believed to be very young starbursts. This paper extends the first X-ray survey of WR
galaxies by Stevens & Strickland by studying WR galaxies identified subsequent to the
original WR galaxy catalogue of Conti.
Out of a sample of 40 new WR galaxies a total of 10 have been observed with the ROSAT
PSPC, and of these seven have been detected (NGC 1365, NGC 1569, I Zw 18, NGC 3353,
NGC 4449, NGC 5408 and a marginal detection of NGC 2366). Of these, all are dwarf
starbursts except for NGC 1365, which is a barred spiral galaxy possibly with an active
nucleus. We also report on observations of the related emission-line galaxy IRAS 0833+6517.
The X-ray properties of these galaxies are broadly in line with those found for the original
sample; they are X-ray overluminous compared with their blue luminosity and have thermal
spectra with typically kT , 0:4¹1:0 keV. There are some oddities: NGC 5408 is very
overluminous in X-rays, even compared with other WR galaxies; I Zw 18 has a harder Xray spectrum; NGC 1365, although thought to contain an active nucleus, has X-ray properties
that are broadly similar to other WR galaxies, and we suggest that the X-ray emission from
NGC 1365 is due to starburst activity.
A good correlation between X-ray and blue luminosity is found for the WR galaxy sample
as a whole. However, when just dwarf galaxies are considered there is little evidence of
correlation. We discuss the implications of these results on our understanding of the X-ray
emission from WR galaxies and suggest that the best explanation for the X-ray activity is
starburst activity from a young starburst region.
216
I. R. Stevens and D. K. Strickland
Table 1. The extended sample of WR galaxies discussed in the paper. Included are new WR galaxies that
were not included in the Conti (1991) catalogue, which have been observed with the ROSAT PSPC, and WR
galaxies in the Conti catalogue which have other reported X-ray observations. Details of the related
emission-line galaxy IRAS 0833+6517 are also included.
Galaxy
name
(1)
Morphology
(2)
2000 coordinates
RA
Dec
(hh mm ss)
(◦ 0 00 )
(3)
(4)
Distance
Quality
(Mpc)
(5)
Exposure
time
(ks)
(6)
129
20.0
2.2
339
3.44
10
13.1
3.7
32
8.0
13.9
6.2, 2.6
4.7, 3.1
12.0
2.3
16.8
4.4, 2.5
7.8
4.7
5.2, 2.9
3
1
1
3
1
1
1
1
2
1
(7)
01 04 01
03 33 36
04 30 49
06 00 44
07 28 24
09 34 02
10 45 23
12 28 12
13 48 22
14 03 19
¹30
¹36
þ64
¹39
þ69
þ55
þ55
þ44
¹42
¹41
Other WR galaxies
NGC 3125
NGC 6764
S
SBb
10 06 33
19 08 18
¹29 56 08
þ50 55 00
12
32
–
–
4
4
Related EL galaxies
IRAS 0833+6517
–
08 38 23
þ65 07 14
76
5.0
2
42
08
51
19
11
14
57
05
21
23
30
23
00
06
00
25
33
30
14
18
Column 1: The names of the target galaxies used in this paper. Alternate names for some of the objects are
given in the text.
Column 2: Morphological type: the listed type (where available) is taken from the SIMBAD data base.
Columns 3 and 4: Galaxy position – J2000 Coordinates.
Column 5: Galaxy distance in Mpc (see text for details).
Column 6: Exposure time(s) of the ROSAT observation.
Column 7: Quality code of the ROSAT observations.
1. Source was the intended target of the observation and lies at the centre of the field of view.
2. Source was not intended target but lies within the inner detector ring (radius 20 arcmin).
3. Source was not intended target and lies outside the inner detector ring.
4. Other X-ray observations of WR galaxies in the Conti catalogue (see text for details).
ROSAT PSPC. The extended sample of WR galaxies will be
described in more detail in Section 2, but it enables us to more
than double the number of X-ray detections of WR galaxies. This
new enlarged sample gives us a rather better view of the X-ray
properties of young starbursts, as exemplified by WR galaxies, and
enables us to paint a rather more complex picture than presented in
SS98 (though with the same underlying message – that the X-ray
emission is probably primarily due to hot gas produced by starburst
activity, such as a superbubble or superbubbles). In this paper we
shall also briefly discuss X-ray observations of related emissionline galaxies, though the current sample size is too small to be of
great use.
The paper is organized as follows. In Section 2 we describe the
enlarged WR galaxy sample and reprise the method of analysis; in
Section 3 we present results for those galaxies observed with the
ROSAT PSPC (both detections and non-detections). In Section 4 we
briefly discuss other relevant X-ray observations of WR galaxies in
the literature and also observations of the related emission-line
galaxy IRAS 0833+6517, in Section 5 we discuss the significance of
the results from the larger sample, and in Section 6 we summarize
our findings.
2
THE ENLARGED SAMPLE
The Conti (1991) catalogue listed 36 WR galaxies,1 of which 14 had
been observed with the ROSAT PSPC, and of these seven were
detected (SS98). Since then we have found an extra 40 WR galaxies
in the literature, and we have determined which of these have been
observed by the ROSAT PSPC.
Many of the additional WR galaxies come from Masegosa,
Moles & del Olmo (1991), who searched for WR emission features
in all galaxies in the Spectroscopic Catalogue of H II Galaxies
(SCHG; Terlevich et al. 1991). Out of around 500 galaxies,
Masegosa et al. (1991) found that around 10 per cent were WR
galaxies. There were a number of duplications of the Conti (1991)
catalogue, but this procedure identified 26 additional WR galaxies.
Further new WR galaxies are discussed in Drissen, Roy & Moffat
(1993), Contini, Davoust & Conside`re (1995), Steel et al. (1996),
Izotov et al. (1996), Thuan, Izotov & Lipovetsky (1996), Leitherer
et al. (1996), Ohyama, Taniguchi & Terlevich (1997), Heckman et
al. (1997), Izotov et al. (1997) and Martin (1998). Contini (1998)
has also provided two additional examples of WR galaxies and also
a larger number of emission-line galaxies with narrow He II l4686
emission. Note that, as in Conti (1991), we have not included nearby
spiral galaxies which have been observed to have WR stars in H II
regions (i.e. NGC 300, Breysacher et al. 1997 or the H II region
IC131 in M33, Long et al. 1996). However, we do include the spiral
1
Counting NGC 1741 and HCG 31A as a single galaxy. As discussed in
SS98, the ROSAT PSPC is unable to resolve these two interacting galaxies
and we count them as a single object.
q 1998 RAS, MNRAS 301, 215–230
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WR galaxies observed with ROSAT
EQ 0102–310
–
NGC 1365
SBbc
NGC 1569
I
Tol 0559–393
I
NGC 2366
I
I Zw 18
–
NGC 3353
Im
NGC 4449
I
Tol 1345–420
–
NGC 5408
I
X-ray survey of Wolf–Rayet galaxies – II
2
Note that in the original Conti catalogue I Zw 18 was classified as an
emission-line galaxy with narrow HeII l4686, but has since been reclassified
as a WR galaxy by Izotov et al. (1997).
q 1998 RAS, MNRAS 301, 215–230
Table 2. Some related parameters for the WR and emissionline galaxy sample. A discussion of these parameters can be
found in Section 2.
Galaxy
name
log LB
log LFIR
( erg s¹1 ) ( erg s¹1 )
NLyc wð HbÞ
˚)
(s¹1 ) (A
EQ 0102–310
NGC 1365
NGC 1569
Tol 0559–393
NGC 2366
I Zw 18
NGC 3353
NGC 4449
Tol 1345–420
NGC 5408
–
44.29
42.07
44.11
41.34
–
42.69
42.17
–
41.87
–
44.32
41.74
–
42.21
41.54
42.85
42.91
41.99
41.98
53.6
–
51.9
53.5
–
50.8
51.8
51.4
52.0
51.4
153
–
35
24
299
69
175
11
67
259
NGC 3125
NGC 6764
42.60
43.62
42.25
43.62
51.6
52.1
100
23
IRAS 0833+6517
44.26
44.18
54.4
–
structure. As in SS98 we assume that H0 ¼ 75 km s¹1 Mpc¹1 and
q0 ¼ 0:5 throughout the paper. We fit the integrated X-ray spectra
from the galaxy with a single-temperature Raymond–Smith model.
We constrain the X-ray absorbing column to be greater than the
Galactic absorbing column (NHStark –Stark et al. 1992). We estimate
errors for the fitted spectral parameters (kT, NH and Z) at the
68 per cent confidence level for one parameter of interest. The
quoted values for X-ray luminosity are for the 0:1¹2:5 keV waveband, and have been corrected for absorption.
We adopt the following procedure to estimate errors for the X-ray
luminosities for the detected galaxies. We assume that the dominant
source of error in LX is from errors in NH (which is probably a
reasonable assumption). From the fitting procedure we have 1j
bounds on NH . We freeze NH at both the upper and lower limits, then
fit again and determine LX at the upper and lower NH bounds. This
defines our upper and lower bounds for LX.
Some additional relevant parameters for the extended galaxy
sample are given in Table 2. The far infrared luminosity, LFIR , is
evaluated from the 60- and 100-mm IRAS fluxes, using the same
expression as in SS98. The values for NLyc and w( Hb) listed in
Table 2 are taken from a range of sources: I Zw 18, NGC 3125,
NGC 6764 and IRAS 0833+6517 – Conti (1991); Tol 0559–393,
Tol 1345–420 and NGC 5408 – Terlevich et al. (1991); NGC 1569
and NGC 4449 – Kennicutt (1992); EQ 0102–310 – Masegosa,
Moles & Campos-Aguilar (1994); NGC 2366 – Kennicutt, Balick
& Heckman (1980); NGC 3353 – Steel et al. (1996). We do
however caution that the values of NLyc and w( Hb) do not form
a completely homogeneous sample (for instance, data taken using
apertures of different sizes etc).
3
3.1
X - R AY O B S E RVAT I O N S : W R G A L A X I E S
NGC 1365
The SBbc spiral galaxy NGC 1365 has been classified as a Seyfert 2
(Turner, Urry & Mushotzky 1993) or Seyfert 1.8 (Maiolino & Rieke
1995). It is a member of the Fornax cluster, and we assume a
distance of 20 Mpc.
NGC 1365 contains a large number of giant H II regions (Alloin et
al. 1981), and, as discussed by Phillips & Conti (1992), one of these
(region L4), which is about 25 arcsec to the NE of the active
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galaxy NGC 1365 in our sample, for reasons set out in
Section 3.1.
We note that the detection of a broad He II l4686 feature can be
difficult even with high signal-to-noise ratio (S/N) data. This does
mean that our sample of WR galaxies will tend to be rather
heterogenous. Future studies will have to focus on broader samples
of starburst galaxies and starburst regions such as 30 Dor.
Of the additional 40 WR galaxies in the enlarged sample, a total
of 10 have been observed by the ROSAT PSPC, and their details are
given in Table 1. Seven were the intended targets of the observations
(NGC 1365, NGC 1569, NGC 2366, I Zw 18, NGC 3353, NGC 4449
and NGC 5408) and were detected (though the case is marginal for
NGC 2366), while the other three were observed serendipitously
and were not detected. A similar dichotomy was also found for the
WR galaxies in SS98.
WR galaxies are a subset of emission-line (EL) galaxies. Conti
(1991) listed a small number of examples of related EL galaxies,
namely those with narrow-line He II l4686 emission (as compared
to broad emission for WR galaxies) and those with no He II l4686
emission. The origin of narrow He II l4686 emission is still subject
to debate, and Motch, Pakull & Pietsch (1994) suggested that the Xray ionization is likely not responsible for the bulk of the narrow
He II l4686 emission. The presence of both narrow and broad He II
l4686 emission in some WR galaxies suggests that there may be a
connection between the phenomena, and makes it interesting to
compare the X-ray properties of these galaxies. The sample of EL
galaxies is small, with only a total of nine in both classes listed by
Conti (1991).2 Of these only one has been observed with the ROSAT
PSPC: IRAS 0833+6517 (an EL galaxy with no He II l4686). We
discuss the X-ray properties of IRAS 0833+6517 in Section 4.3.
Contini (1998) lists a further eight EL galaxies with narrow He II
l4686. Of these only one, Mrk 49, a blue compact dwarf galaxy
(also classified as Mrk 1318), has been observed. However, there
seems to be emission from a point source near to but not coincident
with Mrk 49 (the X-ray emission is centred about 45 arcsec away
from the centre of Mrk 49 – too large to be attributable to ROSAT
pointing errors). This X-ray source, which seems to be associated
with a faint optical source, is possibly a background quasar. This
contamination makes it difficult to estimate a flux or upper limit for
Mrk 49, and as a consequence we do not discuss this object further.
In order to extend the sample of WR galaxies further we also
briefly report on other observations of WR galaxies from the
literature (Section 4). This yields two extra detections –
NGC 3125 was observed with the Einstein satellite, but not with
the ROSAT PSPC (though we can use the ROSAT HRI to actually
determine the luminosity), and NGC 6764 was observed during the
All Sky Survey phase of the ROSAT mission.
The basic method of analysis of the ROSAT PSPC is the same as
in SS98 and the full details will not be repeated here. We crosscorrelate the positions of the X-ray sources in each image, as found
by the point source searching package PSS (Allan 1994), with stars
in the Guide Star Catalogue to fix the pointing of the X-ray
observation. We superimpose the X-ray image on optical images
from the STScI Digitized Sky Survey. For NGC 1365, NGC 1569,
NGC 5408 and NGC 3353 two separate ROSAT PSPC observations
are available. Where appropriate we have obtained spectra from
both observations separately to check for spectral variability.
However, we only use the longer observation to show the spatial
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I. R. Stevens and D. K. Strickland
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˚ , due to C III.
nucleus, shows a broad emission line at around 5696 A
According to Phillips & Conti (1992), the strength of this line and
˚ are indicative of the presence of between
the absence of CIV 5808 A
350 and 1400 WC9 WR stars. HST observations of NGC 1365
reveal numerous super-star clusters arranged in an elongated ring
around the nucleus (Kristen et al. 1997).
NGC 1365 was not included in the original WR galaxy catalogue
of Conti (1991), but was included in the subclass of WR barred
spiral galaxies by Contini et al. (1995). NGC 1365 is rather
different from many of the other WR galaxies in the sample
(which are often dwarf galaxies) and rather more similar to
nearby spirals such as M33 or NGC 300 which we have explicitly
excluded from the survey. However, we include NGC 1365 here
principally because of its interesting X-ray properties and the light
that they shed on the nature of NGC 1365. Also, a fuller discussion
of the multiwavelength properties of NGC 1365 will be presented in
Stevens, Forbes & Norris (1998).
NGC 1365 was detected with Einstein with an X-ray luminosity
of LX ¼ 8:2 × 1040 erg s¹1 (corrected for distance; Fabbiano, Kim
& Trinchieri 1992). NGC 1365 has been observed with the ROSAT
PSPC on two occasions, separated by 6 months. Turner et al. (1993),
reporting on the ROSAT observations, found a total of five point
sources near to a strong nuclear source, and found that the emission
from the nuclear regions was not well fitted by an absorbed power law,
but could be better fitted by a model with either a power law plus a
Raymond–Smith thermal component (with kT ¼ 0:6 keV ) or a
power law plus an emission line at an energy of 0.8 keV . It is worth
noting that the fit quality of the spectra for the other Seyfert galaxies
in Turner et al. (1993) was substantially better than for NGC 1365.
Iyomoto et al. (1997) reported on ASCA observations of
NGC 1365, finding evidence for a soft thermal component,
which dominates at low energies, a hard power-law component
and strong Fe K emission at around 6.6 keV. Iyomoto et al.
(1997) interpreted the power-law emission as being due to a lowluminosity AGN.
We have reanalysed both ROSAT PSPC observations, adjusted
the pointing using the BL Lac object 1E0331.3–3629 (a strong
point source about 12 arcmin off axis). The X-ray/optical image of
NGC 1365 is shown in the left panel of Fig. 1 with X-ray contours
superimposed on the Digitized Sky Survey image. The X-ray
contours are from the longer 6.2-ks observation only. As noted by
Turner et al. (1993), there are several point sources in the vicinity of
the nucleus of NGC 1365. In addition to a strong central point
source there is also low-surface-brightness extended emission along
the bar. Whether this is due to unresolved point sources or diffuse
emission is unclear.
It is impossible to resolve emission from the giant H II region L4
that contains the WR stars (or indeed the other H II regions close to
the nucleus). The X-ray-determined position of the central point
source is about 11 arcsec from the radio position of the nucleus
(Sandqvist, Jo¨rsa¨ter & Lindblad 1995) and does not seem to be
explicitly associated with any of the nuclear H II regions.
We collect photons from a radius of 1:2 arcmin to generate an Xray spectrum of the central region. In line with our analysis of the
spectral properties of other WR galaxies we simultaneously fit both
spectra of the nuclear region with a single-temperature Raymond–
Smith model. Interestingly, we find we get a very good fit (x2n , 0:7)
with such a model, with kT ¼ 0:7 keV, a low metallicity (0:06 Z( )
and a column NH ¼ 1:99 × 1020 cm¹2 , almost identical to the Stark
absorbing column (NHStark ¼ 1:92 × 1020 cm¹2 – Table 3). The
derived luminosity, corrected for absorption, is LX ¼
6:1 × 1040 erg s¹1 , with a range of 5:6¹6:7 × 1040 erg s¹ 1.
Figure 1. The X-ray emission from NGC 1365. The top panel shows the Xray emission from NGC 1365. The X-ray contours are from a backgroundsubtracted image with pixel size of 5 arcsec, smoothed with a Gaussian of
FWHM of 10 arcsec. Contour levels increase by a factor of 2 from 5:6 × 10¹3
count s¹1 arcmin¹2 . The bottom panels show the normalized spectra of
NGC 1365 (crosses) superimposed on the best-fitting single-temperature
Raymond–Smith spectra (solid line) for both observations. The longer
observation is in the upper panel. Details of the best fit are given in Table 3.
q 1998 RAS, MNRAS 301, 215–230
X-ray survey of Wolf–Rayet galaxies – II
219
It is notable that the X-ray properties of this active galaxy are
quite similar, in general terms, to those of WR galaxies (see
Section 5). Of particular note is the small local column density
implied by the spectral fits. This strongly suggests that in the
ROSAT band we are not seeing significant X-ray emission from a
buried Seyfert nucleus (which we would expect to be heavily
absorbed), but rather starburst emission from the H II regions
around the nucleus.
3.2 NGC 1569
3.3 NGC 2366/NGC 2363 [Mrk 71]
NGC 2366 is a Magellanic barred galaxy at an assumed distance of
3.44 Mpc (Tolstoy et al. 1995). NGC 2363 is a giant H II region lying
q 1998 RAS, MNRAS 301, 215–230
Figure 2. The X-ray emission from NGC 1569. The top panel shows the Xray emission from NGC 1569. The X-ray contours are from a backgroundsubtracted image with pixel size of 5 arcsec, smoothed with a Gaussian of
FWHM of 10 arcsec. Contour levels increase by a factor of 2 from 5:6 × 10¹3
count s¹1 arcmin¹2 . The bottom panel shows the normalized spectrum of
NGC 1569 (crosses) superimposed on the best-fitting single-temperature
Raymond–Smith spectrum (solid line). Details of the best fit are given in
Table 3.
at the southwestern end of NGC 2366. For the purposes of this study
we shall regard NGC 2366 as a single dwarf starburst galaxy
including the star-forming region NGC 2363, rather than treating
NGC 2363 as a distinct object.
The signature of WR stars in NGC 2363 was originally noted by
Drissen et al. (1993). Gonza´lez-Delgado et al. (1994) undertook a
detailed optical study of NGC 2363 and found broad emission lines
˚ , indicative of WC-type WR stars. Gonza´lezat 4660 and 5810 A
Delgado et al. (1994) also derived a total mass of 3:4 × 105 M( for
the stellar cluster in NGC 2363. Broad wings in several emission
lines, including Ha, extending over a large area, suggest a strong
outflow – probably the blow-out of a superbubble. Radio observations strongly suggest the presence of non-thermal emission in
NGC 2363, though observations with lower resolution suggest that
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NGC 1569 is a nearby Magellanic-type irregular dwarf galaxy in
the M81 group, with extremely blue optical colours. NGC 1569 is
host to several super-star clusters, and the two brightest are
designated NGC 1569-A and NGC 1569-B. WR features have
been observed in the NGC 1569-A cluster (Gonza´lez-Delgado et al.
1998a; Martin 1998) indicating a very young stellar population.
Further to this, De Marchi et al. (1997) have found that NGC 1569A is a double cluster, one component of which has a young
population including WR stars, while the second has an older
population.
NGC 1569 has been well studied in X-rays. Heckman et al.
(1995) presented optical and ROSAT HRI results for NGC 1569,
finding evidence of diffuse hot gas, which they interpreted as being
due to a superwind. They also showed that the extended X-ray
spurs, which they interpreted as being a superwind, were morphologically associated with Ha filaments. Della Ceca et al. (1996)
extended this study with a combined analysis of ROSAT and ASCA
observations, which also suggested the presence of a superwind. In
ROSAT HRI observations Della Ceca et al. (1996) found the central
starburst could be resolved into four clumps. The PSPC observations showed extended emission along the galaxy minor axis,
extending out to 1:9 arcmin (,1:2 kpc). In the ASCA observations
evidence for a harder point-like source was found, which they
suggested may be due to SN remnants or X-ray binaries in the
central starburst regions.
Because of the extensive study that NGC 1569 has received we
shall only present brief results on this galaxy, and look at the
integrated global X-ray properties, as seen with the ROSAT PSPC,
in order to compare and contrast with other WR galaxies. NGC 1569
has been observed twice with the ROSAT PSPC, for 3.1 and 4.7 ks.
We analyse only the longer observation, and adopt a distance of
2.2 Mpc (De Marchi et al. 1997).
An overlay of the X-ray contours on an optical image is shown in
Fig. 2, showing clearly extended X-ray emission centred on the
galaxy. We have collected photons from a region of radius 2 arcmin,
and spectrally fitted them with a single-temperature Raymond–
Smith model. The background-subtracted spectra and best-fitting
model are shown in the right panel of Fig. 2. The best-fitting X-ray
temperature is well constrained to be kT ¼ 0:4 keV. The fitted Xray absorbing column is rather high, with NH ¼ 4:4 × 1021 cm¹ 2,
and with quite a large range. The Galactic absorbing column is also
rather high (NHStark ¼ 2:8 × 1021 cm¹2 ), and the fitted column is not
significantly larger than this. The fitted metallicity is rather low
(Z ¼ 0:005Z( ) as in many of the other WR galaxies. We derive an
intrinsic X-ray luminosity of LX ¼ 3:0 × 1039 erg s¹1 , with a range
of 2:3¹9:0 × 1040 erg s¹1 .
220
I. R. Stevens and D. K. Strickland
the overall emission from NGC 2366 as a whole is dominated by
thermal emission (Yang, Skillman & Sramek 1994).
NGC 2366 has been observed with the ROSAT PSPC, as the
primary target of a 2.3-ks observation. An initial look at the X-ray
image revealed no substantial emission associated with NGC 2366,
however closer analysis suggests that we may have a possible
detection (with a PSS j ¼ 2:9). For the purposes of this paper we
shall assume that this is a detection, though it is rather uncertain.
An overlay of the X-ray contours on an optical image is shown in
Fig. 3, showing faint X-ray emission from the vicinity of the
starburst region NGC 2363. There is also a possible X-ray point
source to the W of NGC 2363, which may also be associated with
NGC 2366.
The source is too faint to obtain a spectral fit, and in order to
determine an estimate of the X-ray luminosity we assume a spectral
model with kT ¼ 0:5 keV, a metallicity of 0:1Z( and a column
equal to the Stark column (4:5 × 1020 cm¹2 ). Using this method we
derive an unabsorbed X-ray luminosity of 6:6 × 1037 erg s¹1 .
However, this may be an underestimate of the real X-ray luminosity
if the actual column is larger.
3.4 I Zw 18 [Mrk 166]
I Zw 18 is a blue compact dwarf galaxy with a broad He II l4686
line. Conti (1991) originally classified this galaxy as having only a
narrow He II l4686 line, but broad emission has now been observed
(Izotov et al. 1997; Legrand et al. 1997). I Zw 18 is notable for being
one of the most metal-poor galaxies known, with a metallicity of
about 1=30Z( . A detailed study of the Ha spatial distribution was
performed by Petrosian et al. (1997), and a detailed multiwavelength study by Martin (1996), who briefly discussed results from
the ROSAT observations described below. Martin (1996) finds
evidence for a superbubble shell, expanding at a velocity of
35¹60 km s¹1 , and suggests that this bubble will eventually burst
out of the galaxy. I Zw 18 was not detected with IRAS (Petrosian et
al. 1997).
Figure 4. The X-ray emission from I Zw 18. The top panel shows the X-ray
emission from I Zw 18. The X-ray contours are from a backgroundsubtracted image with pixel size of 5 arcsec, smoothed with a Gaussian of
FWHM of 10 arcsec. Contour levels increase by a factor 2 from 5:6 × 10¹3
count s¹1 arcmin¹2 . The bottom panel shows the normalized spectrum of
I Zw 18 (crosses) superimposed on the best-fitting single-temperature
Raymond–Smith spectrum (solid line). Details of the best fit are given in
Table 3.
I Zw 18 was the target of a 16.8-ks ROSAT observation. It was
detected and an overlay of the X-ray contours on an optical image is
shown in Fig. 4, showing a point-like source centred on the optical
galaxy. The background-subtracted spectrum was fitted with a
single-temperature Raymond–Smith model. One peculiarity concerns the fitted metallicity. For other WR galaxies, where it has been
possible to obtain a fit, the value of the fitted metallicity has alway
been much lower than solar, typically #0:1Z(. When allowed to
float freely, the fitted X-ray metallicity for I Zw 18 is larger than
solar. With the value of the metallicity constrained to be less than or
equal to solar we find a fit with kT ¼ 1:6 keV, substantially higher
than that obtained for most other WR galaxies, and solar abundances. The fitted column is constrained to be $NHStark , and in the
best fit we find that the value of NH is equal to NHStark . For this case we
q 1998 RAS, MNRAS 301, 215–230
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Figure 3. The X-ray emission from NGC 2366. The X-ray contours are from
a background-subtracted image with pixel size of 5 arcsec, smoothed with a
Gaussian of FWHM of 20 arcsec. Contour levels increase by a factor of 2
from 2:8 × 10¹3 count s¹1 arcmin¹2 .
X-ray survey of Wolf–Rayet galaxies – II
221
Table 3. The derived X-ray spectral properties of the WR galaxies observed by ROSAT. The X-ray spectra are all fitted with a
single-temperature Raymond–Smith model with an absorbing column NH . For sources that are not detected we estimate upper
limits assuming a Raymond–Smith spectral model with kT ¼ 0:5 keV, a metallicity of 0:1Z( and a column equal to the Stark
value NHStark .
LX
X-ray luminosity
( ergs¹1 )
(3)
kT
(keV)
NH
(1020 cm¹2 )
Metallicity
(Z( )
x2 (d.o.f)
(4)
(5)
(6)
(7)
1.92
28.55
4.52
2.47
1.27
0.67
7.50
6:12 × 1040
2:96 × 1039
6:61 × 1037
1:25 × 1039
3:21 × 1039
4:85 × 1039
3:08 × 1040
0:70þ0:05
¹0:05
0:41þ0:28
¹0:13
0.5
1:60þ0:14
¹0:19
0:67þ0:07
¹0:06
1:81þ4:93
¹1:81
0:52þ0:2
¹0:02
1:99þ0:36
¹0:07
43:9þ22:4
¹17:5
4.52
2:47þ0:90
¹0:00
2:66þ0:30
¹0:41
13:15þ33:81
¹3:67
7:50þ0:86
¹0:00
0:06þ0:02
¹0:03
0:005þ0:02
¹0:005
0.1
þ0:0
1:0¹0:09
0:02þ0:01
¹0:01
0:0þ0:14
¹0:0
0:005þ0:002
¹0:002
26.7 (39)
12.0 (16)
–
10.2 (15)
27.6 (17)
4.8 (12)
37.0 (40)
Non-detections: WR galaxies
EQ 0102–310
Tol 0559–393
Tol 1345–420
1.93
4.77
7.27
< 4:4 × 1040
< 3:7 × 1040
< 1:0 × 1039
0.5
0.5
0.5
1.93
4.77
7.27
0.1
0.1
0.1
–
–
–
Detections: other WR galaxies
NGC 3125
NGC 6764
8.19
8.08
1:79 × 1039
2:57 × 1040
0.5
0.5
8.19
8.08
0.1
0.1
–
–
Detections: related EL galaxies
IRAS 0833+6517
4.08
2:81 × 1041
0:58þ0:56
¹0:08
14:0þ48:3
¹11:4
0:02þ0:12
¹0:02
3.11 (5)
(1)
Detections: WR galaxies
NGC 1365
NGC 1569
NGC 2366
I Zw 18
NGC 4449
NGC 3353
NGC 5408
Notes on Table 3:
Column 2: The Stark column density (Stark et al. 1992). This value provides the lower limit for the fitted column.
Column 3: The galaxy X-ray luminosity, corrected for absorption, in the 0:1¹2:5 keV waveband. For those galaxies that were not
detected, the upper limits on the X-ray luminosity (68 per cent confidence level) are quoted.
Columns 4–7: The best-fitting parameters, assuming a one-temperature Raymond–Smith model, with metallicity fitted. The fitted
column NH is constrained to be larger than NHStark .
derive an intrinsic X-ray luminosity of LX ¼ 1:3 × 1039 erg s¹1 ,
with a range of 1:3¹1:6 × 1039 erg s¹1 . The values quoted in Table 3
are for this fit. If we fix the metallicity to be 0:1Z( we obtain a fit
with kT ¼ 5:5 keV. The fitted column and intrinsic luminosity are
otherwise quite similar to the previous fit.
Given the extremely low ‘optical’ metallicity of I Zw 18 this is
puzzling. One possible explanation is that we are using too simple
a model to fit the X-ray data. For instance, if in addition to an Xray-emitting superbubble there is also a contribution from X-ray
binaries then this could result in a spurious value for the
metallicity. This could also explain the higher fitted temperature
for I Zw 18. Better X-ray spectra are clearly needed to resolve this
puzzle.
3.5 NGC 4449
NGC 4449 is a nearby Magellanic-type irregular galaxy which has
undergone a recent burst of star formation. The galaxy has giant H II
regions and extended Ha filaments as well as SN remnants. Hill et
al. (1994) presented a study of the star formation in NGC 4449 and a
review of the observational properties can be found in Della Ceca,
Griffiths & Heckman (1997). IRAS data for NGC 4449 are from
Melisse & Israel (1994).
Vogler & Pietsch (1997) presented results from ROSAT PSPC and
HRI observations of NGC 4449, finding a total of seven point
sources within the galaxy. They associated the brightest of these
with a SN remnant, with a luminosity of ,5 × 1038 erg s¹1 . Vogler
& Pietsch (1997) also found a diffuse extended component to the
q 1998 RAS, MNRAS 301, 215–230
emission, with a temperature of 0:25 keV. Della Ceca et al. (1997)
expanded this analysis with ASCA data, and found that the X-ray
spectrum is rather complex, with perhaps three components.
As NGC 4449 is a well-studied system, and given that we are
interested in the global properties rather than a detailed study, we
shall present only a brief discussion of the PSPC data. In line with
our analyses of other WR galaxies we extract a single integrated
spectrum from the whole galaxy and do not try to analyse the X-ray
point sources individually.
The ROSAT PSPC observed NGC 4449 for 7.8 ks. We extract a
spectrum from a region within a radius of 2:4 arcmin of the
galaxy centre, and this includes all of the point sources in the
galaxy. We then fit this spectrum with a single-temperature
Raymond–Smith model. An overlay of the X-ray contours from
the PSPC observation on a Digitized Sky Survey image is shown
in Fig. 5, and shows the complex X-ray morphology and the
extended emission.
The background-subtracted spectra and best-fitting model are
also shown in Fig. 5. The best-fitting X-ray temperature is
kT ¼ 0:67 keV and we find a metallicity of 0:02Z( . The fitted
column is NH ¼ 2:7 × 1020 cm¹2 , significantly larger than the Stark
value. We derive an intrinsic X-ray luminosity of LX ¼ 3:2 × 1039
erg s¹1 , with an estimated range of 1:9¹3:4 × 1039 erg s¹1 . We note
that the best fit is not of great quality, with x2n ¼ 1:6. This is perhaps
not too surprising given that we know that we are including several
point sources in our integrated spectrum, and that the ASCA
results suggest that different regions have different spectral
characteristics.
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NHStark
Stark column
(1020 cm¹2 )
(2)
Galaxy
Name
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I. R. Stevens and D. K. Strickland
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Figure 5. X-ray emission from NGC 4449. The top panel shows the X-ray
emission contours superimposed on an optical image. The X-ray contours
are from a background-subtracted image with pixel size of 5 arcsec,
smoothed with a Gaussian of FWHM of 10 arcsec. Contour levels increase
by a factor of 2 from 5:6 × 10¹3 counts s¹1 arcmin¹2 . The bottom panel
shows the normalized spectrum of NGC 4449 (crosses) superimposed on the
best fit (solid line). Details of the best fit are given in Table 3.
3.6 NGC 3353 [Haro 3, Mrk 35]
NGC 3353 is a dwarf starburst galaxy at an assumed distance of
D ¼ 13:1 Mpc, of morphological type Im, and a bright infrared
source, with LFIR ¼ 1:7 × 109 L( (Steel et al. 1996).
NGC 3353 has been the subject of a recent optical study by Steel
et al. (1996), who found that NGC 3353 was dominated by several
star-forming regions, in particular a nuclear emission region and a
brighter, off-centre, very blue emission-line region. Steel et al.
(1996) found a large population of WR stars in this second region. A
˚ was found, as well as
broad WR emission feature around 4650 A
˚ , possibly indicative of WC
evidence of a feature around 5870 A
stars. Steel et al. (1996) concluded, both from the presence of WR
stars and from the stellar continuum, that the star formation was
Figure 6. X-ray emission from NGC 3353. The top panel shows the X-ray
emission contours superimposed on an optical image. The X-ray contours
are from a background-subtracted image with pixel size of 5 arcsec,
smoothed with a Gaussian of FWHM of 20 arcsec. Contour levels increase
by a factor of 2 from 2:8 × 10¹3 count s¹1 arcmin¹2 . The bottom panels
show the normalized spectra of NGC 3353 (crosses) superimposed on the
best fit (solid line) for both observations. The longer (4.4-ks) observation is
shown in the upper panel. Details of the best fit are given in Table 3.
q 1998 RAS, MNRAS 301, 215–230
X-ray survey of Wolf–Rayet galaxies – II
223
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recent, with an age of #5 Myr. As noted by Steel et al. (1996),
NGC 3353 was observed with Einstein, with an X-ray luminosity of
LX ¼ 3:5 × 1039 erg s¹1 (corrected for distance, Fabbiano et al.
1992). Fanelli, O’Connell & Thuan (1988) and Kinney et al. (1993)
have described UVobservations of NGC 3353, finding a continuum
dominated by emission from massive stars. ISO observations,
reported on by Metcalfe et al. (1996), found IR emission associated
with the main star-forming regions, and with an extension similar to
that seen in Ha.
There have been two on-axis PSPC observations of NGC 3353,
the first for 4.4 ks, and the second with 2.5 ks, separated by 18
months. We have generated background-subtracted images for both
data sets independently, and extracted source spectra for each
observation. We show the X-ray contours for the longer exposure
superimposed on an optical sky survey image (Fig. 6). We have
been unable to improve the pointing accuracy by identifying optical
counterparts to other X-ray sources in the NGC 3353 field, and
consequently the pointing in Fig. 6 may be slightly in error. The Xray contours are somewhat elongated in a north–south direction and
the peak of the X-ray emission is slightly offset from the optical
centre, and close to the star-forming knot region D in the observations of Steel et al. (1996). Using the pointing of the shorter second
observation results in the X-ray emission being closer to, but still
slightly displaced from, the optical centre. It is clear that we are
seeing emission from NGC 3353, but higher spatial resolution will
be needed to resolve the source of the X-ray emission.
We have simultaneously fitted the spectra for both observations
with a single-temperature Raymond–Smith model, and the results
for both observations are shown in the right panels of Fig. 6. Neither
of the spectra has many counts and so we are unable to obtain a wellconstrained fit. The best-fitting spectrum has kT ¼ 1:8 keV, a very
low metallicity, and an absorbing column of NH ¼ 1:32×
1021 cm¹2 . We note that while the fitted temperature is larger than
for most other WR galaxies it is poorly constrained. The X-ray
luminosity (corrected for absorption) is LX ¼ 4:9 × 1039 erg s¹1 ,
with a range of 4:2¹13:0 × 1039 erg s¹1 .
3.7 NGC 5408 [Tol 1400-41]
NGC 5408 is a low-luminosity dwarf irregular galaxy, with several
giant H II regions, and is a site of very active star formation.
NGC 5408 was classified as a WR galaxy by Masegosa et al.
(1991), although this assertion has been contested (R. Terlevich,
private communication). NGC 5408 has been studied in the optical
by Bohuski et al. (1972), and shows three main nuclear H II regions.
The IRAS data are from Melisse & Israel (1994), and we adopt a
distance of 8 Mpc.
NGC 5408 has been observed by both the Einstein and ROSAT
satellites (Stewart et al. 1982; Fabian & Ward 1993). Fabian & Ward
(1993) found a fit for the X-ray spectra of NGC 5408 with a
Raymond–Smith model with kT , 0:5 keV, a column of
7 × 1020 cm¹2 and a low metallicity (0:02Z( ). The X-ray luminosity was determined to be 2 × 1040 erg s¹1 . Fabian & Ward (1993)
found no evidence of any significant extension in the X-ray
emission.
Two ROSAT PSPC observations of NGC 5408 have been made
(5.2 and 2.9 ks, separated by 5 months). Fabian & Ward (1993)
primarily analysed the longer observation. As an improvement, we
have analysed both observations, and simultaneously fitted both
spectra. Our results are broadly consistent with those of Fabian &
Ward (1993), with no significant variability between the two
observations.
q 1998 RAS, MNRAS 301, 215–230
Figure 7. X-ray emission from NGC 5408. The top panel shows the X-ray
emission contours superimposed on an optical image. The X-ray contours
are from a background-subtracted image with pixel size of 5 arcsec,
smoothed with a Gaussian of FWHM of 10 arcsec. Contour levels increase
by a factor of 2 from 5:6 × 10¹3 count s¹1 arcmin¹2 . The bright star SE of
NGC 5408 is HD 122532 (A0p, mB ¼ 6:0). This star is not detected. To the
SW the bright star is CD¹40 8364 (G5, mB ¼ 10:0). This star is also not
detected. The bottom panels show the normalized spectra of NGC 5408
(crosses) superimposed on the best fit (solid line) for both observations. The
longer (5.2-ks) observation is in the upper panel. Details of the best fit are
given in Table 3.
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I. R. Stevens and D. K. Strickland
The X-ray morphology of NGC 5408 is shown in Fig. 7, showing
that the X-ray emission is broadly centred on the main nuclear H II
regions. The fitted X-ray spectra for both observations are also
shown in Fig. 7. The best-fitting spectrum has kT ¼ 0:52 keV and a
column of NH ¼ 7:5 × 1020 cm¹2 , constrained to be equal to the
Stark value. The fitted metallicity is low, with Z ¼ 0:005Z( . The Xray luminosity (corrected for absorption) is LX ¼ 3:1 × 1040 erg s¹1 ,
with a range of 3:1¹3:4 × 1040 erg s¹1 .
We note that Motch et al. (1994) have reported on ROSAT HRI
observations of NGC 5408. These observations show that the X-ray
emission is not centred on the main H II regions, but is offset by
about 10 arcsec. Further study is required to identify the source of
the X-ray emission in NGC 5408, and there remains the possibility,
as noted by Motch et al. (1994), that the X-ray source is unrelated to
the galaxy.
3.8 Non-detections
4 O T H E R X - R AY O B S E RVAT I O N S O F W R
G A L A X I E S A N D R E L AT E D E L G A L A X I E S
4.1 NGC 6764
NGC 6764 is an SBb spiral galaxy classified as a LINER but also
showing WR emission features in the nucleus. It was classified as a
WR galaxy by Conti (1991). Eckart et al. (1996) have presented a
multiwavelength study of NGC 6764, including a detection from
the ROSAT All Sky Survey. The total observation time was short
(1.4 ks), with a count rate of 0:01 6 0:003 count s¹1 , insufficient for
a reliable spectrum.
In order to obtain an estimate of the X-ray luminosity we have
assumed a model with kT ¼ 0:5 keV Z ¼ 0:1Z( , a purely Galactic
4.2
NGC 3125 [Tol 3, Tol 1004-296]
NGC 3125 is a nearby (D ¼ 12 Mpc) blue compact dwarf, included
in the Conti (1991) catalogue of WR galaxies, and was reported as
being detected with the Einstein IPC by Fabbiano, Feigelson &
Zamorani (1982). However, Fabbiano et al. (1992) suggest that
NGC 3125 was not detected and merely report an upper limit for the
X-ray flux.
If we assume that NGC 3125 was detected with the flux level
reported by Fabbiano et al. (1982), we derive a luminosity (corrected for absorption) LX ¼ 5:7 × 1039 erg s¹1 in the 0.1–2.5 keV
waveband (assuming a spectral model with kT ¼ keV, Z ¼ 0:1Z(
and a Galactic column of 8 × 1020 cm¹2 ).
NGC 3125 was observed for 38 ks with the ROSAT HRI, and we
can use this observation to estimate determine the X-ray luminosity.
The HRI has basically no spectral response, but we can use the
measured count rate, along with an assumption about the spectral
shape (as above), to determine the flux and luminosity. We determine a ROSAT HRI count rate of 1:86 × 10¹3 count s¹1 , which for
the same spectral model as assumed for the Einstein observations
yields an X-ray luminosity (corrected for absorption) of
LX ¼ 1:79 × 1039 erg s¹1 . Clearly if there is substantial local
absorption the true X-ray luminosity of NGC 3125 could be
higher than this, but this value is what we shall assume here.
4.3
IRAS 0833+6517
In the original WR galaxy catalogue Conti (1991) provided examples of related emission-line galaxies, namely those without substantial He II l4686 emission, and those with narrow He II l4686
emission. We have searched the ROSAT data base for PSPC
observations of these galaxies, but as noted in Section 2 there is
only one detection.
IRAS 0833+6517 is a starburst galaxy that shows no He II l4686
emission, and was first noted by Margon et al. (1988), who
commented that the spectral properties of IRAS 0833+6517 were
similar to those of NGC 7714, a WR galaxy, the X-ray properties of
which were reported on in SS98. We assume a distance of 76 Mpc.
Margon et al. (1988) reported on Einstein IPC X-ray observations
of IRAS 0833+6517 and Rephaeli, Gruber & Persic (1995) reported
on HEAO-1 observations. The Einstein IPC luminosity was
reported as LX ¼ 1:0 × 1041 erg s¹1 (Margon et al. 1988). This
value however was for the 0.5–3.0 keV bandpass and assuming a
power-law spectral model with a ¼ 0:5 with an absorbing column
equal to the Stark value.
Gonza´lez-Delgado et al. (1998b) reported on Hopkins Ultraviolet
Telescope and HST observations of IRAS 0833+6517. Spectral
modelling indicates a young starburst, with an age of 6¹7 Myr
(assuming an instantaneous burst of star formation) or 9 Myr
(assuming constant star formation). Evidence of a large-scale outflow associated with the starburst was also found.
These spectral analysis results would suggest that
IRAS 0833+6517 has recently passed through the WR galaxy
phase. Given this relationship to WR galaxies, and that
IRAS 0833+6517 is an interesting object in its own right, and that
ROSAT results on this object have not been discussed in the
literature, we include a brief discussion of this object.
q 1998 RAS, MNRAS 301, 215–230
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Three WR galaxies in the extended sample were observed but not
detected with ROSAT. We estimate X-ray luminosity upper limits
for these systems assuming a model with purely galactic absorption
and a single-temperature Raymond–Smith model with
kT ¼ 0:5 keV and Z ¼ 0:1Z( . The derived X-ray luminosity
upper limits are at the 68 per cent confidence level. Note that if
the actual level of absorption is substantially higher than Galactic
the derived upper limit should be higher.
EQ 0102–310: Classified as a WR galaxy by Masegosa et al.
(1991). It is a comparatively distant galaxy (z ¼ 0:032, Masegosa et
al. 1994). This galaxy was observed serendipitously during a 13.9ks observation of QSO 0101–304. EQ 0102–310 was 34 arcmin
off-axis and was not detected. We derive an X-ray luminosity upper
limit (corrected for absorption) of LX # 4:4 × 1040 erg s¹1 .
Tol 0559–393: A distant galaxy (z ¼ 0:083) in the SCHG
(Terlevich et al. 1991). LFIR in Table 2 is based on only data at
60 mm, the source not being detected at 100 mm. Classified as a WR
galaxy by Masegosa et al. (1991), Tol 0559–393 was observed as
part of a 12-ks observation of the cluster SC 0559–40. Tol 0559–
393 is approximately 40 arcmin off-axis, and was not detected. We
derive an X-ray flux upper limit of LX # 3:7 × 1040 erg s¹1 .
Tol 1345¹420: Classified as a WR galaxy by Masegosa et al.
(1991), with further observations reported on in Masegosa et al.
(1994) and Campbell, Terlevich & Melnick (1986). Tol 1345–420
is an H II galaxy, at a distance of 32 Mpc (Terlevich et al. 1991).
Tol 1345–420 was observed by ROSAT during an 4.7-ks observation of the B2Vnpe star HD 120324 and is 15 arcmin off axis and
close to the detector ring. The source was not detected and we derive
an upper limit of LX # 1:0 × 1039 erg s¹1 .
column (6 × 1020 cm¹2 ) and a distance of D ¼ 32 Mpc. Using this
model to match the observed count rate we obtain an X-ray
luminosity (corrected for absorption) of LX ¼ 2:57 × 1040 erg s¹1 .
X-ray survey of Wolf–Rayet galaxies – II
IRAS 0833+6517 was observed serendipitously with the ROSAT
PSPC during a 5-ks observation of the G1.5Vb star HD 72905.
IRAS 0833+6517 is only 8 arcmin off-axis and well within the inner
detector ring. The source was clearly detected and the X-ray
contours and spectra are shown in Fig. 8, showing strong
point-like emission centred on the galaxy. Spectral fitting yields
an X-ray temperature of kT ¼ 0:58 keV, with Z ¼ 0:02Z( and a
column of 1:40 × 1021 cm¹2 , implying substantial local absorption. The X-ray luminosity (corrected for absorption) is
LX ¼ 2:8 × 1041 erg s¹1 , with a range of 1:1¹12:3 × 1041 erg s¹1 .
The spectral properties are similar to those for NGC 7714, except
that IRAS 0833+6517 is about an order of magnitude more
luminous than NGC 7714.
The ROSAT fluxes are similar to that observed with Einstein
when account is made for different assumed spectral shapes.
q 1998 RAS, MNRAS 301, 215–230
Figure 9. Top panel: the relationship between LX and LB for WR galaxies.
The filled squares are WR galaxies from this paper, open squares are WR
galaxies from SS98, and arrows are WR galaxy upper limits. The solid
triangle is IRAS 0833+6517. The long-dashed line is the LX :LB trend for
nearby spiral galaxies found by Read et al. (1997), the thick solid line is the
trend including all WR galaxy detections from this paper and SS98 and the
dot–dashed line is the correlation excluding the four overluminous dwarf
WR galaxies (see text for details). Bottom panel: the relationship between
LX and far-infrared luminosity LFIR for WR galaxies (with the same symbols
as in the left panel). The long-dashed line is for the Read et al. (1997) sample,
and the thick solid line for all WR galaxy detections.
5
5.1
DISCUSSION
Main trends and correlations
In SS98 we discussed the X-ray properties for the original X-ray
sample of WR galaxies, and now we have a rather larger sample
which expands our view of these objects.
Of the six new WR galaxies where we were able to determine the
X-ray spectral properties, four had fitted temperatures in the range
0:4¹0:8 keV (for single-temperature Raymond–Smith models),
very similar to the findings of SS98. However, two (NGC 3353
and I Zw 18) have higher values of kT, though in the case of
NGC 3353 this is rather uncertain. We noted in SS98 that the more
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Figure 8. The X-ray emission from IRAS 0833+6517. The top panel shows
the X-ray emission from IRAS 0833+6517. The X-ray contours are from a
background-subtracted image with pixel size of 5 arcsec, smoothed with a
Gaussian of FWHM of 10 arcsec. Contour levels increase by a factor of 2
from 5:6 × 10¹3 count s¹1 arcmin¹2 . The bottom panel shows the normalized spectrum of IRAS 0833+6517 (crosses) superimposed on the bestfitting single-temperature Raymond–Smith spectrum (solid line). Details of
the best fit are given in Table 3.
225
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I. R. Stevens and D. K. Strickland
log LX ¼ ð0:79 6 0:19Þ log LB þ ð6:03 6 7:99Þ ;
ð1Þ
where LX and LB are both in erg s¹1 . For this case PCOX ¼ 0:1 per
cent, implying a strong correlation between LX and LB . The
exponent in this relationship is rather smaller than that found in
SS98. However, the relationship is being strongly flattened by four
detections of dwarf WR galaxies at lower LB but with a higher LX
(NGC 5408, NGC 1569, I Zw 18 and NGC 3125). If we exclude
these four galaxies we find a relationship with an exponent of
1:45 6 0:16, very similar to that found in SS98 (with
PCOX ¼ 0:01 per cent).
These results raise the question as to what correlations there are
for dwarf galaxies. Looking at the 11 dwarf galaxies in the
combined WR galaxy sample in isolation from the non-dwarfs we
find an exponent of 0:46 6 0:35 for the LX :LB correlation, but with a
large value of PCOX ¼ 39 per cent. This implies that the X-ray
luminosities of the dwarf galaxies in the sample are much less
correlated with LB than more normal galaxies.
Two potential explanations of this lack of a trend for dwarf WR
galaxies come to mind. First, it could be that it could be that the
origin of the X-ray emission is largely divorced from the causes of
the optical luminosity. An example of such a model would be one
where the X-ray emission from a dwarf galaxy is dominated by an
individual source, such as an X-ray luminous SN. The SN would not
greatly contribute to LB , and hence there would be no correlation
between LX and LB . The long-term X-ray light curve of such a
model would show the galaxy having a rather low quiescent X-ray
luminosity, but with occasional large, short-lived excursions to
large LX as individual SN events occur. This idea will be discussed
further in Section 5.4. A second idea would be that the lack of a
correlation is due to the complex star-formation history of an
individual galaxy. In SS98 we showed how a short-lived burst of
massive star formation could give rise to the LX :LB trend seen in that
paper. If we were to assume that star formation was occurring over a
longer period (see Section 5.2), with, for instance, several star
clusters being formed at different epochs, this would lead to a more
complex relationship between LX and LB . This idea will be
discussed in a separate paper.
In Fig. 9 we also show the relationship between LX and LFIR for
WR galaxies. Again many of the new WR galaxies follow a similar
trend to that in SS98. NGC 5408 is again an exception, lying
substantially above the general trend.
Using all of the detections from this paper and SS98 we find the
following correlation:
log LX ¼ ð0:75 6 0:15Þ log LFIR þ ð7:79 6 6:40Þ ;
ð2Þ
¹1
. The exponent in this
where LX and LFIR are both in erg s
relationship is very similar to that found in SS98. In this case
PCOX ¼ 0:08 per cent implying a very strong correlation. Looking
only at dwarf galaxy detections, we find an exponent of
0:71 6 0:40, but with PCOX ¼ 65 per cent, implying less evidence
of a correlation between LX and LFIR for dwarf galaxies.
The results for NGC 1365 are interesting. Given that NGC 1365
is classified as an active galaxy it is perhaps surprising that its X-ray
spectral properties are rather similar to the other WR galaxies, in
spectral terms. On the LX :LB plot, NGC 1365 lies close to but below
the WR trend line, and rather closer to the trend for nearby spirals in
Read, Ponman & Strickland (1997).
Contini et al. (1995) found that all barred spiral galaxies with WR
features in their sample (seven in total) were highly inclined (with
i , 50¹608; for NGC 1365, i , 608), and suggested that starburst
winds may play a role in removing absorbing material perpendicular to the galaxy disc, making the WR feature more likely to be
observed. The X-ray spectra from the nuclear regions of NGC 1365
show very little local absorption.
In contrast to NGC 1365, the other Seyfert 2 galaxies discussed
by Turner et al. (1993) have rather different luminosity properties.
In particular, these other Seyfert 2 galaxies are typically an order of
magnitude more X-ray luminous. We suggest that the X-ray
emission from NGC 1365 is not primarily from an active nucleus,
and is dominated by starburst-type emission from the H II regions in
the vicinity of the nucleus. We shall discuss the multiwavelength
properties of NGC 1365 in more detail in Stevens et al. (1998).
The results concerning IRAS 0833+6517 are also of note. This
galaxy, while of pertinence to WR galaxies, is also a very interesting
object in its own right. The X-ray spectral properties of
IRAS 0833+6517 are very similar to those of WR galaxies. It is
however, very X-ray luminous, but lies very close to the LX :LB trend
for WR galaxies.
5.2
Age/X-ray luminosity relationship for starbursts
Contini et al. (1997) have recently discussed a possible relationship
between the age and the equivalent width of CO emission for barred
spiral starbursts. This poses the question as to whether there is any
discernible relationship between age and X-ray properties for WR
galaxies. This is an important area, as WR galaxies are likely to be
important tracers of starburst activity due to the presence of massive
short-lived stars.
q 1998 RAS, MNRAS 301, 215–230
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luminous WR galaxies typically had larger fitted X-ray temperatures. These two galaxies do not fit this trend as their values of kT
are far too high. The reason for the high temperature is unclear –
perhaps it is because the spectra are of poor quality, or perhaps we
are seeing emission from two sources, diffuse emission from hot gas
combined with emission from X-ray binaries. Only higher quality
spectral and spatial information will resolve this.
One of the main findings of SS98 was a clear relationship
between LX and LB for WR galaxies, and that WR galaxies were
substantially X-ray overluminous compared with a sample of
nearby galaxies. A correlation between LX and LFIR was also
found for WR galaxies, but in this case there no real difference
between WR galaxies and the nearby galaxy sample.
In Fig. 9 we show the relationship between LX and LB for WR
galaxies, including those discussed in SS98 as well as in this paper.
It is clear that many of the new WR galaxies follow the same trend
found in SS98, and are substantially X-ray overluminous for their
blue luminosity. Compared with SS98 we now have rather more
dwarf WR galaxies. There are, however, some notable discrepancies in Fig. 9. In particular, NGC 5408 is very X-ray overluminous.
NGC 5408 seems to be a rather peculiar object, and we will discuss
it further in Section 5.2. While not as extreme as NGC 5408,
NGC 1569, I Zw 18 and NGC 3125 lie above the general trend
found for WR galaxies. While for NGC 3353 there are quite large
errors on the derived X-ray luminosity, in the cases of NGC 1569
and NGC 5408 the errors are smaller. NGC 1569 could also be a
rather special case of WR galaxy. Most of the other WR galaxies do
not show strong signs of superwind activity as revealed by extended
X-ray emission, while NGC 1569 does. NGC 1569 also seems to be
a rather more advanced starburst, where the starbursting activity has
been going on for longer.
Using all of the detections (but not upper limits) from this paper
and SS98, we find the following correlation (using the ASURV
package – see SS98 and Isobe, Feigelson & Nelson 1986):
X-ray survey of Wolf–Rayet galaxies – II
To explore any possible relationship we plot the dependence of
the ratios LX =LB and LX =LFIR against the Hb equivalent width wðHb.
As discussed by Leitherer & Heckman (1995) and Schaerer &
Vacca (1998), wðHbÞ is age sensitive, declining sharply as the
starburst ages (for an instantaneous starburst). wðHbÞ is also
metallicity-dependent, but the decline of wðHbÞ with age is the
dominant effect. The ratios LX =LB and LX =LFIR should provide a
normalized measure of X-ray activity compared with the stellar
properties of the galaxies.
In the left panel of Fig. 10 we plot the relationship between LX =LB
and wðHbÞ. NGC 5408 has a very large wðHbÞ as well as a large
LX =LB ratio. There are no easily discernible trends in this diagram,
except perhaps large scatter at large wðHbÞ (i.e. very young
starbursts), and a narrowing at smaller wðHbÞ, as the starburst
ages. It is worth noting that the prototypical starburst M82, which
has no Hb emission (and is presumed to be in a more advanced stage
of starburst), has a LX =LB ¼ ¹2:76, comparable to many of the WR
galaxies.
q 1998 RAS, MNRAS 301, 215–230
In the right panel of Fig. 10 we plot the relationship between the
ratio LX =LFIR and wðHbÞ. Again, no significant trend is found,
though there does seem to be a wider scatter for younger starbursts
[larger wðHbÞ], and a narrowing as wðHbÞ deceases. For M82,
LX =LFIR ¼ ¹3:47, again similar to that for many of the WR galaxies.
We have in effect been looking for trends assuming that the starformation epoch was of very short duration. However, while that
may be true in some cases, in other cases there is a more complex
history of star formation. An example is NGC 1569, where star
formation was ongoing for a substantial time, and probably ceased
about 4 Myr ago (Vallenari & Bomans 1996). The effect of a
complex star-formation history on trends such as in Figs 9 and 10
will require further study.
As noted earlier, NGC 5408 lies significantly above the general
trend of the other WR galaxies in Fig. 9, and is remarkably X-ray
luminous, with LX =LB , 0:04. This galaxy also has a very large Hb
equivalent width, suggesting that it is a very young starburst. There
remains the possibility that the X-ray source is unrelated to
NGC 5408, and could be a background QSO. However, the fitted
spectra are typical of starburst galaxies (and fitting with an absorbed
power-law model does not give as good a fit). The probability of a
bright, unrelated, X-ray source being so close to NGC 5408 is small
(using the log N vs log S curve of Gioia et al. 1990, we estimate a
probability of 3 × 10¹4 that an unrelated source with a flux equal to
or greater than that observed for NGC 5408 occurs within a radius
of 2 arcmin around NGC 5408).
We note that very luminous non-nuclear X-ray sources have been
seen in a number of galaxies. For example, Marston et al. (1995)
have presented ROSAT results for M51, finding a total of eight
bright, persistent, sources in the disc, with luminosities in the range
LX ¼ 5¹29 × 1038 erg s¹1 . Marston et al. (1995) suggest several
explanations for these sources: stellar mass black holes in binaries,
young supernova remnants and starburst-type emission. In
Section 5.4 we will discuss the possibility of supernovae powering
the X-ray emission from WR galaxies. Marston et al. (1995) largely
rejected the SNR model on the grounds of lack of observed X-ray
variability. The black hole model is perhaps viable, though in the
case of M51 the observed sources are more luminous than any
comparable source in our Galaxy (and the source in NGC 5408 is an
order of magnitude more luminous than the brightest source in
M51). Several of the sources in M51 are co-located with bright H II
regions, and it is possible that the X-ray sources are due to starburst
activity. This is the model we prefer for our WR galaxies but it does
not explain the anomalously high X-ray luminosity of NGC 5408,
and this object merits further attention.
5.3
Metallicities
Many of the WR galaxies in our sample are dwarf galaxies (11 out
of 16 detected). In addition to providing information about the
starburst phenomena, WR galaxies could also be very informative
about dwarf galaxy evolution.
Spaans & Norman (1997) have presented a model describing the
evolution of dwarf galaxies, which explains the large population of
starbursting dwarfs at a redshift z , 0:5. This paper makes an
interesting prediction concerning the metallicities of dwarf
galaxies, namely that dwarf galaxies should be quiescent with
metallicities of Z < 0:01Z( or Z > 0:1Z( (depending on whether
they have had a major episode of star formation in the past or have
yet to experience their first major starburst phase), or be actively
forming stars with 0:01Z( < Z < 0:1Z( .
Downloaded from http://mnras.oxfordjournals.org/ by guest on October 6, 2014
Figure 10. The relationship between Hb equivalent width and the ratios
LX =LB and LX =LFIR for WR galaxies. For a given metallicity and an
instantaneous starburst, wðHbÞ decreases with time. The closed squares
are WR galaxies from this paper, open squares are WR galaxies from SS98,
arrows are WR galaxy upper limits.
227
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I. R. Stevens and D. K. Strickland
5.4 Can X-ray luminous SNe power the X-ray emission?
In SS98 we discussed possible origins of the X-ray emission from
WR galaxies and concluded that emission from superbubbles
provided the most plausible explanation. One alternative deserving
of more discussion is that a young X-ray luminous SN or a compact
SNR (cSNR – Terlevich 1994) could be responsible for the bulk of
the X-ray emission from some WR galaxies. This could also explain
the lack of correlation between LX and LB for dwarf WR galaxies
(Section 5.1). X-ray luminous SN are believed to be type II supernova events occurring in a dense circumstellar medium (CSM),
probably generated by the wind of the supernova progenitor during
its last phases of evolution (Schlegel 1995). The interaction of the
SN shocks (either forward or reverse shock) with the dense CSM
leads to enhanced X-ray emission. Several examples have been
observed with LX , 1038 ¹1041 erg s¹1 (SN1978K in NGC 1313,
Schlegel, Petre & Colbert 1996; SN1986J in NGC 891, Bregman &
Pildis 1992; SN 1993J in NGC 3031, Suzuki & Nomoto 1995;
SN1988Z in MCG +03-28-022, Fabian & Terlevich 1996). Consequently, an individual X-ray luminous SN could be as luminous as a
typical WR galaxy. Also, the fitted X-ray spectral temperatures for
the SN are comparable to those of WR galaxies, in the range
0:5¹1:0 keV (Schlegel 1995). Further, Vogler & Pietsch (1997)
suggest that the brightest X-ray source in NGC 4449 may in fact be
an X-ray luminous SNR, with a luminosity of ,5 × 1038 erg s¹1 .
Blair, Kischner & Winkler (1983) estimate the age of this SNR to be
,100 yr. Vogler & Pietsch (1997) note that the X-ray source may be
extended and it is not entirely clear whether all the emission from
this source is due to the SNR or whether there are contributions
from nearby sources.
There are a few arguments that can be put forward against the
suggestion that individual young SNe can account for all the X-ray
emission from WR galaxies.
(i) In order to be X-ray luminous the supernova must have
occurred recently (all of the X-ray detected supernovae, with the
exception of the one in NGC 4449, were observed at ages t # 20 yr).
Only a few SNe have been observed in X-ray detected WR galaxies:
NGC 1365 – SN1957C (unknown type) and SN1983V (type Ic);
NGC 5253 – SN1895B (type Ia) and SN1972E (type Ia);
NGC 1614 – SN1996D (type Ic). The observed SNe were mostly
not co-located with the observed X-ray emission. The exception is
SN1996D in NGC 1614. However, this supernova occurred after the
X-ray observations were taken.
(ii) All of the X-ray detected X-ray luminous SN have been
observed as bright radio sources (SN1978K – Ryder et al. 1993;
SN1986J – Rupen et al. 1987; SN1988Z – van Dyk et al. 1993;
SN 1993J – van Dyk et al. 1994). Radio observations of WR
galaxies, where available, tend to show diffuse thermal emission,
and do not show bright point-like non-thermal emission indicative
of SNR (there are some exceptions, such as NGC 2363). It is
possible that if the magnetic field strength is substantially lower in
dwarf galaxies, due to the lack of a galactic dynamo, then we might
not expect to see young SNe as bright radio sources in dwarf
galaxies (although the SN in NGC 4449 has been detected at radio
wavelengths). Clearly, there is a need for better radio maps of many
of the WR galaxies.
(iii) Regarding variability, several WR galaxies have been seen
at different epochs (with Einstein as well as ROSAT) – for a single
X-ray luminous SN we would expect to see substantial variability
between the observations (Chevalier & Fransson 1994). The timescale between ROSAT observations for NGC 1365, NGC 3353 and
NGC 5408 is between 5 and 18 months, and typically ,10¹15 yr
between Einstein and ROSAT observations. As is apparent in Figs 1,
6 and 7, no substantial variability has occurred between the ROSAT
observations, and when account is made of different assumed
spectral shapes there is no evidence of any substantial variability
between the Einstein and ROSAT observations.
(iv) Regarding spatial distribution, with the ROSAT PSPC many
WR galaxies are spatially unresolved, and the emission could be
point-like. Studies with the ROSAT HRI have shown much more
complex or multiple structures in several WR galaxies (i.e.
NGC 1569, Heckman et al. 1995; NGC 5253, Strickland & Stevens
1998b). In these cases the X-ray emission is clearly not dominated
by a single SN.
In summary, we suspect that in our WR galaxies we are not
seeing emission from a single X-ray luminous SN. More detailed
radio maps of many WR galaxies are needed to fully exclude this
possibility, as well as to estimate the number of older SN remnants.
It is possible that in some cases, particularly dwarf WR galaxies, a
single X-ray luminous SN could dominate the X-ray emission, and
could be a partial explanation for the lack of a trend between LX and
LB noted in Section 5.1. However, as a mechanism young SNe
probably cannot account for the X-ray emission from the whole
class of galaxy. High spatial resolution studies with AXAF will help
to determine the fraction of X-ray emission from WR galaxies that
comes from diffuse emission and from point sources.
6
SUM M A RY AND C O NC L USI O N S
In this paper we have significantly expanded the sample of WR
galaxies observed at X-ray energies and continued our investigations of the X-ray properties of WR galaxies, This paper confirms,
in many respects, the trends found in SS98; that the X-ray
luminosity of WR galaxies is well correlated with LB and that
WR galaxies are significantly X-ray overluminous compared with a
nearby sample of galaxies. No significant trends have been found
between X-ray indicators of activity (LX =LB or LX =LFIR ) and the
q 1998 RAS, MNRAS 301, 215–230
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Because all of the dwarf galaxies in our sample are currently
starbursting, we might expect them to have metallicities in the range
0:01Z( < Z < 0:1Z( . However, the observational picture is rather
unclear. Of seven dwarf galaxies with fitted metallicities, only one
has Z > 0:1Z( (NGC 5253; SS98). However, the large uncertainties
on the fitted metallicities mean that for all seven objects the fitted
metallicity is consistent with the range above.
In addition, we note the uncertainties caused by using X-ray
determinations of metallicities. The X-ray-emitting material,
potentially coming from massive star winds and supernovae, may
not be representative of the galaxy as a whole. However, simulations show that much of the X-ray emitting material comes from
shocked gas swept up by the starburst, rather than starburst material
itself (Suchkov et al. 1994). A second and more complex problem
concerns the reliability of X-ray-determined metallicities themselves. Simulations by Strickland & Stevens (1998a) illustrate the
problems that occur when trying to fit an intrinsically multitemperature medium with a single-temperature model, and how the
fitting procedure can substantially underestimate the true metallicity.
While metallicity may be a good indicator of dwarf galaxy
evolution, there are major shortcomings in trying to do this via Xray spectroscopy. Such problems can only be resolved with more
detailed modelling and higher quality data.
X-ray survey of Wolf–Rayet galaxies – II
q 1998 RAS, MNRAS 301, 215–230
In summary, we have greatly expanded the sample of WR
galaxies in our survey. We find that as a class of objects their
integrated X-ray properties include thermal spectra with typically
kT , 0:4¹1:0 keV and an X-ray luminosity well correlated with
LB . As a class of X-ray objects, WR galaxies are X-ray overluminous. However, there are indications that dwarf WR galaxies
have rather distinct luminosity characteristics.
AC K N O W L E D G M E N T S
IRS and DKS acknowledge PPARC funding. The data analysis
presented in this paper made use of the Starlink node at the
University of Birmingham, and the Asterix and Dipso packages.
This research has also made use of data obtained from the Leicester
Database and Archive Service at the Department of Physics and
Astronomy, Leicester University, UK.
The Digitized Sky Surveys were produced at the Space Telescope
Science Institute under US Government grant NAG W-2166. The
images of these surveys are based on photographic data obtained
using the Oschin Schmidt Telescope on Palomar Mountain and the
UK Schmidt Telescope.
We thank the referee, Roberto Terlevich, for helpful comments.
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