Mon. Not. R. Astron. Soc. 294, 523–547 (1998)

A ROSAT survey of Wolf–Rayet

Ian R. Stevensଙ and David K. Stricklandଙ

School of Physics and Space Research, University of Birmingham, Edgbaston, Birmingham, B15 2TT

Accepted 1997 July 22. Received 1997 June 30; in original form 1996 August 12 Downloaded from https://academic.oup.com/mnras/article/294/4/523/1025955 by guest on 26 September 2021 ABSTRACT We present results from a ROSAT Position Sensitive Proportional Counter (PSPC) survey of the X-ray emission from Wolf–Rayet (WR) galaxies, a class of galaxies believed to be young starbursts (with ages of t#4–6 Myr), many of which are blue compact dwarf galaxies. Of the 36 WR galaxies listed in the catalogue of Conti, a total of 14 have been observed deliberately or serendipitously with the ROSAT PSPC, and of these, seven have been detected. The derived X-ray luminosities of WR galaxies range over nearly three orders of # Å 38 21 Å 41 21 magnitude, from LX 4 10 s to 2 10 erg s . The X-ray spectra of the WR galaxies can typically be well-fitted with a single temperature Raymond–Smith spectral model, with a temperature in the range kT:0.3–1.0 keV, with the general trend that the more X-ray-luminous WR galaxies have hotter spectra. WR galaxies are significantly X-ray-overluminous for their blue luminosity, compared with a sample of nearby spiral and starburst galaxies. In addition, the X-ray luminosity of

WR galaxies correlates well with the far-infrared luminosity LFIR and the number of

Lyman continuum NLyc. No strong correlation was found with the equivalent width of the WR emission feature around l4686 Å, the presence of which essentially defines the class of galaxies. There is little evidence of extended X-ray emission. Various explanations for the observed properties of WR galaxies are explored, and we conclude that the X-ray emission provides strong evidence that a large fraction of the observed X-rays are coming from a hot superbubble formed by the combined action of stellar winds from massive early-type in the central starburst cluster. These results are consistent with, and add weight to, the view that WR galaxies are young starbursts, in which the duration of the -forming epoch was very short, and that we are viewing them a few Myr after the initiation of the starburst. As such, WR galaxies represent an important epoch in the evolution of starburst galaxies.

Key words: stars: Wolf–Rayet – ISM: jets and outflows – galaxies: starburst – galaxies: stellar content – X-rays: galaxies.

galaxies with a bright nucleus that is bluer than expected for 1 INTRODUCTION its morphological type, which emits strong narrow emission Wolf–Rayet (WR) galaxies are a subset of emission-line (or lines similar to low ionization H II regions as a consequence H II) galaxies, and are defined as ‘those galaxies in whose of photoionization by the radiation from hot 40 42 21 integrated spectra a broad emission feature at He II l4686, stars, with a typical Ha luminosity of 10 –10 erg s ’ attributed to WR stars, has been detected’ (Conti 1991). On (Gonz´alez-Delgado et al. 1995). These definitions of star- the other hand, starburst galaxies can be defined as ‘spiral burst and WR galaxies should be considered as general criteria rather than as hard rules. For instance, local galaxies ଙ E-mail: [email protected] (IRS); such as M33 or M101 contain giant H II regions where WR [email protected] (DKS) stars have been detected in the integrated spectra, but we

© 1998 RAS 524 I. R. Stevens and D. K. Strickland shall follow Conti (1991) and not include such galaxies in example Conti 1991 and Vacca & Conti 1992) and studies of our sample. In addition, while some of our sample galaxies individual objects (see comments on individual galaxies in (NGC 5253 and Mrk 33) appear to have to have some of the Sections 4 and 5). The purpose of this paper is to present an characteristics of (dwarf) elliptical galaxies, they are clearly X-ray survey of a sample of WR galaxies, using the ROSAT undergoing starburst activity. X-ray Telescope (XRT) and PSPC, which are well suited to Conti (1991) produced the first catalogue of WR galaxies, the study of WR galaxies. The spatial resolution of this and we shall use this sample as our baseline for this X-ray instrument is very good compared with previous instru- study, as it also provides a collation of other relevant data. A ments (the 90 per cent enclosed radius at 1 keV for the few additional WR galaxies have been discovered since the PSPC is 27 arcsec), and the modest spectral resolution publication of the Conti catalogue (for example, Masegosa, allows better spectral fitting than was possible with the Ein- Moles & del Olmo 1991, Contini, Davoust & Consid`ere stein Imaging Proportional Counter. 1995 and Thuan, Izotov & Lipovetsky 1996), but we shall For nearby starbursts, ROSAT has been able to resolve not discuss these objects here. extended X-ray emission around galaxies, which is indica- In this paper we shall report on an X-ray study of a tive of a superwind or galactic-scale outflow driven by the Downloaded from https://academic.oup.com/mnras/article/294/4/523/1025955 by guest on 26 September 2021 sample of WR galaxies, using observations made with the starburst (for example M82, Strickland, Ponman & Stevens ROSAT Position Sensitive Proportional Counter (PSPC). 1997; NGC 253, Read, Ponman & Strickland 1997; NGC X-ray studies of galaxies have revealed a wealth of informa- 2146, Armus et al. 1995; NGC 3628, Fabbiano, Heckman & tion about energetic phenomena such as X-ray binaries, the Keel 1990). The WR galaxies in this sample are typically hot phase of the (ISM), superbubbles further away than these galaxies (cf. Table 1), and this, and galactic-scale winds. As there is a general consensus coupled with the fact that WR galaxies are likely younger that the phenomena of starburst and WR galaxies are starbursts, means that we do not expect to see such closely related, and that a WR is probably a starburst extended emission around the sample galaxies. galaxy observed at an early stage in the evolution of the In this study of WR galaxies we shall make extensive starburst, it seems sensible to undertake an X-ray study of comparisons with the ROSAT XRT survey of Read et al. WR galaxies and to compare their X-ray properties with (1997), which studied 17 nearby spiral galaxies, including both normal ‘quiescent’ galaxies and starbursts. In physical some starbursts. The Read et al. (1997) sample provides us terms, the defining characteristics for a WR galaxy is for a with a reasonably extensive sample of galaxies, analysed in a large number of WR stars to be present (or at least an similar manner, with which to compare the X-ray properties unusually large proportion of WR stars compared with the of WR galaxies. number of O stars). WR stars are believed to be the descen- There are several interrelated goals to this work. the first dants of the most massive stars (with initial masses is to provide an overview of the X-ray emission properties of P40 Mᖿ), and their lifetimes as WR stars are typically less WR galaxies as a test of the hypothesis that they are young than 106 yr, although this is dependent on . As starbursts. A second goal is to explore the X-ray evolution discussed by Conti (1991), WR galaxies form a rather of starbursts. A third is to compare the optical and X-ray heterogenous sample, ranging from isolated galaxies morphology of WR galaxies, and a fourth to study the through interacting/merging galaxies to IR-luminous emis- growth of superbubbles and superwinds in starbursts. sion-line galaxies. In some systems the WR stars are found Consequently, we do not attempt a detailed analysis of each in a star-forming nucleus, while in others there is a single observation, but attempt to provide an overview of the X- giant H II region. WR galaxies form a subset of H II galaxies, ray emission from this class of galaxy. Some results from and are often blue compact dwarf galaxies (BCDGs). Heck- ROSAT observations of individual galaxies in this sample man et al. (1995), reporting on observations of the star- have already been published (for example, NGC 5253, Mar- bursting H II galaxy NGC 1569, discuss the importance of tin & Kennicutt 1995; NGC 4861, Motch, Pakull & Pietsch BCDGs in the context of galaxy evolution and the X-ray 1994; Fourniol, Pakull & Motch 1996). background (see also the discussion in Fabian & Ward 1993 The paper is organized as follows. In Section 2 we discuss on ROSAT observations of NGC 5408). the selection of galaxies in this ROSAT sample, as well as the Vacca & Conti (1992) concluded from their study of 10 other relevant parameters for the galaxies. In Section 3 we WR galaxies that the observed stellar characteristics of describe the method of analysis for the ROSAT data. In these objects can only be reconciled with the constraints of Section 4, for those galaxies actually detected with ROSAT, stellar evolution if the massive star content was formed in a we present the results of the X-ray observations along with burst of star formation of less than 106 yr duration and about a general description of galaxy characteristics. In Section 5 a few 106 yr ago (see also Arnault, Kunth & Schild 1989 and we briefly discuss those galaxies not detected and derive Schaerer 1995). Therefore, WR galaxies are amongst the X-ray flux upper limits. In Section 6 we discuss the X-ray youngest examples of the starburst phenomenon, observed properties of WR galaxies and correlations with the proper- at a propitious moment when the most massive stars are ties at other . In Section 7 we discuss the origin evolving from O stars to WR stars. This fact, that WR of the X-ray emission, with a particular emphasis on X-ray galaxies may well be a coeval sample of young starbursts, emission from superbubbles, and in Section 8 we discuss makes them a particularly important class of objects to and summarize our results. study, and may yield considerable insight into the temporal evolution of starbursts, superbubbles and the development 2 THE WR GALAXY SAMPLE of superwinds. In recent years, there have been several studies of WR We have selected our X-ray sample of WR galaxies in the galaxies, both treating them as a class of objects (for following manner. First we use as our baseline the WR

© 1998 RAS, MNRAS 294, 523–547 X-ray survey of Wolf–Rayet galaxies 525

Table 1. The X-ray catalogue of Wolf–Rayet galaxies observed by the ROSAT satellite. Downloaded from https://academic.oup.com/mnras/article/294/4/523/1025955 by guest on 26 September 2021

Notes to Table 1. Column 1: the names used throughout this paper are those used in Conti (1991). 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 2 and 4: galaxy position from Conti (1991). Note that in Table 1 of Conti (1991) the position of Mrk 178 is given wrongly. Column 5. galaxy distance in Mpc. the data are primarily from Conti (1991) except NGC 5253, where we use data from Saha et al. (1995, see text for details). Column 6: exposure time of the ROSAT observation. Column 7: quality code of the ROSAT observation. 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 of the inner detector ring.

galaxy catalogue of Conti (1991), consisting of a total of 36 Number of WR galaxies in Conti (1991) 36 galaxies.1 For these galaxies we then searched the ROSAT Number observed by ROSAT in pointed mode 14 data base at Leicester University to find those galaxies that Number with WR galaxy as ROSAT target 7 have been observed with the ROSAT PSPC. In several cases All seven galaxies that were the intended target of the the WR galaxies were the intended targets, while in others ROSAT observations have been detected. In contrast, none the galaxy serendipitously happened to be in the field of of the seven galaxies that were serendipitously observed view. When the galaxy is not the intended target there are were detected, and we have derived X-ray flux upper limits two potential problems; the first is that if the intended target for these galaxies. Details of the final sample of 14 WR is (for instance) a bright star the length of observation may galaxies observed with ROSAT are shown in Table 1, along be insufficient to adequately detect the galaxy and secondly with their positions (in 2000 coordinates), distance in Mpc when the WR galaxy is some distance off-axis (for example, and some details of the ROSAT observations. outside the inner detector ring), the point-spread function In Fig. 1 we show a histogram of the blue magnitudes of the PSPC is much larger and again the galaxy is less likely (m ) of all the WR galaxies in the Conti (1991) catalogue, to be detected. B and identify those that have been observed and detected by The statistics of our WR galaxy sample as oberved by ROSAT. As we shall show in Section 6, there is a strong ROSAT are as follows. correlation between the blue luminosity (LB) of WR

1 galaxies and their X-ray luminosity. As a consequence, mB In table 1 of Conti (1991), 37 galaxies are listed. However, two should be a reasonable predictor of the observed X-ray flux galaxies in Conti’s catalogue, NGC 1741 and HCG 31A, are both part of (HCG) number 31, have been from the galaxy. It is clear from Fig. 1 that there is a clear observed in the same ROSAT field and are not spatially resolved in distinction between the distribution of those WR galaxies X-rays. They appear to be in the process of colliding or merging that were detected and those that were not. The only excep- : and we shall treat them as effectively one galaxy under the title tion to this is Mrk 178, with mB 13.9, which was not NGC 1741. detected. This can be contrasted with NGC 1614, with with

© 1998 RAS, MNRAS 294, 523–547 526 I. R. Stevens and D. K. Strickland

: 21 21 constant of H0 75 km s Mpc . In the subsequent sec- tions we make use of the following parameters.

IRAS fluxes: S12, S25, S60 and S100 We have collected the fluxes in each of the four IRAS wave- bands for our sample. Out of the 14 WR galaxies in the sample, nine have detections in sufficient wavebands to be useful. Most of the data are available via SIMBAD, and are supplemented by data from Rego et al. (1993) for Mrk 309. We use the IRAS fluxes to estimate the far-infrared lumino-

sity LFIR of the WR galaxy (see below), and in an IRAS colour–colour diagram as a discriminator for the dust emis- sion from the galaxy (see Fig. 12 later). Downloaded from https://academic.oup.com/mnras/article/294/4/523/1025955 by guest on 26 September 2021

Far-infrared luminosity: L Figure 1. A histogram of the blue magnitudes mB of all WR FIR galaxies in the Conti (1991) catalogue (solid line). The subset of We estimate the far-infrared luminosities of the galaxies WR galaxies that have been detected with ROSAT are marked with from the IRAS measurements, according to the expression diagonal hatching, while the subset that have been observed but not detected are marked with horizontal hatching. from Devereux & Eales (1989): : Å 5 2 ǹ LFIR(Lᖿ) 3.65 10 D (2.58S60 S100), (1) where D is the distance in Mpc, S and S are the IRAS 60- m :14.0, which was detected. There are two likely causes 60 100 B and 100-lm fluxes in Jy and L (or in particular the ratio for this. First, the L :L correlation for WR galaxies is FIR X B L /L ) is an indication of the strength of recent star- steeper than linear (Section 6.1) so that intrinsically higher FIR B forming activity. luminosity galaxies (such as NGC 1614) are more likely to be detected than fainter galaxies (such as Mrk 178) even if Blue luminosity: L they have the same mB, and secondly, the length of the B observation of Mrk 178 was only 2.9 ks, compared with 10 ks Conti (1991) lists the apparent (mB) and absolute (MB) blue for NGC 1614. magnitudes for the whole sample of WR galaxies, and we For the catalogue of Conti (1991)‚ and from other refer- convert this into a blue luminosity. The WR galaxies span a ences listed below, we have compiled a set of relevant para- large range in LB, from very faint blue galaxies, such as Mrk meters for our sample, which we can then correlate with the # Å 41 21 178, Mrk 750 and Pox 186, with LB 2–4 10 erg s , up X-ray properties of WR galaxies. These parameters are to very luminous galaxies, such as NGC 1614, NGC 1741 shown in Table 2 and described below. Where available, we # Å 44 21 and Mrk 309, with LB 0.8–2 10 erg s . use the type classification from the SIMBAD data base. Note that in the case of NGC 5253 we have used a different luminosity: LSN distance and mB from those listed in Conti (1991), and have scaled all the relevant parameters accordingly. Note that, in The supernova luminosity is a measure of the power accordance with Conti (1991), we have assumed a Hubble injected into the galaxy by supernovae (SN). As WR

Table 2. Other related parameters for the Wolf–Rayet galaxy sample. See Section 2 for a detailed discussion of the parameters in this table.

© 1998 RAS, MNRAS 294, 523–547 X-ray survey of Wolf–Rayet galaxies 527 galaxies are starbursts, we use the prescription of Devereux will therefore be a measure of the relative number of hot b & Eales (1989) for LSN that: stars in the galaxy. Large w(H ) means that the hot star population makes up a sizeable portion of the galaxy and L :0.0153[L 20.076L ]S, (2) SN FIR B vice versa. The data for w(Hb) are taken from Conti where S is the appropriate scaled rate of SN from van den (1991). Bergh & Tammann (1991). See Read & Ponman (1995) for a discussion of the rationale for using this expression. We Number of Lyman continuum photons: N have in effect assumed that all of the WR galaxies are Lyc b starburst galaxies. If we adopt the criteria that if LFIR / The H line flux can be used to estimate the number of  LB 0.38 then the galaxy is a starburst (Read et al. 1997), Lyman continuum photons. As discussd by Conti (1991), then only NGC 4861 is not a starburst. It is also worth noting under the assumptions of pure recombination for the for- that the X-ray properties of NGC 4861 are somewhat dif- mation of the lines, that the temperature : 4 : 4 23 ferent from the other WR galaxies (see Section 4.5). is Te 10 K and the electron density is ne 10 cm , and that the emission line comes from an optically thick region, Downloaded from https://academic.oup.com/mnras/article/294/4/523/1025955 by guest on 26 September 2021 then the number of Lyman continuum photons can be esti- Einstein X-ray luminosity: LEIN X mated as Four galaxies in our sample (NGC 1614, NGC 4861, NGC N :2.06Å1012L(Hb). (3) 5253 and NGC 7714) were observed with the Einstein X-ray Lyc observatory. As a cross-check on our analysis we have taken NLyc is an estimate of the radiation emitted below the EIN values of the Einstein X-ray luminosity, LX , from the cata- Lyman limit and is a quantitative estimate of the number of logue of Fabbiano, Kim & Trinchieri (1992, corrected for ionizing stars in the galaxy. The largest contribution to NLyc EIN distance where appropriate). Note that these values of LX is from the earliest O type stars, with smaller contributions were calculated assuming a temperature of kT:5 keV, from later O stars and B supergiants. In WR galaxies, WR which is a poor approximation to the spectral temperature stars will also make a significant contribution to NLyc. The of WR galaxies (Section 4). values of NLyc used here are from Conti (1991).

WR emission-feature equivalent width: w(WRE) 3 METHOD OF ANALYSIS Conti (1991) did not tabulate values of the equivalent width of the WR emission feature at l#4650 Å because of the The ROSAT PSPC data on all of the WR galaxies in the inconsistent means of measurement. The WR emission X-ray sample were obtained from the LEDAS data base at feature potentially consists of the contribution from several Leicester University, UK, and have been reduced in a uni- different lines, most notably He I l4686, but also N III l4640 form manner using the ASTERIX software package. The and C III/IV l4650. Several papers list measurements of the majority of the galaxies (nine out of 14) lie within the inner equivalent width of the entire emission feature (the WR detector ring of the ROSAT PSPC (cf. Table 1). The data ‘bump’), while some just measure the equivalent width of were initially cleaned to remove periods of high background the He II l4686 line. We have attempted to obtain values and then binned into a background-subtracted spectral that are measurements of the whole WR emission feature in cube. The point-source detection program PSS (Allan 1994) the range 4600–4700 Å. The data used are taken from the was used to search for each WR galaxy and to estimate the following sources. detection significance (s), and also to locate any other X-ray Hen2-10, Mrk 33, Mrk 750, II Zw 62, POX 186, Mrk 309: sources near the galaxy (which we have attempted to Kunth & Joubert (1985) identify). The contours from the galaxy X-ray emission were NGC 1741: Kunth & Schild (1986) superimposed on an optical image of the field (size 7.5 Minkowski’s object, NGC 7714: van Breugel et al. (1985) arcmin by 7.5 arcmin, taken from the ‘Digitized Sky Sur- IRAS 0100322238: Armus, Heckman & Miley (1988) vey’). The X-ray images have been smoothed before con- NGC 4861: Dinnerstein & Shields (1986) touring in order to reduce noise. The images are initially For NGC 1614, Mrk 178 and NGC 5253, no suitable formed in 5-arcsec pixels and are then smoothed with a sources for the equivalent width of the fature were found in Gaussian of full width at half-maximum (FWHM) of typi- the literature. We do not pretend that this procedure gives cally 10 arcsec (20 arcsec in some cases). In cases where the us a fully consistent determination of the equivalent width WR galaxy was not detected, we have used the PSS package of this feature, but we include it as a potentially interesting to estimate X-ray flux upper limits. If we assume a Ray- parameter for WR galaxies. The flux in this line (or lines) mond–Smith spectral model for the non-detected galaxy, gives an indication of the number of WR stars, and the with kT:0.5 keV and an absorbing column equal to the equivalent width gives an indication of how dominant this Stark value (Stark et al. 1992) then we can estimate an contribution is in terms of the galaxy as a whole. upper limit for the X-ray luminosity of the galaxy. Clearly if the true absorbing column for the galaxy is much larger than the Stark value then we may well be underestimating the Hb equivalent width: w(Hb) true upper luminosity limit. To account for this, we have The Hb flux is largely derived from massive ionizing stars, performed regression fits using just the detections as well as and the flux in the Hb line is a measure of the total number some including the upper limits (Section 6). In all cases the of ionizing stars (see discussion on the number of Lyman X-ray luminosity upper limits are at the 68 per cent confi- b continuum photons NLyc below). The equivalent width of H dence level.

© 1998 RAS, MNRAS 294, 523–547 528 I. R. Stevens and D. K. Strickland

Table 3. The derived X-ray spectral properties of the Wolf–Rayet galaxies observed by ROSAT. Downloaded from https://academic.oup.com/mnras/article/294/4/523/1025955 by guest on 26 September 2021

Notes to Table 3. Column 2: this is the Stark column density. Column 3: the PSS detection significance. Column 4: the quoted X-ray luminosities have been corrected for absorption. For those galaxies that were

detected we have estimated the errors in LX according to the procedure set out in Section 3. For those galaxies that were not detected, the upper limits on the X-ray luminosity (68 per cent confidence level) are quoted. Column 5: the fitted X-ray temperature.

Column 6: the fitted X-ray absorbing column. Note that the value of NH in the X-ray fits is constrained to be

larger than the Galactic NH column (column 1). This constraint was applied only in the cases of NGC 1614 and NGC 1741. Column 7: the fitted metallicity in terms of solar metallicity. In the cae of Mrk 33 it was necessary to fix the metallicity to obtain a fit.

For the detected WR galaxies we have binned-up source less than 0.5 dex (except in the case of Mrk 33, where large spectra. The spectra have been taken from a circular region errors in NH result in large errors in LX). of typical radius 1.2 arcmin around the centre of the galaxy, Results from the analysis of the X-ray observations of and have then been fitted with a single-temperature Ray- WR galaxies, both detections and upper limits, are given in mond–Smith plasma model, allowing for an absorbing col- Table 3. In Section 4 we discuss the ROSAT results for those umn and allowing the metallicity to vary. The absorbing WR galaxies that were detected, and in Section 5 we briefly column for the spectral fit is constrained to be larger than discuss those that were not. the Stark column in the direction towards the galaxy. The errors for the fitted spectral parameters (kT, NH and Z) in Table 3 are at the 68 per cent confidence level for one 4 RESULTS: X-RAY DETECTIONS parameter of interest. The usual correlation trend between 4.1 NGC 1614 (Mrk 617, II Zw 15, Arp 186) kT and NH is that an overestimate in NH tends to result in an underestimate of kT and vice versa. This is illustrated in the NGC 1614 is believed to be a merging galaxy undergoing a ROSAT results for NGC 5253 in Martin & Kennicutt (1995, starburst. The galaxy is observed to have a bright optical their fig. 5). Note that in some cases we were forced to fix centre, with two roughly symmetrical inner spiral arms (Neff certain of the spectral model parameters in order to obtain et al. 1990, Armus, Heckman & Miley 1990). The outer a fit. It is rather more problematic to estimate errors for the regions of the galaxy are more disturbed, having a large X-ray luminosities for the detected galaxies. To do this we curved extension to the south-east of the nucleus and a have assumed that the dominant source of error in LX is linear tail to the south-west of the nucleus. These features from errors in NH (which is probably a reasonable assump- can just be seen in the optical image presented in Fig. 2. s tion). From the fitting procedure, we have 1 bounds on NH. NGC 1614 is classified as SBbp and is also a very powerful : Å 11 We freeze NH at both the upper and lower limits, then fit IR galaxy, with LFIR 1.8 10 Lᖿ, assuming a distance of again and determine LX at the upper and lower NH bounds. 64 Mpc (Conti 1991). A broad WR emission feature was

This defines our upper and lower bounds for LX. The esti- first reported in Vacca & Conti (1992). NGC 1614 has also mates of the error bounds for LX are quoted in Table 3. We been studied in the IR by Keto et al. (1992) and Puxley & note that errors in LX estimated by this method are typically Brand (1994).

© 1998 RAS, MNRAS 294, 523–547 X-ray survey of Wolf–Rayet galaxies 529 Downloaded from https://academic.oup.com/mnras/article/294/4/523/1025955 by guest on 26 September 2021

Figure 2. X-ray emission from NGC 1614. The top panel shows the X-ray emission contours superimposed on an optical image. The X-ray contours are from an 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 2.8Å1023 count s21 arcmin22. The lower panel shows the normalized spectrum of NGC 1614 (crosses) superimposed on the best fit (solid line). Details of the best fit are given in Table 3.

NGC 1614 has been observed with ROSAT in a 10-ks X-ray emission in Fig. 2, but they are of low significance and observation (usable time 9.1 ks), and was at the centre of as such there is no evidence for extended emisson from the pointing. NGC 1614 has been clearly detected with NGC 1614. There is also no evidence of any X-ray emission about 160 count in the extracted spectrum. The X-ray/opti- from the tidal tail to the south-west of NGC 1614. The X-ray cal morphology of NGC 1614 is shown in the upper panel of spectrum is well-fitted with a single temperature Raymond– Fig. 2. The X-ray morphology is centred on the brightest Smith model with kT:0.81 keV. The absorbing column for optical emission. There are some apparent extensions to the this fit was constrained to be no lower than the Galactic NH

© 1998 RAS, MNRAS 294, 523–547 530 I. R. Stevens and D. K. Strickland value, implying very little local absorption. The fitted spec- & Vacca (1994) have reported on HST observations of Hen trum of NGC 1614 is shown in the lower panel of Fig. 2. The 2-10, finding that the star formation activity resolves into system is luminous, with an X-ray luminosity (corrected for smaller knots (of size #10 pc) within the starburst region, : Å 41 21 5 6 absorption) of LX 1.8 10 erg s . An estimate of the with masses of 10 –10 Mᖿ and ages of around 5 Myr. error for LX is given in Table 3. We have analysed a 9.3-ks PSPC pointed observation of A previous X-ray observation of NGC 1614 by the Hen 2-10 (9.2 ks usable), and the results are shown in Fig. 4. Einstein satellite (Fabbiano et al. 1992) found an X-ray Hen 2-10 is clearly detected, with around 160 count in the EIN# Å 41 21 luminosity of LX 3 10 erg s (corrected for dis- extracted spectrum. The X-ray emission is centred on the tance). However, as this luminosity was derived assuming a galaxy, and there is no evidence for any source extension. thermal bremsstrahlung spectrum with kT:5 keV, there The X-ray spectrum is well-fitted with a single temperature : : Å 21 are likely to be substantial errors in the Einstein determina- Raymond–Smith model with kT 0.42 keV, NH 3 10 22 : 40 21 tion. consequently the factor of 2 difference between the cm and LX 10 erg s . An estimate of the error for LX two X-ray luminosities is not of great significance. is given in Table 3. No optical source with a position coincident with the Downloaded from https://academic.oup.com/mnras/article/294/4/523/1025955 by guest on 26 September 2021 X-ray source to the north-west of Hen 2-10 could be found 4.2 NGC 1741 and HCG 31A (Mrk 1089) in the SIMBAD data base, but there is an 11th magnitude These two galaxies are part of Hickson Compact Group star in the HST Guide Star Catalogue at this location. number 31, an interacting group that has been studied in the optical by Rubin, Ford & Hunter (1990). The two WR galaxies NGC 1741 (also known as HCG 31C) and HCG 4.4 Mrk 33 (Haro 2) 31A are in the process of interacting and probably merging. Mrk 33 is a well-studied blue compact galaxy, which is com- HCG 31A is classified by Rubin et al. (1990) as Sdm and paratively metal-rich (with a metallicity only 2–3 times HCGT 31C as Im. Recent Hubble Space Telescopee (HST) lower than solar). Its general morphology and brightness observations of NGC 1741 by Conti, Leitherer & Vacca profiles are those of an , but it has a bright (1996) have resolved two centres of violent star formation, blue nucleus indicative of intensive star formation. A WR each about 100 times more luminous than 30 Doradus, and emission feature was found in Mrk 33 by Kunth & Joubert both centres of activity can be resolved into smaller knots of (1985). Using the HST, Lequeux et al. (1995) detected an activity. Conti et al. (1996) also make an interesting connec- outflow from the star-forming region with a velocity of 200 tion between the optical spectrum of NGC 1741 and 2 km s 1, and interpreted this as being a galactic wind. recently discovered distant star-forming galaxies (Sec- There are two observations of Mrk 33 in the archive: a tion 8). pointed 4.1-ks PSPC observation of Mrk 33 (usable time There is a short ROSAT observation of NCG 31, with an 4.0 ks), and a 17.6-ks (17.4 ks usable) observation, with Mrk exposure of 1.9 ks. NGC 1741 has been clearly detected, but 33 25 arcmin off-axis. We have analysed both observations, with only about 27 counts in the extracted spectrum. The and use the shorter on-axis observation to obtain an image X-ray/optical morphology of NGC 1741 is illustrated in Fig. and the longer off-axis observation to obtain the X-ray 3, which shows the X-ray contours centred on NGC 1741 spectra. and HCG 31A, but also including HCG 31B and HCG 31D. In the on-axis observation, Mrk 33 is clearly detected There is, however, no evidence for any extension of the (although the PSS significance of detection s:4.8 is lower emission associated with NGC 1741. It is also worth noting than for the other WR galaxies). Mrk 33 is also detected in that there seems to be some X-ray emission associated with the off-axis observation, with around 48 count in the HCG 31G (to the south-east of the main body of the group), extracted spectrum. The best-fitting spectrum has kT:0.36 which is also known as Mrk 1090. 2 keV and N :2.2Å1021 cm 2). We fixed the metallicity at The spectrum for NGC 1741 is poor, and many of the H 0.1 times solar to obtain a fit, and many of the parameters fitted parameters are poorly constrained, with kT:0.75 : Å 20 22 are poorly constrained. The determined X-ray luminosity is keV and NH 4.7 10 cm . The derived X-ray lumino- : Å 40 21 : Å 40 21 then LX 1.4 10 erg s . Because of the substantial sity, corrected for absorption, is LX 7.6 10 erg s . An errors in the fitted value of NH there are correspondingly estimate of the error for LX is given in Table 3. substantial errors in LX (Table 3). The X-ray/optical morphology is shown in Fig. 5. There 4.3 Henize 2-10 seems to be a slight offset between the centroid of the X-ray emission and the optical centre of Mrk 33. However, it is Hen 2-10 was the first emission-line galaxy discovered to sufficiently small (#15 arcsec) that ROSAT pointing errors have evidence for WR stars (Allen, Wright & Goss 1976), could account for this (Briel et al. 1995). There is no and is in many ways considered the prototypical WR galaxy. evidence for any extension to the X-ray emission. It is a BCDG containing two starburst regions separated by 8 arcsec. Hutsemekers & Surdej (1984) confirmed the presence of the broad WR emission feature at around 4.5 NGC 4861 (Mrk 59, I Zw 49) 4660 Å, first seen by Allen et al. (1976). The history of Hen 2-10 is uncertain and it has been suggested that the object is A dwarf , this galaxy is classified as Sm in the really two dwarf irregular galaxies in the process of merging SIMBAD data base, and consists of a series of bright knots (Johansson 1987), but it has also been suggested that Hen (H II regions), lying in a narrow band, culminating in a very 2-10 is a dwarf elliptical experiencing a self-induced period large bright knot at the south-west extreme (the so-called of starburst activity (Corbin, Korista & Vacca 1993). Conti ‘Bright knot‘. Its general morphology is either that of an

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Figure 3. X-ray emission from NGC 1741. The top panel shows the X-ray emission contours superimposed on an optical image. The X-ray contours are from an 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 2.8Å1023 count s21 arcmin22. The lower panel shows the normalized spectrum of NGC 1741 (crosses) superimposed on the best fit (solid line). Details of the best fit are given in Table 3. elongated streak or an edge-on disc (Dinnerstein & Shields tail, possibly indicative of a merger or a recent interaction. 1986). The large bright knot has a diameter of around 1 kpc, Spectrophotometric observations of NGC 4861 have also and, while resembling the 30 Doradus in the LMC, is been presented by Izotov et al. (1996) who found evidence much larger (Barth et al. 1994). A broad WR emission for a galactic wind. NGC 4861 has already been observed as feature was found in this galaxy by Dinnerstein & Shields a strong X-ray source by Einstein (Fabbiano, Feigelson & # 40 (1986). CCD imaging by Dottori et al. (1994) of NGC 4861 Zamorani 1982, Fabbiano et al. 1992), with LX 10 found evidence for a double nucleus as well as a counter- erg s21.

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Figure 4. X-ray emission from Henize 2-10. The top panel shows the X-ray emission contours superimposed on an optical image. The X-ray contours are from an 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 2.8Å1023 count s21 arcmin22. the lower panel shows the normalized spectrum of Hen 2-10 (crosses) superimposed on the best fit (solid line). Details of the best fit are given in Table 3.

Results from ROSAT observations of NGC 4861 have The total observation time in the ROSAT PSPC observa- been presented by Motch et al. (1994), who found that the tion of NGC 4861 was 17.2 ks (usable 16.3 ks). NGC 4861 is spectrum could be best-fitted with either a thermal brems- clearly detected, and there is a total of 190 count in the strahlung spectrum or a power-law. They also noted that extracted spectrum. Our results are very similar to those of they were unable to obtain a good fit with a Raymond– Motch et al. (1994) and are shown in Fig. 6. The X-ray Smith spectrum for larger than 0.05 times solar morphology is elongated along the main axis of the galaxy (see also Fourniol et al. 1996). and has an extent of about 1.6 arcmin by 1.25 arcmin. Analy-

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Figure 5. X-ray emission from Mrk 33. The top panel shows the X-ray contours superimposed on an optical image. The X-ray contours are from an image with pixel size of 5 arcsec, smoothed with a Gaussian of FWHM of 20 arcsec. The contour levels increase by a factor of 2 from 2.8Å1023 count s21 arcmin22. The lower panel shows the normalized spectrum of Mrk 33 (crosses) superimposed on the best fit (solid line). Details of the best fit are given in Table 3.

: Å 39 sis of ROSAT High Resolution Imager (HRI) data shows From the ROSAT spectrum, we determine LX 5.9 10 21 : : Å 21 22 that the X-ray emission is primarily coming from two unre- erg s , kT 0.5 keV and NH 1.9 10 cm . We also find solved point sources, coincident with two H II regions, one that a single-temperature Raymond–Smith model does not of which is the ‘Bright Knot’ (Fourniol et al. 1996). With the give as good a fit as a power-law-type spectrum (as noted by lower spatial resolution of the ROSAT PSPC, these two Motch et al. 1994). However, for consistency we stick with sources blur to give the elongated morphology seen in our single-temperature Raymond–Smith results. It could be Fig. 6. that the different spectral shape is indicating that a different

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Figure 6. X-ray emission from NGC 4861. The top panel shows the X-ray emission contours superimposed on an optical image. The X-ray contours are from an 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 2.8Å1023 count s21 arcmin22. The lower panel shows the normalized spectrum of NGC 4861 (crosses) superimposed on the best fit (solid line). Details of the best fit are given in Table 3. production mechanism is responsible for at least part of the irregular galaxies. It is a nearby, low-luminosity galaxy, with X-ray emission, such as an X-ray binary (see Section 7). a burst of star formation occurring in the central regions of the galaxy. We assume a distance of 4.1 Mpc (Saha et al. 1995), which is different from that assumed by Conti (1991). 4.6 NGC 5253 (Haro 10) Where necessary we have converted the relevant param-

NGC 5253 has been classified as being of morphological eters using this distance. We also calculate LB for NGC 5253 type E/S0, but shows features of both elliptical and gas-rich using the value of MB from Saha et al. (1995).

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Figure 7. X-ray emission from NGC 5253. The top panel shows the X-ray emission contours superimposed on an optical image. The X-ray contours are from an 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 1.4Å1023 count s21. The lower panel shows the normalized spectrum of NGC 5253 (crosses) superimposed on the best fit (solid line). Details of the best fit are given in Table 3.

A broad He II l4686 line was seen by Walsh & Roy (1989) found two superbubble-type structures of size #1 kpc. in one of the knots of star formation seen in the central Infrared spectroscopy of NGC 5253 by Lumsden, Puxley & regions of the galaxy. There have been recent multiwave- Doherty (1994) failed to detect IR features of WR stars, but length studies of NGC 5253 by Martin & Kennicutt (1995), did detect weak H2 emission. Marlowe et al. (1995) and Beck et al. (1996). Martin & Results from a 34.1-ks (usable time 33.7 ks) ROSAT Kennicutt (1995) discuss the dynamics of the Ha filaments observation of NGC 5253 have been reported by Martin & seen in the central regions of NGC 5253, finding evidence of Kennicutt (1995). The X-ray/optical morphology is shown in a slowly expanding region, while Marlowe et al. (1995) Fig. 7. There is some evidence that the X-ray emission is

© 1998 RAS, MNRAS 294, 523–547 536 I. R. Stevens and D. K. Strickland slightly extended, but a definitive answer on this will await This galaxy was serendipitously observed as part of a analysis of an HRI observation. The X-ray spectrum is well- 20-ks exposure of the cluster . IRAS 0100322238 fitted with a single-temperature Raymond–Smith spectral is outside the inner ring of the detector (about 30 arcmin : : Å 20 22 model, with kT 0.43 keV, NH 8.0 10 cm and from the centre of the field), and there is no detectable : Å 38 21 LX 4 10 erg s , and results are in good agreement with emission. Assuming a Raymond–Smith spectral model with : : Å 20 those of Martin & Kennicutt (1995). The X-ray spectrum is kT 0.5 keV and an absorbing column of NH 1.6 10 also shown in Fig. 7. cm22 (the Stark value), we derive an upper limit for the unabsorbed X-ray luminosity (68 per cent confidence level) of 2.14Å1040 erg s21. 4.7 NGC 7714 (Arp 284, Mrk 538) NGC 7714 is a typical , classified as type S, 5.2 Minkowski’s object that has undergone a recent close interaction with NGC 7715. A detailed optical study of this system has been made Minkowski’s object is located near the centre of the galaxy by Gonz´alez-Delgado et al. (1995). The 6-cm radio map cluster Abell 194, and in close proximity to the elliptical Downloaded from https://academic.oup.com/mnras/article/294/4/523/1025955 by guest on 26 September 2021 shows a weak double radio structure (separation 1 arcsec). galaxy NGC 541, and is believed to be a starburst triggered A bipolar wind is implied from the isovelocity contours of by an interaction with a radio jet from the nearby elliptical Taniguchi et al. (1988). A weak WR emission feature at galaxy NGC 541 (van Breugel et al. 1985). Minkowski’s l4686 was reported by van Breugel et al. (1985), and the object is also embedded in a bridge of luminous material presence of broad He II l4686 emission has been confirmed connecting NGC 541 to NGC 545/547 (Brodie, Bowyer & by Gonz´alez-Delgado et al. (1995). McCarthy 1985). Broad He II l4686 emission was observed NGC 7714 was detected with the Einstein IPC (Weedman in Minkowski’s object by van Breugel et al. (1985). et al. 1981). Fabbiano et al. (1992) presented a re-analysis of The Einstein satellite also observed the central regions of the Einstein observations, finding a luminosity of 1.2Å1041 Abell 194 (Jones & Forman 1984) and detected several erg s21 for an assumed distance of 59.3 Mpc. Weedman et regions of enhanced emission. It was suggested that one of : Å 40 21 al. (1981) noted that this luminosity was consistent with the these X-ray clumps, with LX 7 10 erg s , was possibly emission of #104 supernova remnants from a volume of associated with Minkowski’s object. van Breugel et al. # radius 300 pc. (1985) argued, on the basis of the LX /LB ratio, that this was In the ROSAT data base there are two PSPC observations unlikely and that the emission was most likely from NGC of NGC 7714, and we only analyse the longer 12.7-ks obser- 541. Our analysis of ROSAT observations supports this. vation (usable time 10.4 ks). The NGC 7714/7715 system is Minkowski’s object was observed as part of a 25-ks PSPC on-axis for this observation. observation of Abell 194. Minkowski’s object is only #1.5 In the upper panel of Fig. 8, the X-ray/optical mor- arcmin from the field centre. It is clear that NGC 541 has phology is shown. There is a distinct optical bridge joining been detected, and that Minkowski’s object has not been NGC 7714 and NGC 7715. However, in X-rays NGC 7714 is detected. We can derive an upper limit to the unabsorbed clearly seen but there is no evidence of any emission from X-ray luminosity (68 per cent confidence level) of Min- NGC 7715. There is also no evidence of any extension to the kowski’s object of 1.44Å1040 erg s21 (assuming a spectral X-ray emission from NGC 7714. The X-ray spectrum of model for Minkowski’s object with kT:0.5 keV and : Å 20 22 NGC 7714 is also shown in Fig. 8, along with the best fit. The NH 3.1 10 cm ). spectrum is well-fitted with a Raymond–Smith model with Spectral fitting for the spectra of NGC 541 (using both : : Å 20 22 kT 0.8 keV, NH 8.4 10 cm and a luminosity of power-law and Raymond–Smith models) yields luminosi- Å 40 21 # 41 22 4.4 10 erg s . An estimate of the error for LX is given in ties for NGC 541 of LX 10 erg s , basically consistent Table 3. The Einstein IPC luminosity of Fabbiano et al. with the Einstein results, and this would strongly suggest (1992) is consistent with this, given the differing assumed that Einstein detected NGC 541 rather than Minkowski’s distances and the respective instrument capabilities. object. The other X-ray point source, north-east of NGC 7714 in Fig. 8, is an AGN (QSO 2333ǹ0154, Bowen et al. 1994). 5.3 Mrk 178 The bright optical star to the south-east of NGC 7714 is HD : : 22195 (F6V, mB 6.12, mV 5.68). HD 22195 is detected as Mrk 178 is a blue compact galaxy consisting of an irregular a point source, but at a low significance. component and a spheroidal component, at a distance of 4.5 Mpc. The younger south-east knot of star formation has a broadened WR emission feature while the NW knot does 5 RESULTS: X-RAY UPPER LIMITS not – see Gonz´alez-Riestra, Rego & Zamorano (1988) for a more complete discussion of this object. Mrk 178 is one of 5.1 IRAS 0100322238 the least intrinsically luminous galaxies in the sample, with 2 : Å 41 21 IRAS 01003 2238 is an extremely far-infrared luminous LB 1.8 10 erg s . # Å 11 galaxy (LFIR 6 10 Lᖿ – see Table 2). The first report of The source was observed during a 2.9-ks observation of a WR feature at around 4660 Å in this galaxy was by Armus Abell 1314. Mrk 178 is about 20 arcmin from the field centre et al. (1988), finding broad He II l4686 and N III l4640 and is not detected. We derive an upper limit to the unab- emission. They interpreted this emission feature as being sorbed X-ray luminosity (68 per cent confidence level) of # 5  Å 37 21 caused by the presence of 10 WR stars in this galaxy. LX 2.1 10 erg s (assuming a Raymond–Smith spec- This system is by far the most distant of our sample, with a tral model with kT:0.5 keV and a Stark column of : : Å 20 22 of z 0.1176 and a distance of 470 Mpc. NH 1.4 10 cm ).

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Figure 8. X-ray emission from NGC 7714. The top panel shows the X-ray emission contours superimposed on an optical image. The X-ray contours are from an 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 2.8Å1023 count s21 arcmin22. The lower panel shows the normalized spectrum of NGC 7714 (crosses) superimposed on the best fit (solid line). Details of the best fit are given in Table 3.

Mrk 750 in the field of view. The longest is a 17-ks observa- 5.4 Mrk 750 tion of b Leonis. Mrk 750 was outside the inner detector An emission-line galaxy with a broad He II l4686 feature ring, and was not detected. We derive an upper limit to the first noted by Kunth & Joubert (1985). It has been classified unabsorbed X-ray luminosity (68 per cent confidence level)  Å 38 21 as a dI galaxy with a bright star-forming region. The galaxy of LX 2.1 10 erg s (assuming a Raymond–Smith : : Å 20 is at a distance of about 11 Mpc. spectral model with kT 0.5 keV and NH 2.1 10 In the ROSAT archive are three PSPC exposures with cm22).

© 1998 RAS, MNRAS 294, 523–547 538 I. R. Stevens and D. K. Strickland

limits (resulting from our lack of knowledge of the absorb- 5.5 II Zw 62 ing column) we have performed regression fits on the whole An emission-line galaxy with a WR emission feature, sample (upper limits included) and also just on the detec- initially noted by Kunth & Joubert (1985). The galaxy is at a tions. We shall primarily quote the results for just the X-ray distance of 52 Mpc, and is one of the fainter galaxies in the detections, which we regard as being the most reliable. sample. In general terms, the main results from Section 4 can be II Zw 62 was observed during a 14.9-ks exposure of the summarized as follows. Out of a total of 14 galaxies galaxy NGC 4365. II Zw 62 is about 30 arcmin from the field observed by the ROSAT PSPC, seven have been detected. # Å 38 centre, and was not detected. We derive an upper limit to The luminosities of WR galaxies range from LX 4 10 the unabsorbed X-ray luminosity (68 per cent confidence erg s21–2Å1041 erg s21. A common feature is that the spec-  Å 39 21 level) of LX 1.6 10 erg s (assuming a Raymond– tra, in most cases, can be well-fitted with a single-tempera- Smith spectral model with kT:0.5 keV and ture Raymond–Smith model with a temperature in the : Å 20 22 NH 1.48 10 cm ). range of kT:0.3–1.0 keV, and that the more luminous WR galaxies tend to have hotter fitted temperatures. Also, no Downloaded from https://academic.oup.com/mnras/article/294/4/523/1025955 by guest on 26 September 2021 evidence for any extension in the X-ray emission was seen, 5.6 POX 186 except in the case of NGC 4861, where the X-ray emission An emission-line galaxy with a WR emission feature origin- was elongated along the major axis of the galaxy. The ally detected by Kunth & Joubert (1985). The galaxy lies at derived absorbing columns were typically larger than the a distance of around 14 Mpc. Stark value, implying some local absorption in the galaxy. In the ROSAT archive are four short observations of The fitted metallicities were low, typically in the range of Alpha Vir, the longest of which has an exposure time of 4.5 0.05–0.15 Zᖿ (Table 3). However, in view of the uncertain- ksec. In this observation, POX 186 is about 30 arcmin from ties in the accuracy of ROSAT metallicities (cf. Bauer & the centre of the field. The source is not detected and we Bregman 1996) the derived values should be treated with derive an upper limit to the unabsorbed X-ray luminosity caution.  Å 38 21 (68 per cent confidence level) of LX 2.4 10 erg s (assuming a Raymond–Smith spectral model with kT:0.5 : Å 20 22 6.1 Blue luminosity correlations keV and NH 4.4 10 cm ). In Fig. 9 we show the correlation between the X-ray lumi- nosities of WR galaxies (corrected for absorption) and their 5.7 Mrk 309 (IV Zw 121) A broad WR emission feature was first found in this narrow- emission-line galaxy by Osterbrock & Cohen (1982). IRAS results for Mrk 309 are from Rego et al. (1993). Mrk 309 is one of the more distant galaxies in the sample at 166 Mpc. This galaxy was observed during a 18-ks observation of MU Peg. Mrk 309 was located about 36 arcmin from the centre of the field. The source is not detected and we derive an upper limit to the unabsorbed X-ray luminosity (68 per  Å 41 21 cent confidence level) of LX 1.9 10 erg s (assuming a Raymond–Smith spectral model with kT:0.5 keV and : Å 20 22 NH 5.7 10 cm ).

6 GALAXY SAMPLE CORRELATIONS In this section we discuss the global properties of our sample of WR galaxies, and correlations between their X-ray properties and luminosities at other wavebands. As we have several upper limits in our determinations of X-ray luminosities, we make use of the ASURV statistical package (Feigelson & Nelson 1985; Isobe, Feigelson & Nelson 1986) Figure 9. The correlation between the ROSAT X-ray luminosities to investigate correlations between the X-ray luminosity of WR galaxies and their blue luminosity LB. Shown in this diagram and other relevant parameters listed in Table 2. We make are the data for the WR galaxies that were detected (filled use of two tests. The first is a linear regression fit using the squares), the upper limits for those WR galaxies not detected parametric EM algorithm, which provides an estimate (arrows), and results from the ROSAT XRT survey of nearby (along with errors) of the best regression fit. The second is galaxies by Read et al. (1997 – unfilled squares). Also shown are the regression fits for the L :L relationship for WR galaxies, for a correlation test, using the Cox proportional hazard model. X B detections only (solid line) and including upper limits (dashed This correlation test yields a probability (P ) that no COX line). The best fit from the results of David et al. (1992) is also correlation exists between the variables (see Isobe et al. shown (dot–dashed line). The Cox probability of there being no 1986 for details). Results are shown in Figs 9–14, and a correlation in the data for WR galaxies is shown both for the case summary of the correlations is given in Table 4. Because of when all data points are included, and when only detections are the potential uncertainties in the X-ray luminosity upper included.

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Figure 10. The correlation between the ROSAT X-ray luminosities Figure 12. IRAS colour–colour diagrams for WR galaxies: the of WR galaxies and their far-infrared luminosity LFIR. Shown in S12 /S25 and S60 /S100 ratios for the WR galaxies (with limits if appro- this diagram are the data for the WR galaxies that were detected priate) are plotted (solid squares), as well those as for the ROSAT (filled squares), the upper limits for those WR galaxies not sample of Read et al. (1997 – open squares). In addition, we plot detected (arrows) and results from the ROSAT XRT survey of the line from Helou (1986), which marks the line where the FIR nearby galaxies of Read et al. (1997 – unfilled squares). The regres- emission comes equally from hot and cold dust. sion fits for the LX :LFIR relationship for WR galaxies are also shown; for detections only (solid line) and including upper limits (dashed line). The best fit from the results of David et al. (1992) is also shown (dot–dashed line). The Cox probability of there being no correlation in the data for WR galaxies is shown both for the case when all data points are included, and when only detections are included.

Figure 13. Comparison of X-ray luminosities for WR galaxies determined by ROSAT with those determined by Einstein (from Fabbiano et al. 1992), along with the regression fit.

blue luminosity LB. For comparison, we also show LX :LB results from the ROSAT survey of nearby spiral galaxies from Read et al. (1997), and from the results of David, Jones & Forman (1992). The most striking thing about this Figure 11. The correlations between the ROSAT X-ray luminosi- diagram is that the WR galaxies are consistently X-ray- ties of WR galaxies and the ratio of the far-infrared to blue lumino- overluminous for their LB by around an order of magnitude, sity LFIR /LB. Shown in this diagram are the data for those WR as compared with the comparison galaxy samples. galaxies that were detected (filled squares), upper limits for WR We find for our sample of detected WR galaxies the fol- galaxies that were not detected (arrows), and results from a lowing correlation: ROSAT XRT survey of nearby galaxies by Read et al. (1997 – unfilled squares). The regression fits are also shown; including only : < 2 < log LX (1.56 0.20) log LB (27.72 8.67), (4) detections (solid line) and including upper limits (dashed line). 21 The Cox probability of there being no correlation in the data for where LX and LB are both in erg s . This regression fit WR galaxies is shown both for the case when all data points are includes only detections. When upper limits are included, included, and when only detections are included. the steepness of the slope decreases slightly (though not

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Figure 14. Additional correlations for the WR galaxy sample. In all four cases we plot the data points for those WR galaxies that were detected (solid squares), upper limits for the WR galaxies that were not detected (arrows), and in the case of the supernovae luminosity LSN, the results from Read et al. (1997 – open squares). In all cases we also plot the regression fits for the case when only X-ray detections are included (solid line) and when upper limits are included (dashed line). The values for the Cox’s probability for both cases are also shown. Top left panel shows the correlation between the X-ray luminosity and the equivalent width of the WR emission feature w(WRE); top right panel the correlation with the number of Lyman continuum photons NLyc; bottom left the correlation with the supernova b luminosity LSN; bottom right the correlation with the equivalent width of H .

< 2 significantly) to 1.52 0.16 (see Fig. 9 and Table 4). For LX :LB results for the bright FIR galaxies are the nearest, both cases, PCOX is very low (see Table 4) indicating that LX although the slope for WR galaxies is significantly steeper. is strongly correlated with LB. The slope of the LX :LB correlation is slightly lower for WR For comparison, ROSAT survey of nearby spirals (Read galaxies than for the Read et al. (1997) sample, but the et al. 1997) found the following regression fit for the LX :LB difference is not significant. correlation (see also Fig. 9): The sample of galaxies in Read et al. (1997) also includes : ǹ0.18 2 ǹ6.40 several starbursts, and in Fig. 9 the starbursts in this sample log LX (1.7720.16) log LB (37.6427.39). (5) tend to be those galaxies with larger LB. It is therefore David, Jones & Forman (1992) found the following corre- interesting that the WR galaxies have distinctly different X- lation between LX and LB for their sample of bright FIR ray properties from starbursts in nearby spirals. It is, how- galaxies: ever, also worth noting that the lone galaxy from the Read

ǹ0.06 ǹ2.6 et al. (1997) sample lying above the LX :LB trend for WR log L :(1.122 ) log L 2(8.62 ). (6) X 0.09 B 3.9 galaxies is the starburst M82. the results from this figure will

Several facts are apparent from the LX :LB relationship be discussed more in Section 7. determined for WR galaxies compared with that for earlier samples. WR galaxies are substantially overluminous com- pared with all of the earlier determined correlations. The 6.2 Far-infrared correlations

2 In Fig. 10 we show the correlation between the X-ray lumin- When we include the error estimates for LX (Table 3) in a regres- son fit for LX and LB, including only the X-ray detections (using the osity (corrected for absorption) and the far-infrared lumin- ODRPACK software), we also get a fit consistent with these values. osity LFIR, along with the regression fits. In Fig. 10 we also

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Table 4. Correlations with X-ray luminosity for a range of different parameters. Regression fit results for a straight line : ǹ of the form log Y M log X C, where M is the slope and C the intercept, except for the correlation of LX with w(WRE) b : ǹ and w(H ), where the relationship is of the form log Y MX C. PCOX is the Cox’s probability that no correlation exists (Isobe et al. 1986). Downloaded from https://academic.oup.com/mnras/article/294/4/523/1025955 by guest on 26 September 2021

show the best-fitting line for the LX :LFIR correlation for the of galaxy. The correlation between LX :LFIR /LB is not as sample of bright FIR galaxies of David et al. (1992). good for earlier fits, but some trends are apparent. First and For our sample of WR galaxies, the regression fit for the foremost, the WR galaxies tend typically to have higher total intrinsic X-ray luminosity versus the far-infrared lumi- values of LFIR /LB compared with the Read et al. (1997) nosity LFIR is (including only detections) sample, indicating that they are active star-forming regions. : < ǹ < The single galaxy from the Read et al. (1997) sample at high log LX (0.75 0.19) log LFIR (7.79 8.31), (7) values of LFIR /LB is again M82. When only detections are 21 where LX and LFIR are both in erg s . When upper limits included, we get a positive correlation between LX and LFIR / : are included, the fit becomes shallower, with a slope of LB with PCOX 6 per cent. However, the inclusion of the < 0.55 0.21, though it is not a significant change. For both upper limits results in PCOX rising to 90 per cent, indicating cases PCOX is low, indicating a strong correlation between LX that there is likely no correlation. and LFIR. To illustrate further some of the FIR properties of our The ROSAT survey of nearby spirals of Read et al. (1997) sample of WR galaxies, in Fig. 12 we show an IRAS colour– found the following regression fit for LX :LFIR: colour diagrams for the WR galaxies. The line in Fig. 12 is

ǹ0.06 ǹ2.13 the dividing line where the contribution to the FIR emission log L :(0.882 ) log L ǹ(1.902 ), (8) X 0.05 FIR 2.26 comes equally from hot and cold dust (Helou 1986; cf. where all luminosities are in erg s21. In addition, David et al. Calzetti et al. 1995). Above this line, the majority of the FIR

(1992) found the following correlation between LX and LFIR emission comes from hot dust, and below this from cool for their sample of bright FIR galaxies: dust. It is clear than in WR galaxies the majority of the FIR

ǹ0.06 ǹ3.0 emission comes from hot dust. The two galaxies from the log L :(0.952 ) log L 2(1.12 ). (9) X 0.05 FIR 2.6 Read et al. (1997) sample roughly collocated with the WR

It is clear that the trend for LX :LFIR for our WR galaxies galaxies in Fig. 12 are the starburst galaxies M82 and NGC is similar to that found by Read et al. (1997) and David et al. 253. (1992). In Fig. 11 we have plotted the correlation of the X-ray 6.3 Other correlations luminosity versus LFIR /LB. This ratio is in some sense the strength of the starburst – the higher the value, the stronger In this subsection, we discuss some of the other correlations the relative strength of the starburst compared with the size for the X-ray properties of WR galaxies.

© 1998 RAS, MNRAS 294, 523–547 542 I. R. Stevens and D. K. Strickland

As a check, in Fig. 13 we compare our ROSAT-deter- 7 ORIGIN OF THE X-RAY EMISSION mined luminosities with those determined by Einstein (from Fabbiano et al. 1992, corrected for distance where appro- In this section we discuss the origin of the X-ray emission priate). There are only four sources that have been detected from WR galaxies. In normal galaxies, the X-ray emission by both instruments (NGC 1614, NGC 4861, NGC 5253 and will have several origins: point sources (primarily X-ray NGC 7714). It is, however, clear from Fig. 13 that there is a binaries or individual supernova), the combined contribu- good correlation between the two instruments, and tion from populations of stars (both high- and low-mass : PCOX 4.5 per cent. The fact that ROSAT and Einstein have stars) and genuinely diffuse emission from a hot interstellar different energy bands is largely negated by the softness of medium or corona. In WR or starburst galaxies, there is the the sources as determined by ROSAT. likelihood of X-ray emission from the starburst region itself Additional examples of the correlations between the and the hot superbubble or superwind caused by the star- X-ray luminosities of WR galaxies and other interesting burst. In AGNs there is also X-ray emission from the black parameters are shown in Fig. 14. The upper left panel of hole at the centre of the galaxy. Downloaded from https://academic.oup.com/mnras/article/294/4/523/1025955 by guest on 26 September 2021 Fig. 14 shows the correlation of LX with the equivalent width David et al. (1992), in their discussion of the X-ray of the WR emission feature. If only the detections are properties of bright FIR galaxies, segregate the X-ray emis- included then there is a trend, with larger equivalent width sion into two components: an old component consisting of : being correlated with larger X-ray luminosity (PCOX 28 per moderately old low-mass stars, including low-mass X-ray cent). However, if the upper limits are included then this binaries (LMXRBs) and Type I supernovae, and a young trend reverses, and large equivalent width correlates with component consisting of massive X-ray binaries (MXRBs), : lower luminosities (PCOX 9 per cent). However, these Type II supernovae and early-type stars. David et al. (1992) correlations are rather weak and are probably indicating no suggest that the old component should dominate in quies- significant correlation. cent galaxies, while the young component dominates in star- The upper right panel shows the correlation between the forming galaxies, and concluded that in their sample of

X-ray luminosity and the number of Lyman continuum galaxies the young component dominates when LFIR / P photons NLyc. There is a strong correlation detected, with LB 1.1. It is worth noting that four of the seven detected low values of PCOX. A similar, roughly linear trend is found WR galaxies do fulfil this criteria, but three do not (Hen both when the upper limits are included and when they are 2-10, NGC 4861 and NGC 5253). In the following sub- excluded. As noted in Section 2, NLyc is a measure of the sections we discuss critically the possible origins of the number of ionizing stars, with contributions from O and X-ray emission in WR galaxies. WR stars in the case of WR galaxies. This plot would seem to indicate that the stronger the starburst the stronger the X-ray emission. 7.1 An active nucleus The lower left panel in Fig. 14 shows the correlation between LX and the supernova luminosity LSN for the WR Fourniol et al. (1996) have presented results on several H II galaxy sample (solid squares) and from the Read et al. galaxies, and have suggested that the X-ray emission from

(1997) sample (open squares). Because LSN is primarily some of the more luminous galaxies in their sample (NGC determined by LFIR (equation 2), it is not surprising that 1614 for example) may be the result of an active nucleus. there is a good correlation. The correlation with only the However, we do not consider this a likely explanation for detections included is substantially better than that where the X-ray emission from WR galaxies for the following the upper limits are included. However, in general terms the reasons. First, AGNs tend to be more luminous than WR WR galaxies tend to lie at higher X-ray luminosities com- galaxies (see for example Fabbiano et al. 1992), although pared with the sample of nearby spirals. Also, the slope for there is some overlap between the samples. Secondly, this correlation is significantly less than unity. AGNs tend to have power-law X-ray spectra rather than The lower right panel shows the correlation between the thermal spectra. Fits to WR galaxies with power-law spec- b b H equivalent width w(H ) and LX. This equivalent width tral models are typically not as good and give the wrong is a measure of the relative contribution of hot stars to the power-law index. Thirdly, while it is difficult to exclude the total optical luminosity of the galaxy. There is an inverse possibility of an active nucleus in some of the more lumin- : correlation when only detections are included (PCOX 4 per ous WR galaxies, we would regard the weight of evidence as cent), so that galaxies where the relative hot star contribu- being in favour of a starburst/superbubble origin for the tion is dominant are typically less X-ray-luminous than X-ray emission (see Section 7.6). those where the hot star fraction is lower. Including upper limits results in a similar gradient, but a substantially higher value of P . This behaviour could be accounted for by a COX 7.2 Galactic coronae simple model whereby the mass of stars formed in the star- bursts in WR galaxies were comparable. Larger galaxies, Another possibility for the X-ray emission is that it is the which have larger quiescent (or pre-starburst) luminosities, result of an extended hydrostatic halo or corona. Forman, will be both more X-ray-luminous and have a smaller star- Jones & Tucker (1985), in their analysis of Einstein observa- burst fraction, and hence lower w(Hb). tions of bright early-tpe galaxies, found that hot (#107 K) These correlations presented in this section provide an gaseous coronae were a common feature of such galaxies 2 intersting picture of the characterisitics of WRE galaxies. In with MB 19. For these systems, Forman et al. (1985) f 2 the following section, we discuss some of the possibilities for found roughly that LX LB, and they concluded that the the origin of the X-ray emission in WR galaxies. hydrostatic coronae could be formed over the lifetime of the

© 1998 RAS, MNRAS 294, 523–547 X-ray survey of Wolf–Rayet galaxies 543 galaxy by gradual accumulation of hot gas from the normal 7.5 Isolated supernovae evolution of its stellar component. We do not consider this a likely possibility to explain A similar line of reasoning to that presented above also much of the X-ray emission from WR galaxies, although it is rules out the idea that a population of a few hundred or a difficult to completely rule out a coronal contribution. First, few thousand individual supernova remnants throughout and perhaps the most telling evidence against a coronal the central region of the galaxy could make a dominant origin, the WR galaxies in our sample lie significantly above contribution to the X-ray emission from WR galaxies (see the trend for the sample of Forman et al. (1985). Secondly, Martin & Kennicutt 1995, and also Fabbioano et al. some of the galaxies are too faint to have an observable 1982). corona, although of the seven detected WR galaxies, four of As discussed by Tenorio-Tagle (1994), a single supernova 2 # 6 8 23 them have MB 19 (NGC 1614, NGC 1741, NGC 7714 in a dense (n 10 –10 cm ) environment (a compact and Hen 2-10). Thirdly, the derived LX :LB relationship for ) can generate X-ray luminosities of the coronae in Forman et al. (1985) is steeper than that for WR order seen in WR galaxies for a short time-scale. Indeed, a galaxies. recent ROSAT HRI observation of SN 1988Z has detected Downloaded from https://academic.oup.com/mnras/article/294/4/523/1025955 by guest on 26 September 2021 # 451 21 this supernova, with a luminosity of LX 10 erg s (Fabian & Terlevich 1996), more luminous than most WR 7.3 X-ray binaries galaxies in this sample. However, the X-ray luminosity of such a SN remnant will decline very rapidly (on a time-scale Given the range of luminosities of the WR galaxies 38 41 21 of a few tens of years) and hence compact SN remnants are (4Å10 –2Å10 eg s ), it is possible that much of the X- unlikely to be the explanation for the X-ray emission from ray emission may result from a small perturbation of X-ray WR galaxies as a whole. binaries. This model is difficult to exclude, and it is likely that in some cases X-ray binaries do contribute to the over- all emission. However, the spectra of X-ray binaries are 7.6 Superbubbles typically much harder than the kT#0.5 keV observed for While individual supernova cannot be a major contributor WR galaxies (see for example Griffiths & Padovani 1990), to the X-ray emission, if sufficient hot stars are formed in a and this, together with the additional correlations dis- small region near the centre of the galaxy then their com- covered, tends to weigh against X-ray binaries being the bined stellar winds and eventual supernova explosions will dominant source of X-ray emission. However, it should be lead to the formation of a hot over-pressured bubble that noted that the ultrasoft components of some black hole will expand and, if sufficiently energetic, break out of the candidates (BHCs) have roughly similar spectral character- galaxy to form a superwind such as that seen around M82. istics to WR galaxies (Inoue 1991). A population of BHCs The X-ray emission in this case is from a dynamic, expand- may make a contribution to some of the less luminous WR ing bubble of hot gas rather than from a static corona (see galaxies. As several BHCs have been found in the LMC, it is Mac Low & McCray 1988). not unlikely that WR galaxies will indeed have such sources While for a single O star the efficiency of conversion of present in them. radiative luminosity to X-ray emission is small, with Given the preferred model of WR galaxies (very young 2 2 L :L #10 6–10 7, the fraction of the total bolometric starbursts), we would expect the possibility of some X bol luminosity of the star that is converted into kinetic energy in MXRBs. By the time we are observing a substantial popula- the is much higher (of the order of 1 per cent). tion of WR stars, the most massive stars should have If this kinetic energy is then converted into thermal energy reached the end of their life to explode as supernova and (via a shock) then this hot gas could provide a source for the form neutron stars, opening the possibility for the formation X-ray emission. The collective action of winds (and subse- of MXRBs. Higher resolution X-ray spectral observations quent supernovae) in a cluster of hot stars will be to effi- in harder energy bands are necessary to detect or constrain ciently thermalize the ejected material into a hot bubble. a harder X-ray component from such binaries, and would To illustrate how a superbubble model could account for clearly be a worthwhile ASCA study. some of the observed properties of WR galaxies, we have performed the following simple calculations, which are designed to be indicative of the general behaviour of WR 7.4 X-ray emission from single stars galaxies. More detailed calculations of the evolution of Early-type stars are known to be X-ray emitters with superbubbles are underway, and will be reported on else- # 30 34 21 # 26 27 LX 10 –10 erg s and LX :Lbol 10 –10 . The X-ray where. In addition, more detailed models of specific temperatures are comparable to those observed in the WR galaxies are also required. galaxies (for example Chlebowski, Harnden & Sciortino We initially calculate the mass and energy input into the 1989). Lower mass stars are also X-ray emitters, and can surrounding medium from a cluster of stars, using the stellar emit more of their luminosity at X-ray energies, with evolution grids discussed in Meynet (1995). We use the # 23 LX :Lbol 10 . However, such low-mass stars can only evolutionary grids with Z:Zᖿ and with the ‘standard’ mass- make up a small fraction of the total stellar luminosity of the loss rates. We assume that the burst of star formation lasts galaxy. While it is likely that populations of individual stars 106 yr (although the evolutionary tracks are not substantially may make a small contribution, the large LX :LB (and different from those with an instantaneous starburst except O LX :Lbol) ratios for WR galaxies make it unlikely that stellar at times 1 Myr), and that the total mass of stars formed is : Å 7 populations make a major contribution (see Martin & Ken- Mtot 5 10 Mᖿ. This value is larger than the inferred total 6 nicutt 1995, and also Fabbiano et al. 1982). mass of stars formed in NGC 5253 (2.5Å10 Mᖿ, Martin &

© 1998 RAS, MNRAS 294, 523–547 544 I. R. Stevens and D. K. Strickland

Kennicutt 1995). However, NGC 5253 is one of the lower ing, equation (10) is valid only for the case of constant luminosity WR galaxies, and the models in this section are energy input. As is apparent from Fig. 15, the energy input designed to represent general trends in WR galaxies, rather form any realistic stellar cluster will be very time-variable. than to model an individual system. We assume two dif- Consequently, a proper treatment of the X-ray evolution of ferent forms for the initial mass function (IMF) for the the superbubble will require a more sophisticated calcula- stellar cluster. One is a Salpeter-type IMF (dN/dMfM2a), tion, including, in addition to a realistic starburst cluster, a with a:2.35, and the other an IMF biased towards the realistic density distribution around the starburst (cf. Tomi- formation of high-mass stars, with a:1. The lower and saka & Ikeuchi 1988; Suchkov et al. 1994). However, in the : upper mass limits of stars in the clusters are ML 1 M and spirit of simplicity, we shall assume that the energy input : MU 120 Mᖿ respectively. In these simulations the peak in into the ISM in equation (10) at a time t is the average value the number of WR stars occurs at around 3–5 Myr. The WR of E˙ up to that time. We assume that the starburst domi- to O star number ratio is typically high (P0.1) between t:3 nates both the X-ray and bolometric luminosity of the and 5.5 Myr after the start of the starburst (see Meynet galaxy. More refined calculations could assume that the 1995). The rate of energy input and the stellar cluster bolo- starburst occurs within a ‘normal’ galaxy that has a quies- Downloaded from https://academic.oup.com/mnras/article/294/4/523/1025955 by guest on 26 September 2021 metric luminosity as a function of time for the two different cent bolometric luminosity L*bol and a quiescent X-ray lumi- a: IMFs are shown in Fig. 15. For the 1 IMF, the cluster is nosity L*X according to the relationship for normal substantially more luminous and injects more energy into galaxies. the ISM. However, as the massive stars finish their life-cycle In Fig. 16 we plot the X-ray luminosity of the cluster as a and explode as supernovae, the luminosity and energy input function of Lbol for both IMFs. The evolution in both cases from the cluster drops sharply. We only present results for is qualitatively similar. The initial starburst evolution leads the bolometric luminosity of the cluster. More detailed cal- to a rise in Lbol with a corresponding increase in LX. How- : culations in different wavebands, and for different param- ever, at around t 3 Myr the cluster Lbol begins to decrease, eters, for the starburst will be presented in a later paper, but while the X-ray luminosity from the superbubble carries on we note that for a starburst cluster the evolution of LB and increasing, as more energy is pumped into the hot bubble by

Lbol is similar (Leitherer & Heckman 1995). stellar winds and the first supernovae, as the most massive Adapting the results of Martin & Kennicutt (1995) we stars reach the end of their life-cycle. find the following relationship for the evolution of the X-ray From Fig. 16 it is clear that the starburst will result in the luminosity of the superbubble: X-ray luminosity of the galaxy being larger than expected for its luminosity at the ages expected for WR galaxies (i.e. L :2.12Å1036E˙ 33/35n17/35t19/35, (10) X 38 0 6 a few Myr), and this is what is observed. This simplified ˙ where E38 is the mechanical or kinetic energy input in the model can thus explain the observed positions of WR 38 21 surrounding ISM in units of 10 erg s , and t6 is the age of galaxies in the LX :LB plane, with the main variable being : 23 the starburst in Myr. We assume n0 10 cm for the hydro- the total mass of stars formed in the starburst. It is also gen number density of the surrounding ISM. Strictly speak- worth noting that a starburst in which the star formation is

Figure 15. The evolution of the bolometric luminosity Lbol (erg Figure 16. A schematic picture of the X-ray evolution of a star- s21) and energy input E˙ (erg s21) from a starburst stellar cluster, for burst galaxy. The two tracks are for the two different starburst two different IMFs; a:1 (solid line), and a:2.35 (dashed line). IMFs; a Salpeter IMF with a:2.35 (dashed line) and an IMF 7 The total mass of stars formed in the cluster is 5Å10 Mᖿ, with the biased to high-mass star formation with a:1 (solid line). The ticks star formation lasting 1096 yr. marks on the track are at times t:1, 2, 4, 6 and 10 Myr.

© 1998 RAS, MNRAS 294, 523–547 X-ray survey of Wolf–Rayet galaxies 545 of much longer duration will follow a different evolutionary corresponds to 240 pc at a distance of 10 Mpc, and hence track in the LX :Lbol (or LX :LB) plane, with the starburst should be able to resolve the superbubble for many of the bolometric luminosity continuing to rise, and not showing WR galaxies in the sample, or put some very tight con- the turn-round apparent for the starburst models with short straints on their sizes. Clearly, the size of the superbubble bursts of star formation (cf. Leitherer & Heckman 1995). depends on both the strength of the starburst and the The fact, noted in Section 6, that the more X-ray-luminous density of the surrounding medium, and this only represents WR galaxies tend to have somewhat higher X-ray tempera- a rough estimate of the size of the superbubble in an indivi- tures is also consistent with the superbubble model, as dual WR galaxy. f ˙ 8/35f 8/33 kT E38 LX (Martin & Kennicutt 1995). Note that the For the case of NGC 4861, in addition to being extended, observed kT:LX relationship is consistent with this this galaxy seems to have a different spectral shape com- (Table 4). pared with the other WR galaxies, and this may be a conse- Clearly, more detailed calculations of the evolution of the quence of emission from an X-ray binary in addition to or starburst are needed, both in terms of the luminosity evolu- instead of thermal emission from the superbubble. tion in different bands (blue, FIR etc.), such as In this section we have presented a relatively simple Downloaded from https://academic.oup.com/mnras/article/294/4/523/1025955 by guest on 26 September 2021 those by Leitherer & Heckman (1995), but also in terms of picture of the X-ray properties and evolution of superbub- hydrodynamic calculations of the formation and growth of bles, and find general agreement with observed properties the superbubble and subsequent X-ray emission. However, of WR galaxies. We have not attempted a detailed model of the above simplified analysis does suggest that at least some a single WR galaxy, and as shown by Martin & Kennicutt of the properties of WR galaxies can be explained by a (1995) in their discussion of superbubble models in relation superbubble model. to NGC 5253, there are still some discrepancies to be ironed The superbubble model can also explain the derived low out. More detailed hydrodynamic models of the evolution values of metallicity. The X-ray emission from superbubbles of superbubbles are clearly needed, including the time- will be a composite of emission from the hot material of the dependent mass/energy input from a realistic stellar cluster, superbubble, which has been processed through stars and and mass-loading of the flow. Such models, in conjunction supernovae (and will have high metal abundances) and ISM with spectral synthesis codes, will be crucial in understand- material swept up by the expanding shell (which may have ing WR galaxies. very low metal abundances). Modelling will be required to determine whether the composite spectrum from these two 8 DISCUSSION AND SUMMARY different phases is consistent with the fitted spectra. As is clear from Fig. 9, the X-ray properties of WR In this paper we have presented the first X-ray survey of galaxies in relation to their LB are substantially different WR galaxies. A total of seven systems have been detected, from those of nearby starburst galaxies in the Read et al. with X-ray upper limits for seven more. The detected WR (1997) sample. We speculate that the reason for this is that galaxies had X-ray luminosities in the range the star-forming period in WR galaxies is of shorter dura- 4Å1038–2Å1041 erg s21, and the X-ray luminosity scales f 1.56 tion than for ‘normal’ starbursts. As suggested by Vacca & with the blue luminosity (LB), with LX LB . The X-ray Conti (1992), for a galaxy to be seen as a WR galaxy, at spectra were typically quite soft, with kT#0.3–1.0 keV for some point in its evolution the star-forming period must be a single-temperature Raymond–Smith model, with more short (O106 yr). A consequence of this is that WR galaxies luminous WR galaxies having hotter spectra. An important will then lie away from ‘normal’ starburst galaxies in the result is that the X-ray luminosities of WR galaxies are

LX :LB plane, which is what is seen. Thus, the X-ray proper- substantially larger than those of normal galaxies with the ties of WR galaxies lend strong support to the notion that same LB. A strong correlation is also found with the FIR WR galaxies are very young starbursts in which the star- luminosity, Although in this cae the WR galaxies do not forming period was of short duration. It is also worth noting have larger X-ray luminosities for the same FIR luminosity. that broadly similar X-ray properties to those of WR We have investigated other correlations, and find a strong galaxies have also been found in the extragalactic H II relationship between the X-ray luminosity and the number region NGC 5408 (Fabian & Ward 1993), who interpreted of Lyman continuum photons NLyc. their results using a similar model to that presented here. In general, we prefer the fits produced when only detec- We note that while we do not see any significant extended tions were included. The inclusion of the X-ray upper limits emision from any WR galaxies other than NGC 4861, this in regression fits typically led to higher values of PCOX, and does not compromise the superbubble picture developed occasionally to some different correlations. It is possible here. As we are looking at young starbursts, the superbubble that this is because we have systematically underestimated has probably not grown to a sufficient extent to be seen as the absorbing column used to derive the X-ray upper limits. extended by the ROSAT PSPC. Using a similar sort of analy- To investigate this further will require observations of a sis to that in equation (10), the radius of the superbubble is larger sample of WR galaxies. given by (Weaver et al. 1977, Martin & Kennicutt 1995) We have discussed the various possibilities for the origin 2 of the X-ray emission from WR galaxies, and have con- R(pc):66E˙ 1/5n 1/5t3/5. (11) 38 0 6 cluded that a superbubble origin is the most likely. We take : For an age in the range 4–6 Myr, and assuming n0 10 this as strong evidence that WR galaxies are indeed young cm23, we find that the radius of the superbubble is of order starbursts, and have presented simple calculations to illu- 300–600 pc, which is smaller that the ROSAT PSPC resolu- strate the expected X-ray evolution of young starbursts as tion at the typical distances of WR galaxies. However, the they form superbubbles. As the X-ray properties and the ROSAT HRI has a detect cell resolution of 5 arcsec, which presence of a large number of WR stars are consistent with

© 1998 RAS, MNRAS 294, 523–547 546 I. R. Stevens and D. K. Strickland a model in which the starburst occurred over a short period Armus L., Heckman T. M., Miley G. K., 1990, ApJ, 364, 471 of time, this means that WR galaxies may be especially Armus L., Heckman T. M., Weaver K. A., Lehnert M. D., 1995, important in helping to understand the evolution of star- ApJ, 445, 666 bursts, as they may represent a reasonable coeval sample. Arnault P. H., Kunth D., Schild M., 1989, A&A, 224, 73 Barth C. S., Cepa J., Vilchez J. M., Dottori H. A., 1994, AJ, 108, Conti et al. (1996) have made the interesting connection 2069 between the UV spectra of the WR galaxy NGC 1741 and Bauer F., Bregman J. N., 1996, ApJ, 457, 382 the spectra of luminous star-forming galaxies at high red- Beck S. C., Turner J. L., Ho P. T. P., Lacy J. H., Kelly shift. Further, they suggest that the ‘burst mode’ of star D. M., 1996, ApJ, 457, 610 formation (i.e. short-duration bursts of star formation) that Bowen D. V., Osmer S. J., Blades J. C., Tytler D., Cottrell L., Fan we see in WR galaxies could be an applicable model for the X.-M., Lanzetta K. M., 1994, AJ, 107, 461 star formation at high z. In the light of our investigation of Briel U. G. et al., 1995, The ROSAT Users’ Handbook. MPE, the X-ray properties of nearby WR galaxies, it would be very Garching interesting to extend our X-ray studies out to larger z. Brodie J. P., Bowyer S., McCarthy P., 1985, ApJ, 293, L59 There are several other directions of study to be pursued Calzetti D., Bohlin R. C., Kinney A. L., Storchi-Bergmann T., Downloaded from https://academic.oup.com/mnras/article/294/4/523/1025955 by guest on 26 September 2021 Heckman T. M., 1995, ApJ, 443, 136 in our X-ray studies of WR galaxies. One obvious path Chlebowski T., Harnden F. R., Sciortino S., 1989, ApJ, 341, 427 would be to extend the X-ray survey work presented here to Conti P. S., 1991, ApJ, 377, 115 include additional WR galaxies, from the catalogue of Conti Conti P. S., Vacca W. D., 1994, ApJ, 423, L97 (1991) as well as from Masegosa et al. (1991) and Contini et Conti P. S., Leitherer C., Vacca W. D., 1996, ApJ, 461, L87 al. (1995). With the resultant higher spatial resolution, Contini T., Davoust E., Consid`ere S., 1995, A&A, 303, 440 observations with the ROSAT HRI will be important to Corbin M. R., Korista K. T., Vacca W. D., 1993, AJ, 105, 1313 constrain the size of the superbubble region (Section 7.6). David L. P., Jones C., Forman W., 1992, ApJ, 388, 82 ASCA observations of WR galaxies will also be productive, Devereux N. A., Eales S., 1989, ApJ, 340, 708 allowing a more reliable determination of metallicities. A Dinnerstein H. L., Shields G. A., 1986, ApJ, 311, 45 systematic investigation of the X-ray properties of the Dottori H, Cepa J., Vilchez J., Barth C. S., 1994, A&A, 283, 753 parent sample of WR galaxies, i.e. H II or emission-line Fabbiano G., Feigelson E., Zamorani G., 19982, ApJ, 256, 397 galaxies, would also be very productive (Terlevich et al. Fabbiano G., Heckman T., Keel W. C., 1990, ApJ, 355, 442 1991), as well as a comparison of properties with a larger Fabbiano G., Kim D.-W., Trinchieri G., 1992, ApJS, 80, 531 sample of classical starbursts. The X-ray survey work clearly Fabian A. C., Terlevich R., 1996, MNRAS, 280, L5 needs to be supported by the determination of relevant Fabian A. C., Ward M. J., 1993, MNRAS, 263, L51 parameters at other wavelengths. Theoretical calculations Feigelson E., Nelson P. I., 1985, ApJ, 293, 192 of the evolution of superbubbles will be important, when Forman W., Jones C., Tucker W., 1985, ApJ, 293, 102 coupled with realistic models for the cluster evolution and Fourniol N., Pakull M. 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