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A ROSAT Survey of Wolf–Rayet Galaxies

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Mon. Not. R. Astron. Soc. 294, 523–547 (1998) A ROSAT survey of Wolf–Rayet galaxies Ian R. Stevensw and David K. Stricklandw 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 t14–6 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 1 Å 38 Ð1 Å 41 Ð1 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 photons 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 stars 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 star-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 ultraviolet radiation from hot 40 42 Ð1 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 w 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 interstellar medium (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 galaxy 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 E40 M>), 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 metallicity. 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).
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