Discovery of a Large Population of Ultraluminous X-Ray Sources in The
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Draft version October 17, 2018 Preprint typeset using LATEX style emulateapj v. 08/13/06 DISCOVERY OF A LARGE POPULATION OF ULTRALUMINOUS X-RAY SOURCES IN THE BULGE-LESS GALAXIES NGC 337 AND ESO 501-23 Garrett Somers1, Smita Mathur1,2, Paul Martini1,2, Linda Watson3, Catherine J. Grier1 and Laura Ferrarese4 Draft version October 17, 2018 ABSTRACT We have used Chandra observations of eight bulge-less disk galaxies to identify new ultraluminous X-ray source (ULX) candidates, study their high mass X-ray binary (HMXB) population, and search for low-luminosity active galactic nuclei (AGN). We report the discovery of 16 new ULX candidates in our sample of galaxies. Eight of these are found in the star forming galaxy NGC 337, none of which are expected to be background contaminants. The HMXB luminosity function of NGC 337 implies a +4:4 −1 star formation rate (SFR) of 6.8−3:5 M yr , consistent at 1.5σ with a recent state of the art SFR determination. We also report the discovery of a bright ULX candidate (X-1) in ESO 501-23. X-1's +0:19 +7:07 20 −2 spectrum is well fit by an absorbed power law with Γ = 1:18−0:11 and NH = 1.13−1:13 × 10 cm , +0:05 −12 −1 −2 implying a 0.3-8 keV flux of 1:08−0:07 × 10 erg s cm . Its X-ray luminosity (LX ) is poorly constrained due to uncertainties in the host galaxy's distance, but we argue that its spectrum implies 40 −1 LX > 10 erg s . An optical counterpart to this object may be present in HST imaging. We also identify ULX candidates in IC 1291, PGC 3853, NGC 5964 and NGC 2805. We find no evidence of nuclear activity in the galaxies in our sample, placing a flux upper limit of 4 × 10−15 erg s−1 cm−2 on putative AGN. Additionally, the type II-P supernova SN 2011DQ in NGC 337, which exploded 2 months before our X-ray observation, is undetected. Subject headings: galaxies: nuclei − galaxies: bulge-less − X-rays: galaxies − X-rays: binaries 1. INTRODUCTION some galaxies (King et al. 2004, see also Roberts 2007), Ultraluminous X-ray sources (ULXs) are extra- have disfavored the theory that IMBHs dominate the ULX population. Other authors have suggested strong Galactic, off-nuclear X-ray objects which radiate at & 2 × 1039 erg s−1 in the 0.3-10 keV range, assuming beaming from sub-Eddington accretion onto a StMBH isotropic emission (for a recent review, see Feng & Soria (e.g. King et al. 2001), although the discovery of emission 2011). This is a remarkable luminosity, as it represents nebulae around several ULXs argues for mostly isotropic the Eddington limit for an accretion disk around the emission (Pakull & Mirioni 2003). most massive stellar mass black holes (StMBHs) known An alternative explanation for ULXs involves super- Eddington accretion onto a stellar mass black hole, per- in the Galaxy (∼ 20 M ; e.g. Remillard & McClintock 2006). Over one hundred ULX candidates have now been haps with mild beaming (e.g. King et al. 2001, Begel- identified in the local universe (Winter et al. 2006, Swartz man 2002). This requires a different accretion paradigm et al. 2011), though the number with well-sampled spec- than the classic thin disk models (Shakura & Sunyaev tra remains low. The most natural explanation for these 1973), such as the class of optically thick, slim disk mod- objects are accreting black holes with masses in the range els (e.g. Abramowicz et al. 1988, Watari 2000, Pouta- 2 5 nen et al. 2007), which can radiate at several times Ed- 10 − 10 M , often referred to as intermediate mass black holes (IMBHs; Colbert & Mushotzky 1999). This dington. This may result in a previously unknown black explanation gained popularity because it does not vi- hole binary (BHB) accretion state dubbed the ultralu- olate the Eddington limit, and explains ULXs in the minous state (Gladstone et al. 2009) which may charac- well known framework of black hole binaries. Indeed, terize StMBH binaries for a fraction of their lifetime. In arXiv:1304.0447v1 [astro-ph.CO] 1 Apr 2013 early investigations of ULXs found evidence of cool accre- this frame work, ULXs would represent the high-mass tion disks and, in some cases, quasi-periodic oscillations end of the high mass X-ray binary (HMXB) distribu- (QPOs) in their spectra, both indicative of IMBH bina- tion, radiating in their most extreme state. This idea ries (see Miller & Colbert 2004 for a review). In particu- is supported by the discovery of luminous blue stellar lar, the object ESO 243-49 HLX-1 is one of the strongest counterparts to several ULXs, a necessary component of known IMBH candidates (Farrell et al. 2009). However, a HMXB (Liu et al. 2004, Levan & Goad 2008, Jonker recent arguments, such as the unphysically large amount et al. 2012). Furthermore, ULXs are preferentially found of mass required to reproduce the ULX populations in in regions of recent star formation and low metallicity (Kaaret 2005, Swartz et al. 2009, Pakull & Mirioni 2002), 1 Department of Astronomy, The Ohio State University, 140 W. likely nurseries of massive StMBHs and binary compan- 18th Ave, Columbus, OH 43210; [email protected] ions (Belczynski et al. 2010). Only dynamical mass mea- 2 Center for Cosmology and Astroparticle Physics, The Ohio surements can definitively confirm the nature of these State University, 191 W Woodruff Avenue, Columbus, OH 43210, USA mysterious objects, so for now the controversy persists. 3 Harvard-Smithsonian Center for Astrophysics, 60 Garden If some ULXs contain IMBHs, their study may pro- Street, Cambridge, MA 02138, USA vide insight into the formation of super massive black 4 Hertzberg Institute of Astrophysics, 5071 West Saanich Road, holes (SMBH), which likely grow from intermediate mass Victoria, BC, V9E 2E7, Canada 2 seeds. The relationship between IMBHs and SMBHs is The observations were undertaken with the Chan- of great importance to the study of galaxy evolution, as dra X-ray Observatory (Weisskopf et al. 2000) Advanced observations of active galactic nuclei (AGN) and their CCD Imaging Spectrometer (ACIS) between November host environments provide strong evidence that galaxies 2010 and January 2012. Each galaxy was imaged for 9.93 and their central black holes co-evolve. In particular, the ks using the ACIS S-array chips S2 through S4, with the MBH{σBulge relationship and the MBH{M Bulge relation- galaxy centered on the back illuminated S3 chip. Fol- ship (Ferrarese & Merritt 2000, Gebhardt et al. 2000, lowing the observations, Chandra X-ray FITS files were H¨aring& Rix 2004) imply that the evolutionary history downloaded from the Chandra Data Archive and ana- of a galaxy and its central massive black hole may be con- lyzed with the Chandra Interactive Analysis of Obser- nected. Since these correlations are fundamental to our vations suite (CIAO; Fruscione et al. 2006) version 3.4. understanding of hierarchical galaxy formation and evo- Following the method of Ghosh et al. (2008), we pro- lution in the ΛCDM universe (e.g. Menci et al. 2004), the duced level 2 event files from the acquired level 1 event full parameter space of MBH, M Bulge, and σBulge must files by applying grade, status, and good time interval be explored. Although the MBH{σBulge relation holds filters, and limiting our observations to the 0.3-8 keV in many mass regimes, it should break down in bulge- range. We searched for contaminating background flares less galaxies if the formation of a SMBH is connected to by binning the exposure of each chip into 30 time bins the presence of a bulge. Yet in some cases, SMBHs have (∼ 300s each). If the count rate of a bin deviated from been identified in bulge-less galaxies by their X-ray AGN the mean by more than 10%, that bin was excluded from signatures (Filippenko & Ho 2003, Satyapal et al. 2007, further analysis. The details of each observation, includ- Shields et al. 2008, Ghosh 2009, Secrest et al. 2012, Araya ing the background flare-subtracted exposure times, are Salvo et al. 2012). This has led some to hypothesize that listed in Table 2. SMBHs in late type galaxies are formed through differ- Next, we used the CIAO task Wavdetect on each ent processes than black holes associated with spheroidal chip to identify potential X-ray objects. At the location systems (Kormendy & Bender 2011). Bulge-less galax- of each candidate, counts were extracted from a 95% en- ies therefore provide a crucial laboratory for studying ergy enclosed circle, 2.300 for on-axis point sources, and galaxy growth in a regime where our understanding of larger for off-axis point sources. The background con- SMBH/galaxy co-evolution may differ from the standard tribution was removed using an annulus with an inner model (Mathur et al. 2012). Although statistics about (outer) radius of 2× (5×) the source extraction circle's the prevalence of bulge-less AGN are beginning to ac- radius. For sources that lay on the edge of the CCD, a cumulate, more late type galaxies must be observed to half annulus was used to determine the background and probe the low mass end of the MBH{σBulge relation. the effects of off-CCD dithering were taken into account. In this paper we discuss Chandra observations of eight If another detected source fell within the annulus, it was nearby, late-type galaxies (see Table 1) selected from excised before the count rate was determined. Wavde- among the twenty objects studied in Watson et al.