arXiv subjects

William R. Forman

Publications and source records attributed to William R. Forman.

At least 91 records · Page 5Linked to original sources

arXiv2012

The diverse hot gas content and dynamics of optically similar low-mass elliptical galaxies

The presence of hot X-ray emitting gas is ubiquitous in massive early-type galaxies. However, much less is known about the content and physical status of the hot X-ray gas in low-mass ellipticals. In the present paper we study the X-ray gas content of four low-mass elliptical galaxies using archival Chandra X-ray observations. The sample galaxies, NGC821, NGC3379, NGC4278, and NGC4697, have approximately identical K-band luminosities, and hence stellar masses, yet their X-ray appearance is strikingly different. We conclude that the unresolved emission in NGC821 and NGC3379 is built up from a multitude of faint compact objects, such as coronally active binaries and cataclysmic variables. Despite the non-detection of X-ray gas, these galaxies may host low density, and hence low luminosity, X-ray gas components, which undergo a Type Ia supernova (SN Ia) driven outflow. We detect hot X-ray gas with a temperature of kT ~ 0.35 keV in NGC4278, the component of which has a steeper surface brightness distribution than the stellar light. Within the central 50 arcsec (~3.9 kpc) the estimated gas mass is ~3 x 10^7 M_sun, implying a gas mass fraction of ~0.06%. We demonstrate that the X-ray gas exhibits a bipolar morphology in the northeast-southwest direction, indicating that it may be outflowing from the galaxy. The mass and energy budget of the outflow can be maintained by evolved stars and SNe Ia, respectively. The X-ray gas in NGC4697 has an average temperature of kT ~ 0.3 keV, and a significantly broader distribution than the stellar light. The total gas mass within 90 arcsec (~5.1 kpc) is ~2.1 x 10^8 M_sun, hence the gas mass fraction is ~0.4%. Based on the distribution and physical parameters of the X-ray gas, we conclude that it is most likely in hydrostatic equilibrium, although a subsonic outflow may be present.

astro-ph.HE↗

arXiv2012

Exploring the unusually high black hole-to-bulge mass ratios in NGC4342 and NGC4291: the asynchronous growth of bulges and black holes

We study two nearby, early-type galaxies, NGC4342 and NGC4291, that host unusually massive black holes relative to their low stellar mass. The observed black hole-to-bulge mass ratios of NGC4342 and NGC4291 are ~6.9% and ~1.9%, respectively, which significantly exceed the typical observed ratio of ~0.2%. As a consequence of the exceedingly large black hole-to-bulge mass ratios, NGC4342 and NGC4291 are ~5.1 sigma and ~3.4 sigma outliers from the M_BH - M_bulge scaling relation, respectively. In this paper, we explore the origin of the unusually high black hole-to-bulge mass ratio. Based on Chandra X-ray observations of the hot gas content of NGC4342 and NGC4291, we compute gravitating mass profiles, and conclude that both galaxies reside in massive dark matter halos, which extend well beyond the stellar light. The presence of dark matter halos around NGC4342 and NGC4291 and a deep optical image of the environment of NGC4342 indicate that tidal stripping, in which >90% of the stellar mass was lost, cannot explain the observed high black hole-to-bulge mass ratios. Therefore, we conclude that these galaxies formed with low stellar masses, implying that the bulge and black hole did not grow in tandem. We also find that the black hole mass correlates well with the properties of the dark matter halo, suggesting that dark matter halos may play a major role in regulating the growth of the supermassive black holes.

astro-ph.CO↗

arXiv2012

Chandra observations of NGC4342, an optically faint, X-ray gas-rich early-type galaxy

Chandra X-ray observations of NGC4342, a low stellar mass (M_K=-22.79 mag) early-type galaxy, show luminous, diffuse X-ray emission originating from hot gas with temperature of kT~0.6 keV. The observed 0.5-2 keV band luminosity of the diffuse X-ray emission within the D_25 ellipse is L_0.5-2keV = 2.7 x 10^39 erg/s. The hot gas has a significantly broader distribution than the stellar light, and shows strong hydrodynamic disturbances with a sharp surface brightness edge to the northeast and a trailing tail. We identify the edge as a cold front and conclude that the distorted morphology of the hot gas is produced by ram pressure as NGC4342 moves through external gas. From the thermal pressure ratios inside and outside the cold front, we estimate the velocity of NGC4342 and find that it moves supersonically (M~2.6) towards the northeast. Outside the optical extent of the galaxy we detect ~17 bright (L_0.5-8keV > 3 x 10^37 erg/s) excess X-ray point sources. The excess sources are presumably low-mass X-ray binaries (LMXBs) located in metal-poor globular clusters (GCs) in the extended dark matter halo of NGC4342. Based on the number of excess sources and the average frequency of bright LMXBs in GCs, we estimate that NGC4342 may host roughly 850-1700 GCs. In good agreement with this, optical observations hint that NGC4342 may harbor 1200 +/- 500 GCs. This number corresponds to a GC specific frequency of S_N = 19.9 +/- 8.3, which is among the largest values observed in full-size galaxies.

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arXiv2012

A Transient Sub-Eddington Black Hole X-ray Binary Candidate in the Dust Lanes of Centaurus A

We report the discovery of a bright X-ray transient, CXOU J132527.6-430023, in the nearby early-type galaxy NGC 5128. The source was first detected over the course of five Chandra observations in 2007, reaching an unabsorbed outburst luminosity of 1-2*10^38 erg/s in the 0.5-7.0 keV band before returning to quiescence. Such luminosities are possible for both stellar-mass black hole and neutron star X-ray binary transients. Here, we attempt to characterize the nature of the compact object. No counterpart has been detected in the optical or radio sky, but the proximity of the source to the dust lanes allows for the possibility of an obscured companion. The brightness of the source after a >100 fold increase in X-ray flux makes it either the first confirmed transient non-ULX black hole system in outburst to be subject to detailed spectral modeling outside the Local Group, or a bright (>10^38 erg/s) transient neutron star X-ray binary, which are very rare. Such a large increase in flux would appear to lend weight to the view that this is a black hole transient. X-ray spectral fitting of an absorbed power law yielded unphysical photon indices, while the parameters of the best-fit absorbed disc blackbody model are typical of an accreting ~10 Msol black hole in the thermally dominant state.

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arXiv2012

The Galaxy Optical Luminosity Function from the AGN and Galaxy Evolution Survey (AGES)

We present the galaxy optical luminosity function for the redshift range 0.05<z<0.75 from the AGN and Galaxy Evolution Survey (AGES), a spectroscopic survey of 7.6 sq. deg. in the Bootes field of the NOAO Deep Wide-Field Survey. Our statistical sample is comprised of 12,473 galaxies with known redshifts down to I=20.4 (AB). Our results at low redshift are consistent with those from SDSS; at higher redshift, we find strong evidence for evolution in the luminosity function, including differential evolution between blue and red galaxies. We find that the luminosity density evolves as (1+z)^(0.54+/-0.64) for red galaxies and (1+z)^(1.64+/-0.39) for blue galaxies.

astro-ph.CO↗

arXiv2011

Chandra and ROSAT observations of Abell 194: detection of an X-ray cavity and mapping the dynamics of the cluster

Based on Chandra and ROSAT observations, we investigated the nearby poor cluster Abell 194, which hosts two luminous radio galaxies, NGC547 (3C 40B) and NGC541 (3C 40A). We demonstrated the presence of a large X-ray cavity (r~34 kpc) formed by the giant southern radio lobe arising from 3C 40B in NGC547. The estimated age of the cavity is t=7.9 x 10^7 years and the total work of the AGN is 3.3 x 10^59 erg, hence the cavity power is P_cav=1.3 x 10^44 erg/s. Furthermore, in the Chandra images of NGC545 and NGC541 we detected sharp surface brightness edges, identified as merger cold fronts, and extended tails. Using the pressure ratios between inside and outside the cold fronts we estimated that the velocities of NGC545 and NGC541 correspond to Mach-numbers of M=1.0^{+0.3}_{-0.5} and M=0.9^{+0.2}_{-0.5}, respectively. The low radial velocities of these galaxies relative to the mean radial velocity of Abell 194 imply that their motion is oriented approximately in the plane of the sky. Based on these and earlier observations, we concluded that NGC545 and NGC541 are falling through the cluster, whose center is NGC547, suggesting that Abell 194 is undergoing a significant cluster merger event. Additionally, we detected 20 bright X-ray sources around NGC547 and NGC541, a surprisingly large number, since the predicted number of resolved LMXBs and CXB sources is 2.2 and 4.1, respectively. To explain the nature of additional sources, different possibilities were considered, none of which are satisfactory. We also studied the origin of X-ray emission in Minkowski's Object, and concluded that it is most likely dominated by the population of HMXBs rather than by hot diffuse ISM. Moreover, in view of the galaxy dynamics in Abell 194, we explored the possibility that the starburst in Minkowski's Object was triggered by its past interaction with NGC541, and concluded that it may be a viable path.

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arXiv2011

A combined low-radio frequency/X-ray study of galaxy groups I. Giant Metrewave Radio Telescope observations at 235 MHz and 610 MHz

We present new Giant Metrewave Radio Telescope observations at 235 MHz and 610 MHz of 18 X-ray bright galaxy groups. These observations are part of an extended project, presented here and in future papers, which combines low-frequency radio and X-ray data to investigate the interaction between central active galactic nuclei (AGN) and the intra-group medium (IGM). The radio images show a very diverse population of group-central radio sources, varying widely in size, power, morphology and spectral index. Comparison of the radio images with Chandra and XMM-Newton X-ray images shows that groups with significant substructure in the X-ray band and marginal radio emission at >= 1 GHz host low-frequency radio structures that correlate with substructures in IGM. Radio-filled X-ray cavities, the most evident form of AGN/IGM interaction in our sample, are found in half of the systems, and are typically associated with small, low- or mid-power double radio sources. Two systems, NGC5044 and NGC4636, possess multiple cavities, which are isotropically distributed around the group center, possibly due to group weather. In other systems the radio/X-ray correlations are less evident. However, the AGN/IGM interaction can manifest itself through the effects of the high-pressure medium on the morphology, spectral properties and evolution of the radio-emitting plasma. In particular, the IGM can confine fading radio lobes in old/dying radio galaxies and prevent them from dissipating quickly. Evidence for radio emission produced by former outbursts that coexist with current activity is found in six groups of the sample.

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arXiv2011

Clustering of obscured and unobscured quasars in the Bootes field: Placing rapidly growing black holes in the cosmic web

We present the first measurement of the spatial clustering of mid-infrared selected obscured and unobscured quasars, using a sample in the redshift range 0.7 < z < 1.8 selected from the 9 deg^2 Bootes multiwavelength survey. Recently the Spitzer Space Telescope and X-ray observations have revealed large populations of obscured quasars that have been inferred from models of the X-ray background and supermassive black hole evolution. To date, little is known about obscured quasar clustering, which allows us to measure the masses of their host dark matter halos and explore their role in the cosmic evolution of black holes and galaxies. In this study we use a sample of 806 mid-infrared selected quasars and ~250,000 galaxies to calculate the projected quasar-galaxy cross-correlation function w_p(R). The observed clustering yields characteristic dark matter halo masses of log (M_halo [h^-1 M_sun]) = 12.7^+0.4_-0.6 and 13.3^+0.3_-0.4 for unobscured quasars (QSO-1s) and obscured quasars (Obs-QSOs), respectively. The results for QSO-1s are in excellent agreement with previous measurements for optically-selected quasars, while we conclude that the Obs-QSOs are at least as strongly clustered as the QSO-1s. We test for the effects of photometric redshift errors on the optically-faint Obs-QSOs, and find that our method yields a robust lower limit on the clustering; photo-z errors may cause us to underestimate the clustering amplitude of the Obs-QSOs by at most ~20%. We compare our results to previous studies, and speculate on physical implications of stronger clustering for obscured quasars.

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arXiv2011

X-ray emission from the Sombrero galaxy: a galactic-scale outflow

Based on new and archival Chandra observations of the Sombrero galaxy (M 104), we study the diffuse X-ray emission in and around its massive stellar bulge. The 2-6 keV unresolved emission from the bulge region closely follows the K-band star light and most likely arises from unresolved stellar sources. At lower energies, however, the unresolved emission reaches a galactocentric radius of at least 23 kpc, significantly beyond the extent of the starlight, clearly indicating the presence of diffuse hot gas. We isolate the emission of the gas by properly accounting for the emission from unresolved stellar sources, predominantly cataclysmic variables and coronally active binaries, whose quasi-universal X-ray emissivity is recently established. We find a gas temperature of ~0.6 keV with little variation across the field of view, except for a lower temperature of ~0.3 keV along the stellar disk. We measure a total intrinsic 0.3-2 keV luminosity of ~2e39 erg/s, which is comparable to the prediction by the latest galaxy formation models for disk galaxies as massive as Sombrero. However, such numerical models do not fully account for internal feedback processes, such as nuclear feedback and stellar feedback, against accretion from the intergalactic medium. On the other hand, we find no evidence for either the nucleus or the very modest star-forming activities in the disk to be a dominant heating source for the diffuse gas. We also show that neither the expected energy released by Type Ia supernovae nor the expected mass returned by evolved stars is recovered by observations. We argue that in Sombrero a galactic-scale subsonic outflow of hot gas continuously removes much of the "missing" energy and mass input from the bulge region. The observed density and temperature distributions of such an outflow, however, continues to pose challenges to theoretical studies.

astro-ph.CO↗

arXiv2011

The Murmur of The Hidden Monster: Chandra's Decadal View of The Super-massive Black Hole in M31

The Andromeda galaxy (M31) hosts a central super-massive black hole (SMBH), known as M31$^\ast$, which is remarkable for its mass ($\sim$$10^8{\rm~M_\odot}$) and extreme radiative quiescence. Over the past decade, the Chandra X-ray observatory has pointed to the center of M31 $\sim$100 times and accumulated a total exposure of $\sim$900 ks. Based on these observations, we present an X-ray study of a highly variable source that we associate with M31$^\ast$ based on positional coincidence. We find that M31$^\ast$ remained in a quiescent state from late 1999 to 2005, exhibiting an average 0.5-8 keV luminosity $\lesssim$$10^{36}{\rm~ergs~s^{-1}}$, or only $\sim$$10^{-10}$ of its Eddington luminosity. We report the discovery of an outburst that occurred on January 6, 2006, during which M31$^\ast$ radiated at $\sim$$4.3\times10^{37}{\rm~ergs~s^{-1}}$. After the outburst, M31$^\ast$ entered a more active state that apparently lasts to the present, which is characterized by frequent flux variability around an average luminosity of $\sim$$4.8\times10^{36}{\rm~ergs~s^{-1}}$. These flux variations are similar to the X-ray flares found in the SMBH of our Galaxy (Sgr A$^\ast$), making M31$^\ast$ the second SMBH known to exhibit recurrent flares. Future coordinated X-ray/radio observations will provide useful constraints on the physical origin of the flaring emission and help rule out a possible stellar origin of the X-ray source.

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arXiv2010

Gas sloshing, cold front formation, and metal redistribution: the Virgo cluster as a quantitative test case

(abridged) We perform hydrodynamical simulations of minor-merger induced gas sloshing and the subsequent formation of cold fronts in the Virgo cluster. We show for the first time that sloshing reproduces all characteristics of the observed cold fronts quantitatively, and we suggest a third cold front at 20 kpc NW of the Virgo core. We identify several new features typical for sloshing cold fronts, most importantly a large-scale brightness asymmetry. We can trace these new features not only in Virgo, but also in other sloshing cold front clusters. By comparing synthetic and real observations, we estimate that the original minor merger event took place about 1.5 Gyr ago when a subcluster of 2-4 \times 10^13 M\odot passed the Virgo core at 100 to 400 kpc distance, where a smaller mass corresponds to a smaller pericentre distance, and vice versa. From the merger geometry, we derive the current location of the disturbing subcluster to be about 1-2 Mpc E of the Virgo core. A possible candidate is M60. Additionally, we quantify the metal redistribution by sloshing and discuss its importance. We verify that the subcluster required to produce the observed cold fronts could be completely ram pressure stripped before reaching the Virgo centre, and discuss the conditions required for this to be achieved. Finally, we demonstrate that the bow shock of a fast galaxy passing the Virgo cluster at ~ 400 kpc distance also causes sloshing and leads to very similar cold front structures. The responsible galaxy would be located about 2 Mpc north of the Virgo centre. A possible candidate is M85.

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arXiv2010

X-ray emission from the Sombrero galaxy: discrete sources

We present a study of discrete X-ray sources in and around the bulge-dominated, massive Sa galaxy, Sombrero (M104), based on new and archival Chandra observations with a total exposure of ~200 ks. With a detection limit of L_X = 1E37 erg/s and a field of view covering a galactocentric radius of ~30 kpc (11.5 arcminute), 383 sources are detected. Cross-correlation with Spitler et al.'s catalogue of Sombrero globular clusters (GCs) identified from HST/ACS observations reveals 41 X-rays sources in GCs, presumably low-mass X-ray binaries (LMXBs). We quantify the differential luminosity functions (LFs) for both the detected GC and field LMXBs, whose power-low indices (~1.1 for the GC-LF and ~1.6 for field-LF) are consistent with previous studies for elliptical galaxies. With precise sky positions of the GCs without a detected X-ray source, we further quantify, through a fluctuation analysis, the GC LF at fainter luminosities down to 1E35 erg/s. The derived index rules out a faint-end slope flatter than 1.1 at a 2 sigma significance, contrary to recent findings in several elliptical galaxies and the bulge of M31. On the other hand, the 2-6 keV unresolved emission places a tight constraint on the field LF, implying a flattened index of ~1.0 below 1E37 erg/s. We also detect 101 sources in the halo of Sombrero. The presence of these sources cannot be interpreted as galactic LMXBs whose spatial distribution empirically follows the starlight. Their number is also higher than the expected number of cosmic AGNs (52+/-11 [1 sigma]) whose surface density is constrained by deep X-ray surveys. We suggest that either the cosmic X-ray background is unusually high in the direction of Sombrero, or a distinct population of X-ray sources is present in the halo of Sombrero.

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arXiv2010

Constraining the Outburst Properties of the SMBH in Fornax A through X-ray, Infrared, and Radio Observations

Combined Spitzer, Chandra, XMM-Newton, and VLA observations of the giant radio galaxy NGC 1316 (Fornax A) show a radio jet and X-ray cavities from AGN outbursts likely triggered by a merger with a late-type galaxy at least 0.4 Gyr ago. We detect a weak nucleus with an SED typical of a low-luminosity AGN with a bolometric luminosity of 2.4x10^42 erg/s. The Spitzer IRAC and MIPS images show dust emission strongest in regions with little or no radio emission. The large infrared luminosity relative to the galaxy's K-band luminosity implies an external origin for the dust. The dust mass implies that the merger spiral galaxy had a stellar mass of 1-6x10^10 M_sun and a gas mass of 2-4x10^9 M_sun. Chandra images show a small ~15"(1.6 kpc) cavity coincident with the radio jet, while the XMM-Newton image shows two large X-ray cavities lying 320"(34.8 kpc) east and west of the nucleus, each approximately 230"(25 kpc) in radius. The radio lobes lie at radii of 14.3'(93 kpc) and 15.6'(101 kcp), more distant from the nucleus than the detected X-ray cavities. The relative morphology of the large scale 1.4 GHz and X-ray emission suggests they were products of two distinct outbursts, an earlier one creating the radio lobes and a later one producing the X-ray cavities. Alternatively, if a single outburst created both the X-ray cavities and the radio lobes, this would require that the morphology is not fully defined by the 1.4 GHz emission. For the more likely two outburst scenario, we use the buoyancy rise times to estimate an age for the more recent outburst of 0.1 Gyr and use the associated PV work done by the expanding plasma to create the X-ray cavities to estimate the outburst's energy of 10^58 ergs. The present size and location of the large radio lobes implies that the AGN outburst that created them happened ~0.4 Gyr ago and released ~5x10^58 ergs. (abridged)

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arXiv2010

Core Gas Sloshing in Abell 1644

We present an analysis of a 72 ks Chandra observation of the double cluster Abell 1644 (z=0.047). The X-ray temperatures indicate the masses are M500=2.6+/-0.4 x10^{14} h^{-1} M_sun for the northern subcluster and M500=3.1+/-0.4 x10^{14} h^{-1} M_sun for the southern, main cluster. We identify a sharp edge in the radial X-ray surface brightness of the main cluster, which we find to be a cold front, with a jump in temperature of a factor of ~3. This edge possesses a spiral morphology characteristic of core gas sloshing around the cluster potential minimum. We present observational evidence, supported by hydrodynamic simulations, that the northern subcluster is the object which initiated the core gas sloshing in the main cluster at least 700 Myr ago. We discuss reheating of the main cluster's core gas via two mechanisms brought about by the sloshing gas: first, the release of gravitational potential energy gained by the core's displacement from the potential minimum, and second, a dredging inwards of the outer, higher entropy cluster gas along finger-shaped streams. We find the available gravitational potential energy is small compared to the energy released by the cooling gas in the core.

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arXiv2009

Host galaxies, clustering, Eddington ratios, and evolution of radio, X-ray, and infrared-selected AGNs

We explore the connection between different classes of active galactic nuclei (AGNs) and the evolution of their host galaxies, by deriving host galaxy properties, clustering, and Eddington ratios of AGNs selected in the radio, X-ray, and infrared. We study a sample of 585 AGNs at 0.25 < z < 0.8 using redshifts from the AGN and Galaxy Evolution Survey (AGES) and data in the radio (WSRT 1.4 GHz), X-rays (Chandra XBootes), and mid-IR (IRAC Shallow Survey). The radio, X-ray, and IR AGN samples show modest overlap, indicating that to the flux limits of the survey, they represent largely distinct classes of AGNs. We derive host galaxy colors and luminosities, as well as Eddington ratios (lambda), for obscured or optically faint AGNs. We also measure the two-point cross-correlation between AGNs and galaxies on scales of 0.3-10 h^-1 Mpc, and derive typical dark matter halo masses. We find that: (1) radio AGNs are mainly found in luminous red galaxies, are strongly clustered (with M_halo ~ 3x10^13 h^-1 M_sun), and have very low lambda <~ 10^-3; (2) X-ray-selected AGNs are preferentially found in galaxies in the "green valley" of color-magnitude space and are clustered similarly to typical AGES galaxies (M_halo ~ 10^13 h^-1 M_sun), with 10^-3 <~ lambda <~ 1; (3) IR AGNs reside in slightly bluer, less luminous galaxies than X-ray AGNs, are weakly clustered (M_halo <~ 10^12 h^-1 M_sun), and have lambda > 10^-2. We interpret these results in terms of a simple model of AGN and galaxy evolution, whereby a "quasar" phase and the growth of the stellar bulge occurs when a galaxy's dark matter halo reaches a critical mass between ~10^12 and 10^13 M_sun. Subsequently, star formation ceases and AGN accretion shifts from radiatively efficient (optical- and IR- bright) to radiatively inefficient (optically-faint, radio-bright) modes.

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arXiv2009

The Star Formation and Nuclear Accretion Histories of Normal Galaxies in the AGES Survey

We combine IR, optical and X-ray data from the overlapping, 9.3 square degree NOAO Deep Wide-Field Survey (NDWFS), AGN and Galaxy Evolution Survey (AGES), and XBootes Survey to measure the X-ray evolution of 6146 normal galaxies as a function of absolute optical luminosity, redshift, and spectral type over the largely unexplored redshift range 0.1 < z < 0.5. Because only the closest or brightest of the galaxies are individually detected in X-rays, we use a stacking analysis to determine the mean properties of the sample. Our results suggest that X-ray emission from spectroscopically late-type galaxies is dominated by star formation, while that from early-type galaxies is dominated by a combination of hot gas and AGN emission. We find that the mean star formation and supermassive black hole accretion rate densities evolve like (1+z)^3, in agreement with the trends found for samples of bright, individually detectable starburst galaxies and AGN. Our work also corroborates the results of many previous stacking analyses of faint source populations, with improved statistics.

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arXiv2008

A Transient Black-Hole Low-Mass X-Ray Binary Candidate in Centaurus A

We report the discovery of a bright transient X-ray source, CXOU J132518.2-430304, towards Centaurus A (Cen A) using six new Chandra X-Ray Observatory observations in 2007 March--May. Between 2003 and 2007, its flux has increased by a factor of >770. The source is likely a low-mass X-ray binary in Cen A with unabsorbed 0.3-10 keV band luminosities of (2-3) x 10^{39} erg s^-1 and a transition from the steep-power law state to the thermal state during our observations. CXOU J132518.2-430304 is the most luminous X-ray source in an early-type galaxy with extensive timing information that reveals transience and a spectral state transition. Combined with its luminosity, these properties make this source one of the strongest candidates to date for containing a stellar-mass black hole in an early-type galaxy. Unless this outburst lasts many years, the rate of luminous transients in Cen A is anomalously high compared to other early-type galaxies.

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arXiv2008

Globular Clusters and X-ray Point Sources in Centaurus A (NGC 5128)

We detect 353 X-ray point sources, mostly low-mass X-ray binaries (LMXBs), in four Chandra observations of Centaurus A (NGC 5128), the nearest giant early-type galaxy, and correlate this point source population with the largest available ensemble of confirmed and likely globular clusters associated with this galaxy. Of the X-ray sources, 31 are coincident with 30 globular clusters that are confirmed members of the galaxy by radial velocity measurement (2 X-ray sources match one globular cluster within our search radius), while 1 X-ray source coincides with a globular cluster resolved by HST images. Another 36 X-ray point sources match probable, but spectroscopically unconfirmed, globular cluster candidates. The color distribution of globular clusters and cluster candidates in Cen A is bimodal, and the probability that a red, metal rich GC candidate contains an LMXB is at least 1.7 times that of a blue, metal poor one. If we consider only spectroscopically confirmed GCs, this ratio increases to ~3. We find that LMXBs appear preferentially in more luminous (massive) GCs. These two effects are independent, and the latter is likely a consequence of enhanced dynamical encounter rates in more massive clusters which have on average denser cores. The X-ray luminosity functions of the LMXBs found in GCs and of those that are unmatched with GCs reveal similar underlying populations, though there is some indication that fewer X-ray faint LMXBs are found in globular clusters than X-ray bright ones. Our results agree with previous observations of the connection of GCs and LMXBs in early-type galaxies and extend previous work on Centaurus A.

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