Globular Clusters and X‐Ray Point Sources in Centaurus A (NGC 5128)

Abstract

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 (GCs) associated with this galaxy. Of the X-ray sources, 31 are coincident with 30 GCs that are confirmed members of the galaxy by radial velocity measurement (two X-ray sources match one GC within our search radius), while one X-ray source coincides with a GC resolved by Hubble Space Telescope (HST) images. Another 36 X-ray point sources match probable, but spectroscopically unconfirmed, GC candidates. The color distribution of GCs 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, although there is some indication that fewer X-ray-faint LMXBs are found in GCs 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 Cen A.