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

TL;DR

This study identifies and analyzes 353 X-ray sources in Centaurus A, revealing their association with globular clusters, and finds that LMXBs are more common in red, massive, and dense clusters, supporting theories of dynamical formation.

Contribution

It provides the first comprehensive correlation of X-ray point sources with globular clusters in Centaurus A, highlighting the influence of cluster properties on LMXB presence.

Findings

LMXBs are preferentially found in red, metal-rich globular clusters.

More luminous (massive) globular clusters host more LMXBs.

X-ray luminosity functions are similar for cluster-associated and unassociated LMXBs.

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 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.