The Low-Mass X-Ray Binary-Globular Cluster Connection. II. NGC 4472 X-Ray Source Properties and Source Catalogs

We present the results of a Chandra/Hubble Space Telescope (HST) study of the point sources of the Virgo Cluster giant elliptical galaxy NGC 4472. We identify 144 X-ray point sources outside the nuclear region, 72 of which are located within the HST fields. The optical data show 1102 sources, of which 829 have colors consistent with being globular clusters (with only four in the restricted central 10" region). Thirty matches are found between the two lists-these are likely to be low-mass X-ray binaries (LMXBs) associated with globular clusters, while 42 of the X-ray sources have no optical counterparts to V<~25 and I<~24, indicating that they are likely to be predominantly LMXBs in the field star population with a small amount of possible contamination from background active galactic nuclei. Thus approximately 40% of the X-ray sources are in globular clusters and ~4% of the globular clusters contain X-ray sources. There is suggestive evidence that the X-ray sources located in blue globular clusters may have harder X-ray spectra than those located in red globular clusters. No statistically significant differences are found between the X-ray properties of the field sources and the X-ray properties of the sources located in globular clusters. This study, along with our previous result from Paper I in this series on the similarity of the spatial profile of the field LMXBs, globular cluster LMXBs, and the globular clusters themselves, suggests that a significant fraction of the observed LMXBs in the field may be created in a globular cluster then ejected into the field by stellar interactions; however, by comparing the results for NGC 4472 with those in several other galaxies, we find tentative evidence for a correlation between the globular cluster specific frequency and the fraction of LMXBs in globular clusters, a correlation that would be most easily explained if some of the field sources were generated in situ. We show that isolated accreting very massive black holes are unlikely to be observable with current X-ray instrumentation and that these sources hence do not contaminate the LMXB population. We discuss the possibility that several point sources near the nucleus and aligned perpendicularly to the radio jet may indicate the presence of a disk wind responsible for the low radiative efficiency observed in the nucleus of this source.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of University for Research in Astronomy, Inc., under NASA contract NAS 5-26555, and on observations made with the Chandra X-Ray Observatory.