The ACS Virgo Cluster Survey III. Chandra and HST Observations of Low-Mass X-Ray Binaries and Globular Clusters in M87

Abstract

The ACIS instrument on board the Chandra X-ray Observatory has been used to carry out the first systematic study of low-mass X-ray binaries (LMXBs) in M87. We identify 174 X-ray point-sources, of which ~150 are likely LMXBs. This LMXB catalog is combined with deep F475W and F850LP images taken with ACS on HST to examine the connection between LMXBs and globular clusters in M87. Of the 1688 globular clusters in our catalog, f_X = 3.6 +- 0.5% contain a LMXB and we find that the metal-rich clusters are 3 +- 1 times more likely to harbor a LMXB than their metal-poor counterparts. In agreement with previous findings for other galaxies, we find that brighter, more metal-rich clusters are more likely to contain a LMXB. For the first time, however, we are able to demonstrate that the probability, p_X, that a given cluster will contain a LMXB depends sensitively on the dynamical properties of the host cluster. Specifically, we use the HST images to measure the half-light radius, concentration index and central density, ρ_0, for each globular, and define a parameter, Γ, which is related to the tidal capture and binary-neutron star exchange rate. Our preferred form for p_X is then p_X \propto Γρ_0^{-0.42\pm0.11} (Z/Z_{\odot})^{0.33\pm0.1}. We argue that if the form of p_X is determined by dynamical processes, then the observed metallicity dependence is a consequence of an increased number of neutron stars per unit mass in metal-rich globular clusters. Finally, we find no compelling evidence for a break in the luminosity distribution of resolved X-ray point sources. Instead, the LMXB luminosity function is well described by a power law with an upper cutoff at L_X ~ 10^39 erg/s. (abridged)