The ACS Virgo Cluster Survey. III.ChandraandHubble Space TelescopeObservations 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, the giant elliptical galaxy near the dynamical center of the Virgo Cluster. These images—with a total exposure time of 154 ks—are the deepest X-ray observations yet obtained of 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 the Hubble Space Telescope (HST) (as part of the ACS Virgo Cluster Survey) to examine the connection between LMXBs and globular clusters in M87. Of the 1688 globular clusters in our catalog, fX = 3.6% ± 0.5% contain an LMXB. Dividing the globular cluster sample by metallicity, we find that the metal-rich clusters are 3 ± 1 times more likely to harbor an LMXB than their metal-poor counterparts. In agreement with previous findings for other galaxies based on smaller LMXB samples, we find the efficiency of LMXB formation to scale with both cluster metallicity Z and luminosity, in the sense that brighter, more metal-rich clusters are more likely to contain an LMXB. For the first time, however, we are able to demonstrate that the probability pX that a given cluster will contain an 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 ρ0 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 pX is then pX ∝ Γρ(Z/Z☉)0.33±0.1. We argue that if the form of pX 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 present a critical examination of the LMXB luminosity function in M87 and reexamine the published LMXB luminosity functions for M49 and NGC 4697. We find no compelling evidence for a break in the luminosity distribution of resolved X-ray point sources in any of these galaxies. Instead, the LMXB luminosity function in all three galaxies is well described by a power law with an upper cutoff at LX ~ 1039 ergs s-1.