We discovered a population of black hole X-ray binaries (XRBs) in the merging Antennae galaxies. We have developed a new methodology (utilizing X-ray and optical data plus N-body simulations) to study the nature of XRBs in nearby galaxies. You can read the full paper here or read the abstract below.
We compare the locations of 82 XRBs detected in the merging Antennae galaxies by Zezas et al., based on observations taken with the Chandra X-Ray Observatory, with a catalog of optically selected star clusters presented by Whitmore et al., based on observations taken with the Hubble Space Telescope. Within the 2 sigma positional uncertainty of 0.58″, we find 22 XRBs are coincident with star clusters, where only 2-3 chance coincidences are expected. The ages of the clusters were estimated by comparing their UBVI,Halpha colors with predictions from stellar evolutionary models. We find that 14 of the 22 coincident XRBs (64%) are hosted by star clusters with ages of 6 Myr or less. Five of the XRBs are hosted by young clusters with ages 10-100 Myr, while three are hosted by intermediate age clusters with t ~ 100-300 Myr. Based on the results from recent N-body simulations, which suggest that black holes are far more likely to be retained within their parent clusters than neutron stars, we suggest that our sample consists primarily of black hole binaries with different ages.