X-ray Binaries and Star Clusters in the Antennae: Optical Cluster Counterparts

TL;DR

This study identifies and analyzes the spatial correlation between X-ray binaries and star clusters in the Antennae galaxies, revealing most are young and likely contain black hole binaries, based on optical and X-ray data comparison.

Contribution

It provides the first detailed comparison of X-ray binary locations with star cluster ages in the Antennae, suggesting black hole binaries are predominant among young clusters.

Findings

22 XRBs are coincident with star clusters, with minimal chance coincidence.

64% of coincident XRBs are in clusters aged 6 Myr or less.

Most XRBs are likely black hole binaries based on cluster ages and simulations.

Abstract

We compare the locations of 82 X-ray binaries (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 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.