Probing the X-Ray Binary Populations of Star-Forming Galaxies

Abstract

X-ray binaries (XRBs) represent a subset of compact objects (e.g. neutron stars and black holes) made visible by their accretion-driven X-ray emission. Approximately 20% of all stellar-mass black holes and neutron stars are expected to pass through an XRB stage at some point in their evolution, making XRBs a crucial evolutionary step for a population gravitational waves progenitors. XRBs with low-mass vs. high-mass donor stars have different formation channels, formation rates, and distributions that are correlated with their environmental properties. In this thesis, I classify the populations of XRBs in nearby, late-type galaxies in M83 and M81 on a source-by-source basis, allowing us to directly test the accuracy of modern XRB models for the first time. Through this work, I find a possible disconnect between the modeled XRB distributions in spiral galaxies and their actual distributions, which appears to fuel a mismatch between the expected number of high-mass vs. low-mass XRBs and the populations I identify through this work. I also conduct the first comprehensive study of XRBs and their relation to both young and old compact star clusters in a broader sample of spiral galaxies, identifying a similar correlation between denser, more massive clusters and XRB formation as is seen in elliptical galaxies. This work yields, for the first time, a statistically significant correlation between X-ray luminosity and cluster mass for young XRB--hosting clusters and an anticorrelation for ancient clusters. Finally, I identify the optical counterparts of 46 ultraluminous X-ray sources across 28 late-type galaxies, contributing 8 new sources to the ongoing effort to classify the donor stars in these extreme X-ray systems.