X-ray source populations in galaxies

TL;DR

High-resolution X-ray observations enable detailed study of diverse X-ray source populations in galaxies, revealing insights into binary evolution, galaxy type differences, and the nature of ultra-luminous X-ray sources.

Contribution

This paper reviews recent advancements in understanding X-ray source populations across different galaxy types using new observational tools.

Findings

X-ray binary populations correlate with galaxy stellar mass and star formation rate.

ULXs may be stellar black holes or intermediate-mass black holes.

Soft X-ray sources' nature remains under debate.

Abstract

Today's sensitive, high-resolution X-ray observations allow the study of populations of X-ray sources, in the luminosity range of Galactic X-ray binaries, in galaxies as distant as 20-30 Mpc. The traditional astronomical tools of photometric diagrams and luminosity functions are now applied to these populations, providing a direct probe of the evolved binary component of different stellar populations. The study of the X-ray populations of E and S0 galaxies has revamped the debate on the formation and evolution of low-mass X-ray binaries (LMXBs) and on the role of globular clusters in these processes. While overall stellar mass drives the amount of X-ray binaries in old stellar populations, the amount of sources in star forming galaxies is related to the star formation rate. Short-lived, luminous, high-mass binaries (HMXBs) dominate these young populations. The most luminous sources in these systems are the debated ULXs, which have been suggested to be ~100-1000 Msol black holes, but could alternatively include a number of binaries with stellar mass black holes. Very soft sources have also been discovered in many galaxies and their nature is currently being debated. Observations of the deep X-ray sky, and comparison with deep optical surveys, are providing the first evidence of the X-ray evolution of galaxies.