Populations of X-ray sources in galaxies

Today's sensitive X-ray observations allow the study of populations of X-ray binaries in galaxies as distant as 20-30 Mpc. Photometric diagrams and luminosity functions applied to these populations provide a direct probe of the evolved binary component of different stellar populations. The study of the X-ray populations of E and SO 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. Though 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 X-ray binaries (HMXBs) dominate these young X-ray populations. The most luminous sources in these systems are the debated ultraluminous X-ray sources (ULXs). 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.