X-ray Source Populations in Galaxies

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 20-30Mpc. 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 X-ray populations. The most luminous sources in these systems are the debated ULXs, which have been suggested to be ~100-1000 M  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. 1. CHANDRA: A NEW PARADIGM This review comes almost two decades after the 1989 Annual Review article on the X-ray emission from galaxies (Fabbiano 1989), and a few words on the evolution of this field are in order. In 1989, the Einstein Observatory (Giacconi et al. 1979), the first imaging X-ray telescope, had opened up the systematic study of the X-ray emission of normal galaxies. The Einstein images, in the ~0.3 – 4 keV range, with resolutions of ~5’’ and ~45’’ (see the Einstein Catalog and Atlas of Galaxies, Fabbiano, Kim & Trinchieri 1992) showed extended and complex X-ray emission, and gave the first clear detection of individual luminous X-ray sources in nearby spiral galaxies, other than the Milky Way. The first ultra-luminous (non-nuclear) Xray sources (ULXs) were discovered with Einstein, and the suggestion was advanced