Annu. Rev. Astron. Astrophys. 2006. 44: 323-366 Copyright © 2006 by Annual Reviews. All rights reserved

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, 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 ultraluminous (nonnuclear) X-ray sources (ULXs) were discovered with Einstein, and the suggestion was advanced that these sources may host > 100 M black holes, a topic still intensely debated. Hot diffuse halos were discovered in elliptical galaxies and used as a means of estimating the mass of the dark matter associated with these galaxies, but their ubiquity and properties were hotly debated. Super-winds from active star-forming galaxies (e.g., M82), an important component of the ecology of the universe, were first discovered with Einstein. All these topics are discussed in the 1989 review.

The subsequent X-ray observatories ROSAT (Truemper 1983) and ASCA (Tanaka, Inoue, Holt 1994) expanded our knowledge of the X-ray properties of galaxies (see, e.g., a review summary in Fabbiano & Kessler 2001), but did not produce the revolutionary leap originated by the first Einstein observations. The angular resolution of these missions was comparable (ROSAT) or inferior (ASCA, with 2 arcmin resolution) to that of Einstein, but the ROSAT spectral band (extending down to ~ 0.1 keV) and lower background provided a better view in some cases of the cooler X-ray components (halos and hot outflows), while the wide spectral band (~ 0.5-10 keV) and better spectral resolution of the ASCA CCD detectors allowed both the detection of emission lines in these hot plasmas and the spectral decomposition of integrated emission components (e.g., Matsushita et al. 1994). Overall, however, many of the questions raised by the Einstein discoveries remained (see Fabbiano & Kessler 2001).

It is only with Chandra's subarcsecond angular resolution (Weisskopf et al. 2000), combined with photometric capabilities commensurable with those of ASCA, that the study of normal galaxies in X rays has taken a second revolutionary leap. With Chandra, populations of individual X-ray sources, with luminosities comparable to those of Galactic X-ray binaries, can be detected at the distance of the Virgo Cluster and beyond; the emission of these sources can be separated from the diffuse emission of hot interstellar gases, both spatially and spectrally; detailed measures of the metal abundance of these gaseous components can be attempted (e.g., Martin, Kobulnicky & Heckman 2002; Soria & Wu 2002; Fabbiano et al. 2004a; Baldi et al. 2006a, b); and quiescent supermassive nuclear black holes can be studied (e.g., Fabbiano et al. 2004b, Pellegrini 2005, Soria et al. 2006).

Here, I will concentrate only on one aspect of the emission of normal galaxies, the study of their populations of discrete X-ray sources, with emphasis on compact accreting binary systems (Table 1). I avoid detailed discussions of the properties of individual nearby galaxies, which are covered in the earlier review by Fabbiano & White (2006, based on publications up to 2003). Also, I do not discuss the properties of the hot interstellar medium (ISM) and of low-level nuclear emission, which were all included in the 1989 review. The field has expanded enough since then that these topics deserve separate reviews. Most of the work discussed in this review is the result of the study of high-resolution Chandra images. Whenever relevant (for the most nearby galaxies, and the spectral and time-variability study of ULXs), I will also discuss observations with XMM-Newton (the European Space Agency X-ray telescope, which has an effective area ~ 3 times larger than Chandra, but significantly coarser angular resolution, ~ 15").

Table 1. Accretion-powered XRBs in galaxies (LX more than a few 10³⁶ erg s^-1)

LMXB	Low Mass X-ray Binaries
	Neutron Star (NS) or Black Hole (BH) + later than type A star
	Time-variable: orbital periods, flares, bursts
	Spectral/luminosity states in BH XRBs
	on average soft spectra with kT ~ 5-10 keV
	Associated with old stellar populations
	Found in the stellar field (bulges) and in Globular Clusters
	Generally believed to be long-lived: lifetimes 108-9 years
	Exception is model of Bildsten & Deloye (2004), Section 3.6
	Discussed in Section 3
HMXB	High Mass X-ray Binaries
	NS or BH + OB star
	Time-variable luminosities and spectra
	Orbital periods, outburst, rapid flaring, pulsations
	On average harder spectra than LMXBs,
	but BH binaries may have similarly soft spectra
	Associated with young stellar populations (e.g., spiral arms)
	Short-lived: lifetimes ~ 106-7 years
	Discussed in Section 4
ULX	Ultraluminous X-ray sources of debated nature
	LX > 1039 ergs s-1 (> Eddington luminosity of NS or ~ 5 M BH)
	Proposed as intermediate mass BH candidates (> 100 M)
	Tend to be found in active star-forming environments
	Discussed in Section 6
SSS	Super-Soft Sources (black body kT ~ 15-80 eV)
	Nuclear burning White Dwarf binaries
	Discussed in Section 5
QSS	Quasi-Soft Sources discovered with Chandra
	kT ~ 100-300 eV
	Some exhibit a hard spectral tail
	Nature is still debated
	Discussed in Section 5, Section 6

This review proceeds as follows: Section 2 is a short discussion of the observational and analysis approaches opened by the availability of high resolution, sensitive X-ray data; Section 3 reviews the results on the old X-ray binary population found in early-type galaxies and spiral bulges; Section 4 addresses the work on the younger X-ray source population of spiral and irregular galaxies; Section 5 and Section 6 discuss two classes of rare X-ray sources, to the understanding of which recent observations of many galaxies have contributed significantly: supersoft sources (SSSs) and ULXs; Section 7 concludes this review with a short discussion of the properties of the galaxies observed in deep X-ray surveys. Section 3, Section 4, and Section 6 are the most substantial. They all start with brief historical introductions, summarize the observational evidence that identifies the X-ray sources with X-ray binaries (XRBs), discuss the X-ray luminosity functions as a means to compare and characterize the XRB populations, address the constraints deriving from the association of the X-ray sources with stellar or other features, and conclude with reviews of the theoretical work and interpretations. Throughout this review, I try to give the reader a feeling for the evolving state of the field by highlighting the different points of view and unresolved questions.