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Deep extragalactic surveys have been successful tools in unraveling the formation and evolution of cosmic building blocks, including galaxies, groups and clusters of galaxies, large-scale structures, and supermassive black holes (SMBH). Deep surveys act as particularly effective "time machines" since fainter objects of a given type generally lie at greater distances and therefore earlier epochs. Furthermore, deep surveys are often able to probe intrinsically less luminous and more typical objects than wide-field, shallower surveys. Finally, the fluxes of all detected objects, both faint and bright, can be summed and compared with the extragalactic background light, which provides an integral census of the emission in the corresponding wavelength range. The multitude of multiwavelength deep surveys centered around the Hubble Deep Fields (e.g., Ferguson, Dickinson & Williams 2000), for example, have demonstrated these characteristics impressively.

This review concentrates on deep extragalactic X-ray surveys in the 0.1-10 keV band accessible to imaging telescopes. Two powerful, currently active X-ray missions, NASA's Chandra X-ray Observatory (hereafter Chandra; Weisskopf et al. 2000) and ESA's X-ray Multi-Mirror Mission-Newton (hereafter XMM-Newton; Jansen et al. 2001), have executed a number of deep extragalactic X-ray observing programs, which comprise by far the most sensitive X-ray surveys ever performed. Building on previous pioneering work with the Einstein, ROSAT, ASCA, and BeppoSAX missions, these surveys resolve the majority of the 0.1-10 keV background. A substantial amount of multiwavelength follow-up work on the detected X-ray sources has also been completed. It is therefore timely to review the status and scientific results of deep extragalactic X-ray surveys. In this review, we focus on surveys reaching flux limits of at least 5 × 10-16 erg cm-2 s-1 (0.5-2 keV) or 1.5 × 10-15 erg cm-2 s-1 (2-10 keV), corresponding to Chandra or XMM-Newton exposures of gtapprox 75 ks (see Figure 1 and Table 1). The equally important wider-field, shallower X-ray surveys are not covered extensively here, although they are mentioned when they especially complement deep surveys; for example, when we are discussing the evolution of active galactic nuclei (AGN) and the growth of the SMBH that primarily power them. Some other in-depth reviews of deep extragalactic X-ray surveys and the cosmic X-ray background (CXRB) are Fabian & Barcons (1992), Hasinger & Zamorani (2000), Brandt et al. (2004b), and Gilli (2004). Rosati, Borgani & Norman (2002a) have recently reviewed X-ray galaxy cluster surveys including some of the key results from deep Chandra and XMM-Newton surveys; we shall not repeat this material in detail here.

Figure 1

Figure 1. Distributions of some well-known extragalactic surveys by Chandra (blue), XMM-Newton (green), and earlier missions (red) in the 0.5-2 keV flux limit versus solid angle, Omega, plane. Circled dots denote surveys that are contiguous. Each of the surveys shown has a range of flux limits across its solid angle; we have generally shown the most sensitive flux limit. The vertical dotted line shows the solid angle of the whole sky.

In the remainder of this section, we briefly review the history of X-ray deep-field research and describe the deepest Chandra and XMM-Newton surveys. We also discuss the observed X-ray number counts and the fraction of the 0.1-10 keV background resolved. In Section 2 we focus on the basic source types found, the observed AGN redshift and luminosity distributions, and the completeness of AGN X-ray selection. Section 3 reviews some key recent results from extragalactic X-ray surveys in targeted areas of scientific interest, and Section 4 briefly examines some outstanding problems and future prospects.

Throughout this review we shall adopt the WMAP consensus cosmology with H0 = 70 km s-1 Mpc-1, OmegaM = 0.3, and OmegaLambda = 0.7 (Spergel et al. 2003), unless otherwise noted.

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