The cosmic X-ray background (XRB) is largely due to accretion onto supermassive black holes integrated over cosmic time. Thus, extragalactic X-ray surveys offer the potential to contribute substantially to our understanding of the physics of Active Galactic Nuclei (AGN) as well as the evolution of the AGN population. Such surveys have dramatically advanced over the past four years, largely due to the flood of data from the Chandra X-ray Observatory (hereafter Chandra) and the X-ray Multi-Mirror Mission-Newton (hereafter XMM-Newton). The superb X-ray mirrors and charge-coupled device (CCD) detectors on these observatories provide
Sensitive imaging spectroscopy in the 0.5-10 keV band, with up to 50-250 times (depending upon the energy band considered) the sensitivity of previous X-ray missions. X-ray surveys have finally reached the depths needed to complement the most sensitive surveys in the radio, submillimeter, infrared, and optical bands.
X-ray source positions with accuracies of 0.3-3". These high-quality positions are essential for matching to (often faint) multiwavelength counterparts.
Large source samples allowing reliable statistical inferences to be drawn about the overall extragalactic X-ray source population. In a fairly deep Chandra or XMM-Newton observation, 100-200 sources can be detected.
The extragalactic survey capabilities of Chandra and XMM-Newton are complementary in several important respects. The sub-arcsecond imaging of Chandra provides the best possible source positions, and with long exposures Chandra can achieve the highest possible sensitivity at energies of 0.5-6 keV; unlike the case for XMM-Newton, even the deepest Chandra observations performed to date do not suffer from significant source confusion. XMM-Newton, in comparison, has a substantially larger photon collecting area than Chandra, allowing efficient X-ray spectroscopy. In addition, XMM-Newton has better high-energy response than Chandra and can carry out the deepest possible surveys from 7-10 keV. Even XMM-Newton, however, does not cover the peak of the X-ray background at 20-40 keV. Finally, the field of view for XMM-Newton is ~ 2.5 times that of Chandra.
Chandra and XMM-Newton have resolved 80-90% of the 0.5-10 keV XRB into discrete sources, extending earlier heroic efforts with missions including ROSAT, ASCA, and BeppoSAX. The main uncertainties in the precise resolved fraction are due to field-to-field cosmic variance (which leads to spatial variation in the XRB flux density) and instrumental cross-calibration limitations. With the recent advances, attention is now focused on (1) understanding the nature of the X-ray sources in detail and their implications for AGN physics, and (2) understanding the cosmological evolution of the sources and their role in galaxy evolution. In this review, we briefly describe the key Chandra and XMM-Newton extragalactic surveys to date (Section 2) and detail some of their implications for AGN physics and evolution (Section 3). In Section 3 we highlight two topics of current widespread interest: (1) X-ray constraints on the AGN content of luminous submillimeter galaxies, and (2) the demography and physics of high-redshift (z > 4) AGN as revealed by X-ray observations. We also discuss prospects for future X-ray surveys with Chandra, XMM-Newton, and upcoming missions (Section 4).
Throughout this paper, we adopt H0 = 70 km s-1 Mpc-1, M = 0.3, and = 0.7 (flat cosmology).