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).