Follow-up studies for the surveys in Table 1 are ongoing, and both Chandra and XMM-Newton continue to generate torrents of superb data. Thus, rapid progress on deep extragalactic X-ray surveys should continue over the next decade. Some key outstanding problems requiring further work include
The detailed cosmic history of SMBH accretion. Have X-ray surveys found the vast majority of actively accreting SMBH, or are they missing substantial numbers of Compton-thick and other X-ray weak AGN? To what degree have the complex X-ray spectra of AGN, combined with limited photon statistics, confused current estimates of obscuration and luminosity? What physical mechanisms are responsible for the observed anti-hierarchical growth of SMBH?
The nature of AGN activity in young, forming galaxies. How common are moderate-luminosity, typical AGN in the z 2-10 universe? Are these AGN feeding and growing in the same way as local AGN? What is the connection between SMBH growth and star formation in submm galaxies?
X-ray measurements of clustering and large-scale structure. What are the detailed clustering properties of X-ray selected AGN out to high redshift, and is there a dependence of AGN fueling upon large-scale environment? How do X-ray groups and clusters evolve out to high redshift, and what does this say about structure formation? How do X-ray, optical, and radio measures of clustering relate?
The X-ray properties of cosmologically distant starburst and normal galaxies. How have the X-ray source populations in these galaxies evolved over cosmic history? Is the relationship between X-ray binary production and star formation indeed universal as indicated by the correlation between X-ray luminosity and star-formation rate?
Table 1 and Figure 1 show that the deep X-ray surveys we have reviewed already cover a respectable amount of the depth versus solid angle "discovery space." However, fully answering the outstanding questions above will likely require improvement along both dimensions of this space. For example, a longer 5-10 Ms Chandra observation could reach 0.5-2 keV flux levels of 5 × 10-18 erg cm-2 s-1 while remaining confusion free and nearly photon limited near the field center. It would search for distant Compton-thick AGN, improve the spectral constraints for and understanding of faint X-ray source populations, and detect a few hundred new distant starburst and normal galaxies. Such a sensitive X-ray observation will not be possible again for 10-20 years until the launches of missions such as XEUS and Generation-X (see Figure 10).
Figure 10. 0.5-2 keV flux limit versus PSF half-power diameter (HPD) for some past (blue), present (red), and future (black) X-ray missions. The PSF HPDs for Constellation-X and XEUS are not precisely determined at present, so these missions are shown as diagonal lines (dotted for Constellation-X and solid for XEUS) at the source confusion limits for the range of PSF HPDs under consideration. The Generation-X mission is only in a preliminary stage of planning at present, and thus its observational capabilities have significant uncertainty. A 10 Ms Chandra exposure can achieve sensitivities comparable to those planned for XEUS and will provide the best available X-ray source positions for 15 yr.
An equally important approach is to survey more solid angle at 0.5-2 keV flux levels of (2-50) × 10-17 erg cm-2 s-1, where currently our understanding of the X-ray universe suffers from limited source statistics and cosmic variance. Several surveys underway specifically target this part of discovery space, such as the Extended Chandra Deep Field-South, the Extended Groth Strip, and COSMOS (see Table 1). These should substantially improve understanding of the X-ray luminosity function at high redshift, the clustering properties of X-ray selected AGN, and the evolution of X-ray groups and clusters.
Finally, a technically challenging but scientifically critical goal is to perform deep extragalactic X-ray surveys at higher energies than those reviewed here. Surveyors of the 10-100 keV band can look forward to observing directly the obscured AGN and other sources that comprise the bulk of the CXRB. The next decade of research in this field should thus be as exciting as the last.
We thank all of our colleagues for educational interactions on deep extragalactic X-ray surveys. Colleagues who specifically helped with the preparation of this review include DM Alexander, FE Bauer, H Brunner, BD Lehmer, V Mainieri, T Miyaji, P Rosati, DP Schneider, AT Steffen, and C Vignali. We acknowledge financial support from NSF CAREER award AST-9983783 and Chandra X-ray Center grants (WNB), and DLR grant 50 OR 0207 (GH).