4. SOME FUTURE PROSPECTS

4.1. Future Prospects for Chandra and XMM-Newton

Future prospects for learning more about AGN physics and evolution via X-ray surveys appear wonderful! Follow-up studies for most of the 40 surveys listed in Table 1 and Table 2 are ongoing, and many exciting results are thus guaranteed even if no more X-ray data are taken. Fortunately, however, both Chandra and XMM-Newton continue to generate torrents of superb new data that can provide even more impressive advances.

Where can the capabilities of Chandra and XMM-Newton be best applied in future observations? Figure 12 presents one useful way of thinking about this issue, via a plot of 0.5-2 keV flux limit versus solid angle for selected X-ray surveys. Key parts of this diagram remain to be explored. For example, very little solid angle has been surveyed at 0.5-2 keV flux levels of (2-20) × 10^-17 erg cm^-2 s^-1, and thus our understanding of the X-ray universe at these flux levels suffers from limited source statistics and likely cosmic variance. These flux levels are below the XMM-Newton confusion limit, and thus multiple 0.25-2 Ms Chandra observations are required. Specific science goals that can be advanced with this approach include (1) pinning down the X-ray luminosity function of moderate-luminosity AGN at z 2-6, (2) tracing AGN clustering out to high redshift; this is ideally done with contiguous, deep coverage, and (3) measuring the evolution and properties of groups and low-luminosity clusters out to z 1. Figure 13a depicts the ongoing Extended Chandra Deep Field-South survey, which has been guided by the philosophy above. It will cover a contiguous ~ 1/4 deg² area at a 0.5-2 keV flux level of (1-2) × 10^-16 erg cm^-2 s^-1, and it should generate 400 new AGN (in addition to the 300 already known in the CDF-S). Almost all of these will have superb HST imaging and multiwavelength coverage.

Figure 12. A selection of extragalactic X-ray surveys in the 0.5-2 keV flux limit versus solid angle, , plane. Shown are the ROSAT All-Sky Survey (RASS), the Einstein Extended Medium-Sensitivity Survey (EMSS), the ROSAT International X-ray/Optical Survey (RIXOS), the XMM-Newton Serendipitous Surveys (XMM Bright, XMM Medium, XMM Faint), the Chandra Multiwavelength Project (ChaMP), the ROSAT Ultra Deep Survey (ROSAT UDS), the 100 ks XMM-Newton survey of the Lockman Hole (XMM LH), Chandra 100 ks surveys, and Chandra 2 Ms surveys (i.e., the CDF-N). Although each of the surveys shown clearly has a range of flux limits across its solid angle, we have generally shown the most sensitive flux limit. The vertical dot-dashed line shows the solid angle of the whole sky. Some key science goals achievable by extending deep Chandra surveys both wider and deeper are also listed.

An equally important guiding philosophy is to observe one field with Chandra as sensitively as possible (see Figure 12). Reaching 0.5-2 keV flux levels of 5 × 10^-18 erg cm^-2 s^-1 is entirely feasible; Chandra could remain nearly photon limited near the field center (see Figure 13b), and source confusion is unlikely even for source densities exceeding 100,000 deg^-2. The total required exposure time on a field is 10 Ms. Specific science goals include (1) determining if there is a significant population of Compton-thick AGN at z 0.5-4 that has been missed to date (see Section 3.1), (2) tightening constraints on moderate-luminosity AGN at z 4-10, (3) detecting hundreds of normal and starburst galaxies out to high redshift (these should outnumber the AGN), and using their X-ray emission as an independent, extinction-free measure of star-formation rate, and (4) obtaining significant numbers of X-ray photons on the faint X-ray source populations currently known, so that X-ray spectral and variability analyses can be applied effectively to determine their nature. Such a sensitive X-ray observation will not be possible again for 10-20 years (see Figure 14)! Performing such an observation now can provide information on the sources that will be the primary targets of future missions such as XEUS and Generation-X; it will thereby bolster the science cases for these missions and aid their optimal design.

Figure 13. (a) Schematic illustration of the Extended Chandra Deep Field-South survey. The underlying grayscale image shows the current CDF-S exposure map. The four large black squares show the coverage of the upcoming four 250 ks Chandra observations. The 63 small gray squares show the coverage of HST ACS observations made by the GEMS project (the GOODS survey provides HST ACS coverage for the central region not covered by GEMS). (b) Chandra 0.5-2 keV image of the central part of the 2 Ms CDF-N centered on the HDF-N (shown in outline). Note that most ( 94%) pixels are black, indicating no background. Chandra is essentially in the photon-limited regime with a 2 Ms exposure, and it can remain in this regime even with an 10 Ms exposure (for 0.5-2 keV sources near the field center).