2.3. AGN Redshift and Luminosity Distributions

Most spectroscopically identified AGN in deep X-ray surveys have z 0-2, although a significant minority have z 2-5. This is partly due to spectroscopic incompleteness bias for faint AGN at z 2, as is apparent by noting the systematic progression of symbol colors and sizes in Figures 4 and 6. The observed redshift distribution is concentrated at significantly lower redshifts than those predicted by pre-Chandra population-synthesis models of the CXRB (see Section 1.1). Application of photometric and other redshift estimation techniques to optically faint X-ray sources partially mitigates this discrepancy, as many of these sources are estimated to lie at z 1-4 (see Section 2.2). However, as will be described further in Section 3.1, there remains a low-redshift enhancement of AGN relative to expectations from pre-Chandra population-synthesis models. About 60% of the 2-8 keV CXRB arises at z < 1.

The observed redshift distributions from deep X-ray surveys also show significant "spikes" in narrow redshift ranges from z 0.5-2.5 (e.g., Barger et al. 2003a; Gilli et al. 2003, 2004); spikes at corresponding redshifts are also seen for sources selected at other wavelengths. These are associated with X-ray source clustering in large-scale sheet-like structures with sizes of 5-10 Mpc. These structures are likely a cause of the apparent cosmic variance mentioned in Section 1.3, and further studies of AGN clustering in deep X-ray surveys should determine if AGN fueling depends significantly upon large-scale environment.

The sources creating most of the 0.1-10 keV CXRB have X-ray luminosities of 10⁴² to a few times 10⁴⁴ erg s^-1 (see Figure 6), comparable to those of local Seyfert 1 galaxies (e.g., NGC 3783, NGC 4051, and NGC 5548) and lower luminosity quasars (e.g., I Zwicky 1). The fraction of AGN showing evidence for significant X-ray obscuration drops with increasing luminosity from 60% at 10⁴² erg s^-1 to 30% at 10⁴⁵ erg s^-1 (e.g., Ueda et al. 2003; Szokoly et al. 2004). A number of X-ray obscured quasars have been found in deep surveys (e.g., Norman et al. 2002; Stern et al. 2002b; Barger et al. 2003a; Szokoly et al. 2004). These generally have X-ray luminosities of 10⁴⁴-10⁴⁵ erg s^-1, just above those of powerful Seyfert galaxies. Some are optically faint or have limited rest-frame spectral coverage (e.g., low-order Balmer lines that can penetrate several magnitudes of extinction are not covered) so that a type 2 optical classification is difficult to prove rigorously. Obscured quasars create 10% of the 0.1-10 keV CXRB.