3.4. High-Redshift (z > 4) AGN Demography and Physics
As is apparent from Figures 5 and 10, deep X-ray surveys can detect z > 4 AGN that are 10-30 times less luminous than those found in wide-field optical AGN surveys such as the SDSS. At least in the local universe, such moderate-luminosity AGN are much more numerous and thus more representative than the rare, highly luminous quasars. Furthermore, unlike the rest-frame ultraviolet light sampled at z > 4 in ground-based AGN surveys, X-ray surveys suffer from progressively less absorption bias as higher redshifts are surveyed. At z > 4, hard 2-40 keV rest-frame X-rays are accessed; these can penetrate large column densities up to several × 1024 cm-2.
Figure 10. Observed-frame, Galactic absorption-corrected 0.5-2 keV flux versus AB1450(1+z) magnitude for z 4 AGN found both in optical and X-ray surveys (the X-ray upper limits shown are all for AGN from optical surveys). The slanted, dashed lines show the ox = -1.5 and ox = -1.8 loci at z = 4.6. Adapted from C. Vignali, W.N. Brandt, D.P. Schneider, et al., 2003, AJ, 125, 2876.
Spectroscopic follow-up of moderate-luminosity X-ray detected AGN at z > 4 is challenging, since such objects are expected to have z magnitudes of 23-26 (provided they have not "dropped out" of the z bandpass entirely). Nevertheless, significant constraints on the sky density of such objects have been set via large-telescope spectroscopy and Lyman-break selection. In the latter case, objects can be selected that either have appropriate optical/near-infrared colors to be at z > 4 or alternatively have no optical/near-infrared detections. The "bottom line" from these demographic studies in the CDF-N and CDF-S is that there are 12 AGN at z > 4 detectable in a 1-2 Ms Chandra field, and that only 4 of these have a z magnitude of < 25 (this limit on the sky density is still ~ 260 times the sky density of z > 4 quasars from the SDSS). These sky-density constraints are sufficient to rule out some pre-Chandra predictions by about an order of magnitude, and the combined X-ray and SDSS results indicate that the AGN contribution to reionization at z 6 is small.
Once high-redshift AGN have been identified, via either X-ray or optical surveys, broad-band spectral energy distribution analyses and X-ray spectral fitting can provide information on their accretion processes and environments. The currently available data, albeit limited, suggest that z > 4 AGN are accreting and growing in roughly the same way as AGN in the local universe; there is no evidence that their inner X-ray emitting regions have been affected by, for example, accretion-disk instabilities or radiation-trapping effects. Figure 10 plots X-ray versus optical flux for z > 4 AGN from both X-ray and optical surveys. The X-ray-to-optical spectral indices, ox, for these objects are consistent with those of AGN in the local universe, once luminosity effects and selection biases are taken into account. These biases and effects likely explain, for example, why the moderate-luminosity, X-ray selected AGN in Figure 10 have notably higher X-ray-to-optical flux ratios than the luminous, optically selected quasars.
Two recent X-ray spectral fitting results on z > 4 AGN are shown in Figure 11. Figure 11a shows the X-ray spectrum of the highest redshift AGN discovered thus far in the CDF-N, a low-luminosity quasar at z = 5.186. It was only possible to obtain a respectable-quality X-ray spectrum for such an object due to the 2 Ms CDF-N exposure. Spectral fitting yields a power-law photon index of = 1.8 ± 0.3, consistent with observations of similar objects at low redshift, and there is no evidence for intrinsic X-ray absorption. Figure 11b shows a "stacked" spectrum of 46 luminous radio-quiet quasars at z = 4.0-6.3 (their median redshift is z = 4.43); this spectrum has 750 counts in total. Joint fitting of the 46 individual spectra, using the Cash statistic, yields a power-law photon index ( = 1.9 ± 0.1) that is again consistent with observations at low redshift. A fairly tight limit on any intrinsic X-ray absorption of NH 9 × 1020 cm-2 is also set. The overall picture emerging, then, is that while the AGN population shows enormous changes in number density over cosmic time, individual AGN X-ray emission regions appear to be remarkably stable entities.
Figure 11. Observed-frame X-ray spectra for (a) the z = 5.186 CDF-N AGN CXOHDFN J123647.9+620941 and (b) 46 radio-quiet quasars at z = 4.0-6.3 that have been stacked together. The best-fitting power-law models with Galactic absorption are also shown; see the text for fitting results. In (a) the lower panel shows the fit residuals in units of sigma, and the vertical dashed line indicates the energy of the (undetected) 6.4 keV iron K line. Adapted from C. Vignali, F.E. Bauer, D.M. Alexander, et al., 2002, ApJ, 580, L105 and C. Vignali, W.N. Brandt, & D.P. Schneider, 2004, astro-ph/0310659.