3.2. Recent X-ray Results on AGN Evolution

Optical studies of AGN evolution have typically focused on luminous quasars. These have been known to evolve strongly with redshift since ~ 1968, having a comoving space density at z 2 that is 100 times higher than at z 0. Figure 6a shows optical luminosity functions in 6 redshift "shells" spanning z = 0.40-2.10 for ~ 16,800 luminous AGN from the 2dF and 6dF surveys. Clear positive evolution with redshift is observed, and pure luminosity evolution (PLE) models provide an acceptable fit to these data. New optical AGN surveys, such as COMBO-17, have recently discovered significant numbers of moderate-luminosity AGN (with M_B > -23) at z 1-4, allowing investigation of their evolution. As for luminous quasars, the AGN found in these surveys also appear to peak in comoving space density at z 2. Both PLE and pure density evolution (PDE) models can acceptably fit the COMBO-17 data alone. Although a systematic combination of the COMBO-17 data with a large sample of higher-luminosity AGN has yet to be published, there are hints that the redshift at which the comoving space density peaks is smaller at lower luminosities.

Figure 6. (a) Optical luminosity functions in 6 redshift "shells" spanning z = 0.40-2.10 for ~ 16,800 luminous AGN from the 2dF and 6dF surveys. Note the clear positive evolution with increasing redshift at high luminosity (i.e., the comoving number density of luminous AGN increases with redshift from z = 0.40-2.10). From S.M. Croom, R.J. Smith, B.J. Boyle, et al., 2004, MNRAS, in press (astro-ph/0403040). (b) X-ray (2-8 keV) luminosity functions in two redshift "shells" (as labeled) for moderate-to-high luminosity AGN from the CDF-N, Abell 370, SSA13, and SSA22 Chandra surveys (see Table 1) as well as several earlier X-ray surveys. The dotted and dashed curves show the maximum possible luminosity functions after allowing for incompleteness of the follow-up spectroscopy. Note the apparent negative evolution with increasing redshift at moderate luminosity. Adapted from L.L. Cowie, G.P. Garmire, M.W. Bautz, et al., 2002, ApJ, 566, L5.

As noted in Section 3.1, the deepest X-ray surveys efficiently select AGN even fainter than those found by COMBO-17 out to high redshift (e.g., see Figure 5). X-ray AGN samples show a clear dependence of AGN evolution upon luminosity, with strong positive evolution only being seen at high luminosities (see Figure 6b). Lower luminosity AGN appear to be about as common at z 0-1 as they ever were, consistent with trend hinted at by COMBO-17. These results are robust to incompleteness of the spectroscopic follow up, although clearly they are still dependent upon the completeness of AGN X-ray selection (see Section 3.1). It appears that while the SMBH in rare, luminous AGN could grow efficiently at high redshift, the SMBH in most AGN had to wait longer to grow.

Figure 7 shows estimates of the comoving spatial density of AGN in three X-ray luminosity ranges as a function of redshift. These have been constructed utilizing a combination of Chandra, ASCA, and HEAO1 surveys at photon energies above 2 keV (with 247 AGN in total). The data are best fit with luminosity-dependent density evolution (LDDE) out to some cutoff redshift (z_c), where z_c increases with luminosity; as a result, the ratio of the peak spatial density to that at the present day is higher for more luminous AGN. At a basic level, LDDE also seems more physically plausible than PLE or PDE; simple PLE models tend to overpredict the number of 10¹⁰ M black holes in the local universe, while simple PDE models tend to overpredict the local space density of quasars.

Figure 7. The comoving spatial density of AGN in three X-ray luminosity ranges as a function of redshift, derived using data from several X-ray surveys. From Y. Ueda, M. Akiyama, K. Ohta, et al., 2003, ApJ, 598, 886.