The Luminosity Function of X‐Ray–selected Active Galactic Nuclei: Evolution of Supermassive Black Holes at High Redshift

Abstract

We present a measure of the hard (2-8 keV) X-ray luminosity function (XLF) of AGNs up to z ~ 5. At high redshifts, the wide area coverage of the Chandra Multiwavength Project is crucial to detect rare and luminous (LX > 1044 ergs s−1) AGNs. The inclusion of samples from deeper published surveys, such as the Chandra Deep Fields, allows us to span the lower LX range of the XLF. Our sample is selected from both the hard (z < 3, f2–8 keV > 6.3 × 10−16 ergs cm −2 s−1) and soft (z > 3, f0.5–2.0 keV > 1.0 × 10−16 ergs cm −2 s−1) energy band detections. Within our optical magnitude limits (r', i' < 24), we achieve an adequate level of completeness (>50%) regarding X-ray source identification (i.e., redshift). We find that the luminosity function is similar to that found in previous X-ray surveys up to z ~ 3 with an evolution dependent on both luminosity and redshift. At z > 3, there is a significant decline in the numbers of AGNs with an evolution rate similar to that found by studies of optically selected QSOs. Based on our XLF, we assess the resolved fraction of the cosmic X-ray background, the cumulative mass density of SMBHs, and the comparison of the mean accretion rate onto SMBHs and the star formation history of galaxies as a function of redshift. A coevolution scenario up to z ~ 2 is plausible, although at higher redshifts the accretion rate onto SMBHs drops more rapidly. Finally, we highlight the need for better statistics of high-redshift AGNs at z gsim 3, which is achievable with the upcoming Chandra surveys.