Black hole masses and Eddington ratios of AGNs at z < 1: Evidence of retriggering for a representative sample of X-ray-selected AGNs

We estimated black hole masses and Eddington ratios for a sample of X-ray-selected active galactic nuclei (AGNs) in the fields covered by the Great Observatory Origins Deep Survey (GOODS). The spanned ranges in redshift (0.4<z<1) and hard X-ray luminosity (1042<~LX<~4×1043 ergs s-1) allow us to study a representative subsample of the main contributors to the 2-10 keV X-ray background. Nuclear and bulge magnitudes in four bands have been measured via a two-dimensional decomposition applied to HST ACS images. Using the black hole versus bulge luminosity relation and the intrinsic nuclear emission, we derived the black hole mass and the AGN bolometric luminosity. We find in our sample that (1) the X-ray-to-optical indices are larger than in optically selected QSOs, as expected due to the X-ray selection); (2) the X-ray bolometric corrections are generally small, suggesting a decrease with the nuclear luminosity; (3) the Eddington ratios are about a factor 10 below the values found at higher redshift and luminosity; (4) the central black holes have rather large masses; and (5) at least for z<~0.8, a scarceness of black holes with mass MBH<=106 Msolar and accretion rate near the Eddington limit: this result could be ascribed to a decline in their number density, or it could suggest a substantial accretion at higher redshift (z>~1) also for these smaller black holes.