Tracing the nuclear accretion history of the red galaxy population

We investigate the evolution of the hard X-ray luminosity of the red galaxy population using a large sample of 3316 red galaxies selected over a wide range in redshift (0.3 < z < 0.9) from a 1.4 deg² region in the Boǒ̈tes field of the NOAO Deep Wide-Field Survey (NDWFS). The red galaxies are early-type, bulge-dominated galaxies and are selected to have the same evolution-corrected, absolute A-band magnitude distribution as a function of redshift to ensure that we are tracing the evolution in the X-ray properties of a comparable optical population. Using a stacking analysis of 5 ks Chandra/ACIS observations within this field to study the X-ray emission from these red galaxies in three redshift bins, we find that the mean X-ray luminosity increases as a function of redshift. The large mean X-ray luminosity and the hardness of the mean X-ray spectrum suggest that the X-ray emission is largely dominated by active galactic nuclei (AGNs) rather than stellar sources. The hardness ratio can be reproduced by either an absorbed (N_H ≈ 2 × 10²² cm^-2) Γ = 1.7 power-law source, consistent with that of a population of moderately obscured Seyfert-like AGNs, or an unabsorbed Γ = 0.7 source, suggesting a radiatively inefficient accretion flow (e.g., an advection-dominated accretion flow). We also find that the emission from this sample of red galaxies constitutes at least 5% of the hard X-ray background. These results suggest a global decline in the mean AGN activity of normal early-type galaxies from z ∼ 1 to the present, which indicates that we are witnessing the tailing off of the accretion activity onto supermassive black holes in early-type galaxies since the quasar epoch.