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 deg ^ { 2 } region in the Boötes 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 R -band magnitude distribution as a function of redshift to ensure 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 suggests that the X-ray emission is largely dominated by AGN rather than stellar sources .
The hardness ratio can be reproduced by either an absorbed ( { N _ { H } \approx 2 \times 10 ^ { 22 } cm ^ { -2 } } ) \Gamma =1.7 power-law source , consistent with that of a population of moderately obscured Seyfert-like AGN , or an unabsorbed \Gamma =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 \sim 1 to the present , which indicates that we are witnessing the tailing off of the accretion activity onto SMBHs in early-type galaxies since the quasar epoch .