The Nuclear Accretion History of Red Galaxies

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We use an X-ray stacking technique to investigate the evolution of the mean X-ray luminosity of 21,000 red galaxies at 0.3 < z < 0.9 as a function of their stellar mass and redshift. The red galaxies are selected from the 9.3 deg² Bootes field of the NOAO Deep Wide-Field Survey (NDWFS). The mean X-ray luminosity is an order of magnitude larger than we would expect from stellar sources alone and therefore must be primarily due to AGN emission. The X-ray luminosities (Lₓ 10⁴1 ergs/s) imply that these must be sources with relatively low accretion rates and/or accretion efficiencies onto their central super-massive black hole (SMBH). The mean X-ray luminosity increases significantly as a function of optical luminosity and stellar mass, implying that more massive galaxies have higher nuclear accretion rates than lower mass galaxies. We calculate that the mean X-ray luminosity evolves as (1+z)³.3+/-1.5. This suggests a global decline in the mean AGN activity of normal early-type galaxies from z 1 to the present. If accreting at typical AGN efficiencies, SMBHs within red galaxies accreted an insignificant proportion of their mass in this time.