AEGIS: The color-magnitude relation for X-ray-selected active galactic nuclei

We discuss the relationship between rest-frame color and optical luminosity for X-ray sources in the range 0.6 < z < 1.4 selected from the Chandra survey of the Extended Groth Strip. These objects are almost exclusively active galactic nuclei (AGNs). While there are a few luminous QSOs, most are relatively weak or obscured AGNs whose optical colors should be dominated by host galaxy light. The vast majority of AGN hosts at z ∼ 1 are luminous and red, with very few objects fainter than M_B = -20.5 or bluer than U - B = 0.6. This places the AGNs in a distinct region of color-magnitude space, on the "red sequence" or at the top of the "blue cloud," with many in between these two modes in galaxy color. A key stage in the evolution of massive galaxies is when star formation is quenched, resulting in a migration from the blue cloud to the red sequence. Our results are consistent with scenarios in which AGNs either cause or maintain this quenching. The large number of red-sequence AGNs implies that strong, ongoing star formation is not a necessary ingredient for AGN activity, as black hole accretion appears often to persist after star formation has been terminated.