The role of AGN in the colour transformation of galaxies at redshifts z~1

Abstract

We explore the role of AGN in establishing and/or maintaining the bimodal colour distribution of galaxies by quenching their star-formation and hence, causing their transition from the blue to the red cloud. Important tests for this scenario include (i) the X-ray properties of galaxies in the transition zone between the two clouds and (ii) the incidence of AGN in post-starbursts, i.e. systems observed shortly after (<1Gyr) the termination of their star-formation. We perform these tests by combining deep Chandra observations with multiwavelength data from the AEGIS survey. Stacking the X-ray photons at the positions of galaxies (0.4<z<0.9) not individually detected at X-ray wavelengths suggests a population of obscured AGN among sources in the transition zone and in the red cloud. Their mean X-ray and mid-IR properties are consistent with moderately obscured low-luminosity AGN, Compton thick sources or a mix of both. Morphologies show that major mergers are unlikely to drive the evolution of this population but minor interactions may play a role. The incidence of obscured AGN in the red cloud (both direct detections and stacking results) suggests that BH accretion outlives the termination of the star-formation. This is also supported by our finding that post-starburst galaxies at z~0.8 and AGN are associated, in agreement with recent results at low-z. A large fraction of post-starbursts and red cloud galaxies show evidence for at least moderate levels of AGN obscuration. This implies that if AGN outflows cause the colour transformation of galaxies, then some nuclear gas and dust clouds either remain unaffected or relax to the central galaxy regions after the quenching their star-formation.