The evolution of AGN host galaxies : from blue to red and the influence of large-scale structures.

Abstract

We present an analysis of 109 moderate-luminosity (41.9<=logL0.5-8.0keV<=43.7) AGNs in the Extended Chandra Deep Field-South survey, which is drawn from 5549 galaxies from the COMBO-17 and GEMS surveys having 0.4<=z<=1.1. These obscured or optically weak AGNs facilitate the study of their host galaxies since the AGNs provide an insubstantial amount of contamination to the galaxy light. We find that the color distribution of AGN host galaxies is highly dependent on (1) the strong color-evolution of luminous (MV<-20.7) galaxies, and (2) the influence of ~10 Mpc scale structures. When excluding galaxies within the redshift range 0.63<=z<=0.76, a regime dominated by sources in large-scale structures at z=0.67 and z=0.73, we observe a bimodality in the host galaxy colors. Galaxies hosting AGNs at z>~0.8 preferentially have bluer (rest-frame U-V<0.7) colors than their z<~0.6 counterparts (many of which fall along the red sequence). The fraction of galaxies hosting AGNs peaks in the ``green valley'' (0.5<U-V<1.0) this is primarily due to enhanced AGN activity in the redshift interval 0.63<=z<=0.76. The AGN fraction in this redshift and color interval is 12.8% (compared to its ``field'' value of 7.8%) and reaches a maximum of 14.8% at U-V~0.8. We further find that blue, bulge-dominated (Sérsic index n>2.5) galaxies have the highest fraction of AGN (21%) in our sample. We explore the scenario that the evolution of AGN hosts is driven by galaxy mergers and illustrate that an accurate assessment requires a larger area survey since only three hosts may be undergoing a merger with timescales <~1 Gyr following a starburst phase.