The Evolution of AGN Host Galaxies: From Blue to Red and the Influence of Large-Scale Structures

We present an analysis of 109 moderate-luminosity (41:9 log L0:5 Y 8:0 keV 43:7) AGNs in the Extended Chan- dra 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 pro- vide an insubstantial amount of contamination to the galaxy light. We find that the color distribution of AGN host gal- axiesishighlydependenton(1)thestrongcolor-evolutionofluminous(MV <20:7)galaxies,and(2)theinfluenceof 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 1⁄4 0:67 and z 1⁄4 0:73, we observe a bimodality in the host galaxy colors. Galaxies hosting AGNs at z k 0:8 preferentially have bluer (rest-frame U V < 0:7) colors than their z P 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 frac- tion 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 (Se ́rsic 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 under- going a merger with timescales P1 Gyr following a starburst phase.