The Evolution and Environments of X-Ray Emitting Active Galactic Nuclei in High-redshift Large-scale Structures

Abstract

We use deep Chandra imaging and an extensive optical spectroscopy campaign on the Keck 10 m telescopes to study the properties of X-ray point sources in two isolate d X-ray-selected clusters, two superclusters, and one "supergroup" at redshifts of z ~ 0.7-0.9. We first study X-ray point sources using the statistical measure of cumulative source counts, finding that the measured overdensities are consistent with previous results, but we recommend caution in overestimating the precision of the technique. Optical spectroscopy of objects matched to X-ray point sources confirms a total of 27 active galactic nuclei (AGNs) within 5 structures, and we find that their host galaxies tend to be located away from dense cluster cores. More than 36% of the host galaxies are located in the "green valley" on a color-magnitude diagram, which suggests they are a transitional population. Based on analysis of [O II] and Hδ line strengths, the average spectral properties of the AGN host galaxies in all structures indicate either ongoing star formation or a starburst within ~1 Gyr, and that the host galaxies are younger than the average galaxy in the parent population. These results indicate a clear connection between starburst and nuclear activity. We use composite spectra of the spectroscopically confirmed members in each structure (cluster, supergroup, or supercluster) to separate them based on a measure of the overall evolutionary state of their constituent galaxies. We define structures as having more evolved populations if their average galaxy has lower EW([O II]) and EW(Hδ). The AGNs in the more evolved structures have lower rest-frame 0.5-8 keV X-ray luminosities (all below 10^(43.3) erg s^(–1)) and longer times since a starburst than those in the unevolved structures, suggesting that the peak of both star formation and AGN activity has occurred at earlier times. With the wide range of evolutionary states and time frames in the structures, we use our results to analyze the evolution of X-ray AGNs and evaluate potential triggering mechanisms.

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