X-ray-emitting active galactic nuclei from z = 0.6 to 1.3 in the intermediate- and high-density environments of the ORELSE survey

Abstract

We studied active galactic nucleus (AGN) activity in 12 large-scale structures (LSSs) in the Observations of Redshift Evolution in Large-Scale Environments (ORELSE) survey, at 0.65 < z < 1.28, using a combination of Chandra observations, optical and NIR imaging and spectroscopy. We located a total of 61 AGNs that were successfully matched to optical counterparts in the LSSs. We found that AGN populations across our sample had more recently had starburst events compared to the overall galaxy populations. We find no relation between AGN activity and location within the LSSs, suggesting triggering mechanisms that depend on global environment are at most sub-dominant. To focus on differences between our AGNs, we grouped them into four sub-samples based on the spectral properties of their parents LSSs. We found one of the sub-samples, SG0023 & SC1604, stood out from the others. AGNs in this sample were disproportionately luminous. These AGNs had the most recent starburst events, in contrast to their parent populations. Additionally, both the AGNs and the overall galaxy population in SG0023 & SC1604 had the largest fraction of close kinematic pairs, which indicates a higher rate of galaxy mergers and interactions. These results suggest that major mergers are driving AGN activity in SG0023 & SC1604, while other processes are likely triggering less luminous AGNs in the rest of our sample. Additionally, minor mergers are unlikely to play a significant role, since the same conditions that lead to more major mergers should also lead to more minor mergers, which is not observed in SG0023 & SC1604.

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