Mass assembly and active galactic nucleus activity at z ≳ 1.5 in the dense environment of XDCP J0044.0–2033

Abstract

Context. XDCP J0044.0−2033 is the most massive galaxy cluster known at z > 1.5 and its core shows a high density of galaxies, which are experiencing mergers and hosting nuclear activity. Aims. We present a multi-wavelength study of a region of 24 kpc × 24 kpc located ∼157 kpc from the center of the cluster, for which we have photometric and spectroscopic observations. Our main goal is to investigate the environmental effects acting on the galaxies inhabiting this high-density region. Methods. We performed source identification and a photometric analysis on high-resolution Hubble Space Telescope (HST) images in the F105W, F140W, and F160W bands and a spectroscopic analysis of the near-infrared (NIR) KMOS data in H and YJ bands. In addition, we analyzed the deep Chandra ACIS-S X-ray exposure. Results. We find that the analyzed region hosts at least nine different sources –six of them confirmed to be cluster members within a narrow redshift range 1.5728 < z < 1.5762– and is therefore denser than the very central, more massive region of the cluster previously analyzed. These sources form two different complexes (Complex M and Complex N) at a projected distance of ∼13 kpc, which are undergoing merging on an estimated timescale of 10−30 Myr. One of the sources shows the presence of a broad Hα emission line and is classified as a Type-1 active galactic nucleus (AGN). This AGN is associated to an X-ray point-like source whose emission appears moderately obscured (with intrinsic absorption NH ∼ 1022 cm−2) and hosts a relatively massive black hole (BH) with a mass of MBH ∼ 107 M⊙, which is accreting with an Eddington ratio of ∼0.2. Conclusions. We conclude that the region analyzed here is consistent with being the formation site of a secondary brightest cluster galaxy (BCG). These findings, together with an in-depth analysis of the X-ray morphology of the cluster, suggest a merging scenario for the entire cluster, with two massive halos both harboring two rapidly evolving BCGs on the verge of being assembled. Our results are also consistent with the scenario in which the AGN phase in member galaxies is triggered by gas-rich mergers and plays a relevant role in the formation of the red sequence of elliptical galaxies observed in the center of local galaxy clusters.