Spectroscopic characterization of the protocluster of galaxies around 7C 1756+6520 at z ~ 1.4

Abstract

Aims. The aim of this paper is the spectroscopic study of 13 galaxies belonging to the field of the protocluster associated with the radio galaxy 7C 1756+6520 at z = 1.4156. In particular, we focus on the characterization of the nuclear activity.Methods. This analysis has been performed on rest-frame optical spectra taken with the Large Binocular Telescope, using the spectrograph LUCI, which is operative in the near-infrared domain. The adopted spectral coverage allowed us to observe emission lines such as Hα, Hβ, [O iii]λ 5007 Å, and [N ii]λ 6583 Å at the redshift of the central radio galaxy. We observed the central part of the protocluster, which is suitable to include the radio galaxy, several spectroscopically confirmed active galactic nuclei (AGN) belonging to the protocluster, and other objects that might be members of the protocluster. Results. For four previously identified protocluster members, we derived the redshift by detecting emission lines that have never detected before for these galaxies. We identified a new protocluster member and eight new possible protocluster members. The stacked spectrum of the galaxies in which we detected the [O iii]λ 5007 Å emission line revealed the second line of the [O iii] doublet at 4959 Å and the Hβ line, which confirms that they belong to the protocluster. By collecting all members identified so far in this work and other members from the literature, we defined 31 galaxies, including the central radio galaxy, around the redshift 1.4152 ± 0.056. This corresponds to peculiar velocities 5000 km s −1 with respect to the radio galaxy. The position versus velocity phase-space diagram suggests that three AGN of the protocluster and the central radio galaxy might be a virialized population that has been coexisting for a long time in the densest core region of this forming structure. This protocluster is characterized by a high fraction of AGN (∼23%). For one of them, AGN1317, we produced two so-called Baldwin, Phillips & Terlevich (BPT) diagrams. The high fraction of AGN and their distribution within the protocluster seem to be consistent with predictions of some theoretical models on AGN growth and feedback. These models are based on galaxy interactions and ram pressure as triggers of AGN activity. Conclusions. The high fraction of AGN belonging to the protocluster suggests that they were likely triggered at the same time, maybe by the ongoing formation of the protocluster. Observations of AGN in this protocluster and in other distant clusters will help clarifying whether the resulting high fraction of AGN is unusual or typical for such structures at high redshift. Our next step will be analyses of previously acquired high-resolution radio data of the central radio galaxy to derive information on the nature of the radio galaxy and connect it with its cosmic evolution.

Multiple AGN activity during the BCG assembly of XDCPJ0044.0-2033 at z~1.6

“…This evidence seems to suggest a crucial role of AGN in the galaxy evolution at this epoch and for these environments. The discovery of multiple AGN activity is common in proto-clusters, where large centrally over-density of AGN are frequently found (Digby-North et al 2010;Galametz et al 2010;Krishnan et al 2017;Casasola et al 2018), but up to now only few works reported multiple AGN activity in high-z galaxy cluster cores, although not at such small distances. For example, Hilton et al (2010) reported the detection of two central X-ray cluster member AGN at a projected distance of ≈90 kpc in the z = 1.46 galaxy cluster XMMXCS J2215.9-1738 and, at higher redshift (z∼2), Gobat et al (2013) found two X-ray AGN at a projected distance of ∼100 kpc in CL J1449+0856.…”