The AGN fraction in high-redshift protocluster candidates selected by Planck and Herschel

Abstract

ABSTRACT A complete understanding of the mass assembly history of structures in the Universe requires the study of the growth of galaxies and their supermassive black holes (SMBHs) as a function of their local environment over cosmic time. In this context, it is important to quantify the effects that the early stages of galaxy cluster development have on the growth of SMBHs. We used a sample of Herschel/SPIRE sources of ∼ 228 red and compact Planck-selected protocluster (PC) candidates to estimate the active galactic nuclei (AGNs) fraction from a large sample of galaxies within these candidates. We estimate the AGN fraction by using the mid-infrared (mid-IR) photometry provided by the WISE/AllWISE data of ∼650 counterparts at high redshifts. We created an AllWISE mid-IR colour–colour selection using a clustering machine learning algorithm and two WISE colour cuts using the 3.4 $\mu$m (W1), 4.6 $\mu$m (W2), and 12 $\mu$m (W3) passbands, to classify sources as AGN. We also compare the AGN fraction in PCs with that in the field to better understand the influence of the environment on galaxy development. We found an AGN fraction of fAGN = 0.113 ± 0.03 in PC candidates and an AGN fraction of fAGN = 0.095 ± 0.013 in the field. We also selected a subsample of ‘red’ SPIRE subsample with a higher overdensity significance, obtaining fAGN = 0.186 ± 0.044, versus fAGN = 0.037 ± 0.010 of ‘non-red sources’, consistent with higher AGN fractions for denser environments. We conclude that our results point towards a higher AGN fraction in PCs, similar to other studies.