The properties of X-ray selected AGN in protoclusters pinpointed by Enormous Lya Nebulae

Abstract

Protoclusters of galaxies are overdense regions of the Universe characterized by large gas reservoirs. Such environments make them perfect laboratories to investigate galaxy-AGN co-evolution and the growth of SMBHs. Galaxies living in such a dense regions are expected to growth efficiently their SMBH, resulting in a higher incidence of AGN than in the field. Some protoclusters exhibit extended Ly$\alpha$ nebulae in their central region, pinpointing the presence of massive gas reservoirs, but whose main powering mechanism is still debated. We aim to investigate the AGN population, and AGN enhancement, in three protoclusters at 2.3 < z < 3.2 which host enormous Ly$\alpha$ nebulae (ELANe). Additionally, we search for the presence of X-ray diffuse emission in the same region of the Ly$\alpha$ nebulae to reveal multi-phase gas in these protoclusters. We use deep (190-270 ks) Chandra observations to identify AGN among the protocluster members and perform X-ray spectral analysis to derive the properties of those sources. We compare the AGN fraction and space density with those observed in other known protoclusters and from the field environment. We find 11 X-ray detected AGN in the three protoclusters. Each structure hosts a central, X-ray powerful (log$(L_{\rm X}/{\rm erg \, s^{-1}}) \sim 45-46$), QSO, while the other X-ray sources are mostly moderately luminous (log$(L_{\rm X}/{\rm erg \, s^{-1}}) \sim 44$) and obscured Compton-Thin AGN. The fraction of AGN in our targets is comparable with estimates for other protoclusters, and significantly higher than what is found for low-redshift clusters. We also find a significant enhancement (2-4 dex) of AGN density with respect to the field and to non active galaxies in the protoclusters. Finally, we find no significant soft X-ray diffuse emissions from the nebulae, thus ruling out gravitational heating as the main powering mechanism of the ELANe.