Semi-empirical models of X-ray AGN in galaxy clusters

Abstract

The realisation that all galaxies host a supermassive black hole (SMBH) at their centres had a profound impact on our current understanding of galaxy formation and evolution. These SMBH grow through the accretion of surrounding material that makes them shine as active galactic nuclei (AGN). Different mechanisms have been proposed to ignite such accretion events, however, which of them is the dominant is still debated. In particular, the environment is expected to play a crucial role as it does in galaxy evolution, e.g., galaxy properties depend on which structures of the cosmic web they inhabit. In this thesis, we use a semi-empirical (SEM) data-driven approach to study the effects of the environment on AGN triggering. The advantage of this approach is the use of state-of-the-art observations to construct realistic mock catalogues instead of using physically motivated recipes that need to be calibrated. Hence, SEM is well-suited for hypothesis testing with a reduced parameter space compared to other modelling strategies. The explicit assumption of our model is that SMBH growth is independent of the halo mass. We compare model predictions against the latest observations on the incidence of AGN in massive galaxy clusters at different cosmic times. We find that AGN activity at low redshift is suppressed in clusters with respect to low-dense regions, however, at high redshift this trend is reversed with AGN being promoted in high-density regions. Enhancement of AGN activity likely happens during infall, before galaxies lose their cold gas. We test claims of preferential activation of AGN in the outskirts of massive galaxy clusters at redshift z ∼ 1. We find that the observed projected overdensities of AGN in the outskirts of high redshift clusters could be explained by cosmic variance, but we cannot reject a physical interpretation. Processes such as ram pressure or increased interaction rates due to lower velocity dispersion could promote AGN activity in the outskirts. Future work includes comparing our results with physically motivated models (e.g., hydrodynamical simulations) and analysing AGN activity in a new cluster sample spanning 0.2 < z < 1 to refine our understanding of environmental impacts on AGN activity.

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