The diverse galaxy counts in the environment of high-redshift massive black holes in Horizon-AGN

TL;DR

This study uses Horizon-AGN simulations to explore galaxy counts around high-redshift massive black holes, revealing large variance and complex environmental effects that influence galaxy observability and quasar surroundings.

Contribution

It provides the first large-scale simulation analysis of galaxy environments around high-redshift black holes, highlighting variance and feedback effects.

Findings

Massive black holes are generally in denser galaxy environments.

Significant variance exists in galaxy counts around these black holes.

AGN feedback can suppress galaxy luminosity and stellar mass, affecting observability.

Abstract

High-redshift quasars are believed to reside in highly biased regions of the Universe, where black hole (BH) growth is sustained by an enhanced number of mergers and by being at the intersection of filaments bringing fresh gas. This assumption should be supported by an enhancement of the number counts of galaxies in the field of view of quasars. While the current observations of quasar environments do not lead to a consensus on a possible excess of galaxies, the future missions JWST, WFIRST, and Euclid will provide new insights on quasar environments, and will substantially increase the number of study-cases. We are in a crucial period, where we need to both understand the current observations and predict how upcoming missions will improve our understanding of BH environments. Using the large-scale simulation Horizon-AGN, we find that statistically the most massive BHs reside in environments with the largest number counts of galaxies. However, we find a large variance in galaxy number counts, and some massive BHs do not show enhanced counts in their neighborhood. Interestingly, some massive BHs have a very close galaxy companion but no further enhancement of the galaxy number counts at larger scales, in agreement with recent observations. We find that AGN feedback in the surrounding galaxies is able to decrease their luminosity and stellar mass, and therefore to make them un-observable when using restrictive galaxy selection criteria. Radiation from the quasars can spread over large distances, which could affect the formation history of surrounding galaxies, but a careful analysis of these processes requires radiative transfer simulations.