Massive black holes in central cluster galaxies

TL;DR

This paper investigates how environmental factors like mergers and gas content influence the co-evolution of massive black holes and their central galaxies in clusters, explaining observed deviations in black hole scaling relations.

Contribution

It introduces a combined analytical and semi-analytical model showing that merger history and gas-poor conditions explain over-massive black holes in cluster galaxies.

Findings

MBHs in cluster galaxies are over-massive compared to standard relations.

Gas-poor mergers contribute to the over-massiveness of black holes.

Environmental effects are key to understanding black hole-galaxy co-evolution.

Abstract

We explore how the co-evolution of massive black holes (MBHs) and galaxies is affected by environmental effects, addressing in particular MBHs hosted in the central galaxies of clusters (we will refer to these galaxies in general as 'CGs'). Recently the sample of MBHs in CGs with dynamically measured masses has increased, and it has been suggested that these MBH masses (M_BH) deviate from the expected correlations with velocity dispersion (sigma) and mass of the bulge (M_bulge) of the host galaxy: MBHs in CGs appear to be `over-massive'. This discrepancy is more pronounced when considering the M_BH-sigma relation than the M_BH-M_bulge one. We show that this behavior stems from a combination of two natural factors, (i) that CGs experience more mergers involving spheroidal galaxies and their MBHs, and (ii) that such mergers are preferentially gas-poor. We use a combination of analytical and semi-analytical models to investigate the MBH-galaxy co-evolution in different environments and find that the combination of these two factors explains the trends observed in current data-sets.