Off-centre supermassive black holes in bright central galaxies

TL;DR

This study shows that supermassive black holes in brightest cluster galaxies are often displaced from the center due to galaxy mergers, affecting their growth and feedback processes, with some offsets lasting billions of years.

Contribution

It combines simulations and orbital integrations to demonstrate the prevalence and longevity of SMBH offsets in BCGs caused by radial galaxy mergers.

Findings

A third of SMBHs are off-centre at present day.

Offsets can last up to 6 billion years.

Off-centre SMBHs can significantly impact galaxy evolution.

Abstract

Supermassive black holes (SMBHs) are believed to reside at the centre of massive galaxies such as brightest cluster galaxies (BCGs). However, as BCGs experienced numerous galaxy mergers throughout their history, the central BH can be significantly kicked from the central region by these dynamical encounters. By combining the Illustris-TNG300 simulations and orbital integrations, we demonstrate that mergers with satellite galaxies on radial orbits are a main driver for such BH displacements in BCGs. BHs can get ejected to distances varying between a few parsecs to hundreds of kiloparsecs. Our results clearly establish that SMBH offsets are common in BCGs and more precisely a third of our BHs are off-centred at $z=0$. This orbital offset can be sustained for up to at least 6 Gyr between $z=2$ and $z=0$ in half of our BCGs. Since the dense gas reservoirs are located in the central region of galaxies, we argue that the consequences of off-center SMBHs in BCGs are to quench any BH growth and BH feedback.