On the hunt for ultramassive black holes in brightest cluster galaxies

TL;DR

This study examines brightest cluster galaxies' placement on the black hole fundamental plane, revealing they may host ultramassive black holes exceeding 10^10 solar masses, challenging existing mass correlations.

Contribution

It demonstrates that standard black hole-host galaxy relations underestimate black hole masses in brightest cluster galaxies, suggesting the existence of ultramassive black holes.

Findings

BCGs are offset from the fundamental plane, indicating underestimated black hole masses.

Black hole masses in BCGs may be underestimated by a factor of 10.

Many BCGs could host black holes exceeding 10^10 solar masses.

Abstract

We investigate where brightest cluster galaxies (BCGs) sit on the fundamental plane of black hole (BH) activity, an established relation between the X-ray luminosity, the radio luminosity and the mass of a BH. Our sample mostly consists of BCGs that lie at the centres of massive, strong cooling flow clusters, therefore requiring extreme mechanical feedback from their central active galactic nucleus (AGN) to offset cooling of the intracluster plasma (L_mech>10^44-45 erg/s). Based on the BH masses derived from the M_BH-sigma and M_BH-M_K correlations, we find that all of our objects are offset from the plane such that they appear to be less massive than predicted from their X-ray and radio luminosities (to more than a 99 per cent confidence level). For these objects to be consistent with the fundamental plane, the M_BH-sigma and M_BH-M_K correlations therefore seem to underestimate the BH masses of BCGs, on average by a factor of 10. Our results suggest that the standard relationships between BH mass and host galaxy properties no longer hold for these extreme galaxies. Furthermore, our results imply that if these BHs follow the fundamental plane, then many of those that lie in massive, strong cool core clusters must be ultramassive with M_BH>10^10M_sun. This rivals the largest BH masses known and has important ramifications for our understanding of the formation and evolution of BHs.