The most massive black holes on the Fundamental Plane of Black Hole Accretion

TL;DR

This study investigates the placement of brightest cluster galaxies on the black hole fundamental plane, revealing that their black hole masses are often underestimated and suggesting the existence of ultramassive black holes that challenge current galaxy-BH co-evolution models.

Contribution

It provides the first detailed analysis of BCGs on the fundamental plane, showing significant offsets and proposing the presence of ultramassive black holes exceeding 10^{10} solar masses.

Findings

BCGs follow a correlation $L_X \\propto L_R^{0.75}$, indicating advection-dominated accretion.

BCGs are offset from the fundamental plane, with underestimated black hole masses by a factor of ~10.

Approximately 40% of BCGs likely host ultramassive black holes > 10^{10} M_\\odot.

Abstract

We perform a detailed study of the location of brightest cluster galaxies (BCGs) on the fundamental plane of black hole (BH) accretion, which is an empirical correlation between a BH X-ray and radio luminosity and mass supported by theoretical models of accretion. The sample comprises 72 BCGs out to $z\sim0.3$ and with reliable nuclear X-ray and radio luminosities. These are found to correlate as $L_\mathrm{X} \propto L_\mathrm{R}^{0.75 \pm 0.08}$, favoring an advection-dominated accretion flow as the origin of the X-ray emission. BCGs are found to be on average offset from the fundamental plane such that their BH masses seem to be underestimated by the $M_\mathrm{BH}-M_\mathrm{K}$ relation a factor $\sim$10. The offset is not explained by jet synchrotron cooling and is independent of emission process or amount of cluster gas cooling. Those core-dominated BCGs are found to be more significantly offset than those with weak core radio emission. For BCGs to on average follow the fundamental plane, a large fraction ($\sim40\%$) should have BH masses $> 10^{10}$ M$_{\odot}$ and thus host ultramassive BHs. The local BH-galaxy scaling relations would not hold for these extreme objects. The possible explanations for their formation, either via a two-phase process (the BH formed first, the galaxy grows later) or as descendants of high-z seed BHs, challenge the current paradigm of a synchronized galaxy-BH growth.