The most massive black holes on the Fundamental Plane of Black Hole Accretion (1710.10268v2)

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.

• The paper finds that core-dominated BCGs exhibit X-ray and radio deviations, suggesting black hole masses may be underestimated by a factor of 10.
• The methodology analyzes 72 BCGs, indicating that approximately 40% could host ultramassive black holes exceeding 10^10 solar masses.
• The results challenge conventional BH-galaxy scaling laws and call for refined mass estimates and revised feedback models in cluster environments.

The Most Massive Black Holes on the Fundamental Plane of Black Hole Accretion

The paper under discussion presents an in-depth analysis of brightest cluster galaxies (BCGs) in the context of the fundamental plane of black hole (BH) accretion. The fundamental plane is a well-established empirical correlation that connects black hole X-ray and radio luminosity to their mass, and is applicable to black holes across a wide range of masses. BCGs are of particular interest due to their position at the center of galaxy clusters and their potential to harbor ultramassive black holes.

Summary and Key Findings

This paper utilizes a sample of 72 BCGs extending out to a redshift of z ≈ 0.3, judged to have reliable nuclear X-ray and radio luminosities. Core-dominated BCGs, characterized by strong radio emissions emanating from the core, show significant deviation from the expected fundamental plane. Specifically, these BCGs present X-ray emissions more indicative of advection-dominated accretion flows (ADAFs). The observed correlation between X-ray luminosity, radio emission, and BH mass for these BCGs does not align with standard predictions unless one considers an underestimation of black hole masses by an approximate factor of 10.

The authors propose that for BCGs to align with the fundamental plane, a substantial portion—approximately 40%—must possess black hole masses exceeding 10^10 solar masses, suggesting the existence of ultramassive black holes. Such massive black holes challenge the conventional BH-galaxy scaling laws. This deviation remains consistent regardless of the emission processes and the level of cluster gas cooling, suggesting that cluster environment or gas content do not significantly impact scaling relations.

Implications and Theoretical Challenges

The implications of these findings are multifaceted. First, the presence of ultramassive black holes in BCGs implies that the traditionally understood BH-galaxy co-evolution does not encompass these extreme cases. With several BCGs hosting black holes significantly above the norm, standard models of synchronized galaxy and BH growth are brought into question. Theories suggesting that such large black holes could form through either a two-phase growth process or from the remnants of high-redshift seed black holes provide a new paradigm for understanding supermassive black hole evolution.

From a practical standpoint, the potential widespread existence of ultramassive black holes in central galaxies of clusters suggests that feedback processes in these environments may be vastly different than previously thought. Active galactic nucleus (AGN) feedback, often invoked to explain energy balances in galaxies, could function differently when ultramassive black holes are involved, affecting surrounding gas dynamics and galactic evolution.

Future Directions

Future research should focus on refining BH mass estimations and understanding the physical mechanisms that could account for the apparent masses derived from the fundamental plane. Investigating whether the high BH masses in BCGs originate from the integration of seed black holes, the effect of tidal stripping, or other environmental factors, could provide insight.

Further observational studies, especially those utilizing both higher precision BH mass measurements and high-resolution radio observations, will be crucial in exploring the validity of current scaling relations among BCGs. Moreover, theoretical models may need to incorporate additional parameters or feedback mechanisms to account for the coexistence of ultramassive black holes and their implications on cluster dynamics and evolution.

In conclusion, the paper presents compelling evidence that the largest black holes, residing in some of the most massive galaxies in the universe, do not conform neatly to expected correlations. This divergence opens the door for new explorations into the mystery of black hole formation, growth, and the intricacies of their interaction with the universe.