Two channels of supermassive black hole growth as seen on the galaxies mass-size plane

TL;DR

This study examines how supermassive black hole masses vary with host galaxy properties, revealing different growth regimes below and above a critical galaxy mass, and highlighting the complex, non-universal nature of black hole-galaxy scaling relations.

Contribution

It identifies a mass-dependent change in black hole growth behavior and proposes a modified relation for massive galaxies, advancing understanding of black hole and galaxy co-evolution.

Findings

Black hole mass variation aligns with the Mbh - sigma relation for galaxies below 2x10^11 Msun.

A change in black hole growth behavior occurs above the critical galaxy mass.

Black holes and galaxies grow synchronously but do not follow a universal relation across all masses.

Abstract

We investigate the variation of black hole masses (Mbh) as a function of their host galaxy stellar mass (Mstar) and half-light radius (Re). We confirm that the scatter in Mbh within this plane is essentially the same as that in the Mbh - sigma relation, as expected from the negligible scatter reported in the virial mass estimator sigma_v^2=GxMstar/(5xRe). All variation in Mbh happens along lines of constant sigma_v on the (Mstar, Re) plane, or Mstar $\propto$ Re for Mstar <2x10^11 Msun. This trend is qualitatively the same as those previously reported for galaxy properties related to stellar populations, like age, metallicity, alpha enhancement, mass-to-light ratio and gas content. We find evidence for a change in the Mbh variation above the critical mass of Mcrit ~ 2x10^11 Msun. This behaviour can be explained assuming that Mbh in galaxies less massive than Mcrit can be predicted by the Mbh - sigma relation, while Mbh in more massive galaxies follow a modified relation which is also dependent on Mstar once Mstar >Mcrit. This is consistent with the scenario where the majority of galaxies grow through star formation, while the most massive galaxies undergo a sequence of dissipation-less mergers. In both channels black holes and galaxies grow synchronously, giving rise to the black hole - host galaxy scaling relations, but there is no underlying single relation that is universal across the full range of galaxy masses.