The Growth Rate of Supermassive Black Holes and Its Dependence on the Stellar Mass of Galaxies at the Present Epoch

TL;DR

This study analyzes the distribution of accretion rates onto supermassive black holes in local galaxies, revealing a universal power-law pattern weakly dependent on galaxy mass, driven mainly by stochastic nuclear processes.

Contribution

It provides the first detailed characterization of the Eddington ratio distribution in local galaxies across a wide mass range, refining previous models and confirming the stochastic nature of SMBH growth.

Findings

The Eddington ratio distribution follows a mass-independent power law with a slope of about 0.7.

The mean SMBH growth time exceeds the Universe's age but by less than an order of magnitude.

The duty cycle of active SMBHs is between 0.2% and 1%, weakly dependent on galaxy mass.

Abstract

We study the distribution of accretion rates onto supermassive black holes (SMBH) in AGNs of the local Universe ($z<0.15$) based on near-infrared and hard X-ray surveys (2MASS and Swift/BAT). Using sufficiently accurate black hole mass estimates, we reliably estimated the Eddington ratio, $\lambda_{Edd}$, for approximately half of the objects in the AGN sample; for the remaining ones we used a rougher estimate based on the correlation of $M_{BH}$ with the galaxy stellar mass $M_\ast$. We found that for a wide range of galaxy masses, $9.28<\log(M_\ast/M_\odot)<12.28$, including the most massive galaxies in the local Universe, the distribution $f(\lambda_{Edd})$ above $\log{\lambda_{Edd}}=-3$ can be described by a power law with $M_\ast$-independent parameters, declining with a characteristic slope $\approx 0.7$ up to the Eddington limit ($\log{\lambda_{Edd}}\sim 0$), where there is evidence for a break. In addition, there is evidence that at $\log{\lambda_{Edd}}<-3$ the dependence $f(\lambda_{Edd})$ has a lower slope or flattens out. The mean characteristic growth time of SMBHs at the present epoch turns out to depend weakly on the galaxy stellar mass and to exceed the lifetime of the Universe but by no more than one order of magnitude. The mean duty cycle of SMBHs (the fraction of objects with $\lambda_{Edd}>0.01$) in the local Universe also depends weakly on $M_\ast$ and is 0.2-1%. These results confirm the trends found in previous studies for the earlier Universe and refine the parameters of the dependence $f(\lambda_{Edd}|M_\ast)$ at $z<0.15$. The revealed universal (weakly dependent on the galaxy stellar mass) pattern of the dependence $f(\lambda_{Edd})$ probably stems from the fact that, at present, the episodes of mass accretion onto SMBHs are associated mainly with stochastic processes in galactic nuclei rather than with global galaxy evolution processes.