Measuring Gas Accretion and Angular Momentum near Simulated Supermassive Black Holes

TL;DR

This study uses high-resolution cosmological simulations to analyze gas and angular momentum transport near supermassive black holes, revealing chaotic, stochastic accretion processes that influence black hole spin evolution.

Contribution

It provides detailed insights into gas dynamics and angular momentum fluctuations in the circumnuclear regions without galaxy mergers or AGN feedback, using state-of-the-art simulations.

Findings

Gas mass fluctuations follow a power-law spectrum with slope -1.

Gas disk angular momentum changes direction on sub-Myr timescales.

Infalling gas clumps may significantly affect SMBH spin evolution.

Abstract

Using cosmological simulations with a dynamic range in excess of 10 million, we study the transport of gas mass and angular momentum through the circumnuclear region of a disk galaxy containing a supermassive black hole (SMBH). The simulations follow fueling over relatively quiescent phases of the galaxy's evolution (no mergers) and without feedback from active galactic nuclei (AGNs), as part of the first stage of using state-of-the-art, high-resolution cosmological simulations to model galaxy and black hole co-evolution. We present results from simulations at different redshifts (z=6, 4, and 3) and three different black hole masses (30 million, 90 million, and 300 million solar masses; at z=4), as well as a simulation including a prescription that approximates optically thick cooling in the densest regions. The interior gas mass throughout the circumnuclear disk shows transient and chaotic behavior as a function of time. The Fourier transform of the interior gas mass follows a power law with slope -1 throughout the region, indicating that, in the absence of the effects of galaxy mergers and AGN feedback, mass fluctuations are stochastic with no preferred timescale for accretion over the duration of each simulation (~ 1-2 Myr). The angular momentum of the gas disk changes direction relative to the disk on kiloparsec scales over timescales less than 1 Myr, reflecting the chaotic and transient gas dynamics of the circumnuclear region. Infalling clumps of gas, which are driven inward as a result of the dynamical state of the circumnuclear disk, may play an important role in determining the spin evolution of an SMBH, as has been suggested in stochastic accretion scenarios.