Measuring Gas Accretion and Angular Momentum near Simulated Supermassive
  Black Holes
  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.
