How Do Massive Black Holes Get Their Gas?

TL;DR

This study uses multi-scale simulations to explore how gas flows from galactic scales down to the vicinity of black holes, revealing diverse morphologies and instabilities that drive accretion and link star formation with AGN activity.

Contribution

It provides a comprehensive, multi-scale simulation framework that captures the complex gas dynamics and instabilities leading to black hole accretion, incorporating star formation and feedback effects.

Findings

High accretion rates (1-10 M_sun/yr) in gas-rich systems can fuel luminous quasars.

Gas morphology varies widely, including spirals, rings, and clumps, with a modest duty cycle.

Correlations between black hole accretion rate and star formation rate depend on scale and nuclear conditions.

Abstract

We use multi-scale SPH simulations to follow the inflow of gas from galactic scales to <0.1pc, where the gas begins to resemble a traditional Keplerian accretion disk. The key ingredients are gas, stars, black holes (BHs), self-gravity, star formation, and stellar feedback. We use ~100 simulations to survey a large parameter space of galaxy properties and subgrid models for the ISM physics. We generate initial conditions for our simulations of galactic nuclei (<~300pc) using galaxy scale simulations, including both major mergers and isolated bar-(un)stable disk galaxies. For sufficiently gas-rich, disk-dominated systems, a series of gravitational instabilities generates large accretion rates of up to 1-10 M_sun/yr onto the BH (at <<0.1pc); sufficient to fuel the most luminous quasars. The BH accretion rate is highly time variable, given fixed conditions at ~kpc. At >~10pc, our simulations resemble the 'bars within bars' model, but the gas exhibits diverse morphologies, including spirals, rings, clumps, and bars; their duty cycle is modest, complicating attempts to correlate BH accretion with nuclear morphology. At ~1-10pc, the gravitational potential becomes dominated by the BH and bar-like modes are no longer present. However, the gas becomes unstable to a standing, eccentric disk or a single-armed spiral mode (m=1), driving the gas to sub-pc scales. Proper treatment of this mode requires including star formation and the self-gravity of both the stars and gas. We predict correlations between BHAR and SFR at different galactic nuclei: nuclear SF is more tightly coupled to AGN activity, but correlations exist at all scales.