Feedback-Dominated Accretion Flows

TL;DR

This paper introduces new two-fluid models of AGN accretion disks where feedback from embedded black holes regulates disk stability, leading to diverse steady states with implications for gravitational wave sources.

Contribution

The study presents a novel nonlocal feedback model involving embedded black holes, differing from prior local prescriptions, and explores its impact on AGN disk stability and structure.

Findings

Discovered steady-state solutions with high Toomre Q values and no star formation.

Embedded black holes can grow significantly during the AGN lifetime.

AGN disk structures differ markedly from traditional 1D models.

Abstract

We present new two-fluid models of accretion disks in active galactic nuclei (AGNs) that aim to address the long-standing problem of Toomre instability in AGN outskirts. In the spirit of earlier works by Sirko & Goodman and others, we argue that Toomre instability is eventually self-regulated via feedback produced by fragmentation and its aftermath. Unlike past semianalytic models, which (i) adopt local prescriptions to connect star formation rates to heat feedback, and (ii) assume that AGN disks self-regulate to a star-forming steady state (with Toomre parameter Q=1), we find that feedback processes are both temporally and spatially nonlocal. The accumulation of many stellar-mass black holes (BHs) embedded in AGN gas eventually displaces radiation, winds and supernovae from massive stars as the dominant feedback source. The nonlocality of feedback heating, in combination with the need for heat to efficiently mix throughout the gas, gives rise to steady-state AGN solutions that can have Q>>1 and no ongoing star formation. We find self-consistent steady-state solutions in much of the parameter space of AGN mass and accretion rate. These solutions harbor large populations of embedded compact objects that may grow in mass by factors of a few over the AGN lifetime, including into the lower and upper mass gaps. These feedback-dominated AGN disks differ significantly in structure from commonly used 1D disk models, which has broad implications for gravitational-wave-source formation inside AGNs.