Rapidly varying accretion and AGN feedback

TL;DR

This paper demonstrates that rapid, stochastic fluctuations in AGN accretion rates can cause large, long-lasting variability in AGN feedback, influencing galaxy cluster environments and explaining observed correlations.

Contribution

It introduces a Langevin-based model showing how short-term accretion variability amplifies feedback fluctuations and explains observed AGN behavior and correlations.

Findings

AGN feedback variability can be amplified by accretion fluctuations.

Rich galaxy clusters exhibit the largest and longest feedback fluctuations.

The model explains the log-normal distribution of accretion rates and duty cycle dependence.

Abstract

Accretion rates onto AGN are likely to be extremely variable on short timescales; much shorter than the typical cooling time of X-ray emitting gas in elliptical galaxies and galaxy clusters. Using the Langevin approach it is shown that, for a simple feedback system, this can induce variability in the AGN power output that is of much larger amplitude, and persists for longer timescales, than the initial fluctuations. An implication of this is that rich galaxy clusters are expected to show the largest and longest-lived fluctuations. Stochastic variations in the accretion rate also mean that the AGN injects energy across a wide range of timescales. This allows the AGN to maintain a much closer balance with its surroundings than if it was periodically activated. The possible non-linear correlation between Bondi accretion rate and jet power, found by Allen et al 2006, can be explained if the instantaneous accretion rate, scaled by jet power, varies log-normally. This explanation also implies that the duty cycle of AGN activity increases with the radiative losses of the surroundings, in qualitative agreement with Best et al 2005.