AGN activity: Self-regulation from backflow

TL;DR

This paper models and simulates the internal backflows within AGN jets, showing they can supply low angular momentum gas to the nucleus, potentially regulating AGN activity independently of star formation.

Contribution

The study introduces a new model for AGN jet backflows, validated by 2D simulations, revealing their role in self-regulating AGN activity through central gas inflows.

Findings

Backflows supply low angular momentum gas to the AGN core.

Mass inflow rates are 0.1-0.8 solar masses per year.

Inflow timescales are approximately 30-50 million years.

Abstract

We study the internal circulation within the cocoon carved out by the relativistic jet emanating from an AGN within the ISM of its host galaxy. Firstly, we develop a model for the origin of the internal flow, noticing that a significant increase of large scale velocity circulation within the cocoon arises as significant gradients in the density and entropy are created near the hot spot (a consequence of Crocco's vorticity generation theorem). We find simple and accurate approximate solutions for the large scale flow,showing that a backflow towards the few inner parsec region develops. We solve the appropriate fluid dynamic equations, and we use these solutions to predict the mass inflow rates towards the central regions. We then perform a series of 2D simulations of the propagation of jets using FLASH 2.5, in order to validate the predictions of our model. The backflows which arise supply the central AGN region with very low angular momentum gas, at average rates of the order of $0.1-0.8 \rm{M}_{\odot} \rm{yr.}^{-1}$, the exact value seen to be strongly dependent on the central ISM density (for fixed input jet power). The time scales of these inflows are apparently weakly dependent on the jet/ISM parameters, and are of the order of $3-5\times 10^{7} \rm{yrs}$. We then argue that these backflows could (at least partially) feed the AGN, and provide a self-regulatory mechanism of AGN activity, that is not directly controlled by, but instead controls, the star formation rate within the central circumnuclear disk.