The small observed scale of AGN--driven outflows, and inside--out disc quenching

TL;DR

This paper explains why AGN-driven outflows are observed within 10 kpc, suggests they can extend further in massive galaxies, and explores their role in galaxy disc star formation and quenching.

Contribution

It demonstrates that observed outflow sizes are consistent with energy-driven models and explores their impact on star formation and inside-out quenching in galaxy discs.

Findings

Outflows are naturally limited to within ~10 kpc due to energy injection levels.

In massive, gas-rich galaxies, outflows can reach several tens of kpc.

Outflows can induce star formation, especially in galaxy outskirts, affecting quenching processes.

Abstract

Observations of massive outflows with detectable central AGN typically find them within radii $\lesssim 10$ kpc. We show that this apparent size restriction is a natural result of AGN driving if this process injects total energy only of order the gas binding energy to the outflow, and the AGN varies over time (`flickers') as suggested in recent work. After the end of all AGN activity the outflow continues to expand to larger radii, powered by the thermal expansion of the remnant shocked AGN wind. We suggest that on average, outflows should be detected further from the nucleus in more massive galaxies. In massive gas--rich galaxies these could be several tens of kpc in radius. We also consider the effect that pressure of such outflows has on a galaxy disc. In moderately gas--rich discs, with gas-to-baryon fraction $< 0.2$, the outflow may induce star formation significant enough to be distinguished from quiescent by an apparently different normalisation of the Kennicutt-Schmidt law. The star formation enhancement is probably stronger in the outskirts of galaxy discs, so coasting outflows might be detected by their effects upon the disc even after the driving AGN has shut off. We compare our results to the recent inference of inside--out quenching of star formation in galaxy discs.