The triggering of local AGN and their role in regulating star formation

TL;DR

This study investigates how local AGN are triggered and their influence on star formation, revealing that merger-driven AGN are common in early-type galaxies and tend to appear after star formation has declined, thus not promptly regulating it.

Contribution

It provides the first detailed analysis of the triggering mechanisms of local AGN and their delayed role in star formation regulation in nearby galaxies.

Findings

Radio AGN are more common in early-type galaxies.

AGN are typically triggered several dynamical timescales after star formation begins to decline.

Merger-driven AGN do not strongly suppress star formation.

Abstract

We explore the processes that trigger local AGN and the role of these AGN in regulating star formation, using ~350 nearby galaxies observed by the mJy Imaging VLBA Exploration at 20cm (mJIVE) survey. The >10^7 K brightness temperature required for an mJIVE detection cannot be achieved via star formation alone, allowing us to unambiguously detect nearby radio AGN and study their role in galaxy evolution. Radio AGN are an order of magnitude more common in early-type galaxies (ETGs) than in their late-type counterparts. The VLBI-detected ETGs in this study have a similar stellar mass distribution to their undetected counterparts, are typically not the central galaxies of clusters and exhibit merger fractions that are significantly higher than in the average ETG. This suggests that these radio AGN (which have VLBI luminosities >10^22 W Hz^-1) are primarily fuelled by mergers, and not by internal stellar mass loss or cooling flows. Our radio AGN are a factor of ~3 times more likely to reside in the UV-optical red sequence than the average ETG. Furthermore, typical AGN lifetimes (a few 10^7 yr) are much shorter than the transit times from blue cloud to red sequence (~1.5 Gyr). This indicates that the AGN are not triggered promptly and appear several dynamical timescales into the associated star formation episode, implying that they typically couple only to residual gas, at a point where star formation has already declined significantly. While evidence for AGN feedback is strong in systems where the black hole is fed by the cooling of hot gas, AGN triggered by mergers appear not to strongly regulate the associated star formation. The inability of the AGN to rapidly quench merger-driven star formation is likely to make merging the dominant mode of star formation in nearby ETGs, in line with evidence for minor mergers being the primary driver of stellar mass growth in these systems.