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

Abstract

We explore the processes that trigger local AGN and the role of these AGN in regulating star formation, using ∼550 nearby galaxies observed by the mJy Imaging VLBA Exploration at 20 cm (mJIVE) survey. The ≳107 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 >1022 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 107 yr) are much shorter than the transit times from blue cloud to red sequence (∼1.5 Gyr). This indicates that nearby (merger-triggered) AGN appear several dynamical time-scales into the associated star formation episode. This implies that such AGN typically couple only to residual gas, at a point where star formation has already declined significantly, and are, therefore, unlikely to play a significant role in regulating the star formation episode.