Delayed triggering of radio Active Galactic Nuclei in gas-rich minor
  mergers in the local Universe
  We examine the processes triggering star formation and Active Galactic
Nucleus (AGN) activity in a sample of 25 low redshift ($z<0.13$) gas-rich
galaxy mergers observed at milli-arcsecond resolution with Very Long Baseline
Interferometry as part of the mJy Imaging VLBA Exploration at 20cm (mJIVE-20)
survey. The high ($>10^7$ K) brightness temperature required for an mJIVE-20
detection allows us to unambiguously identify the radio AGN in our sample. We
find three such objects. Our VLBI AGN identifications are classified as
Seyferts or LINERs in narrow line optical diagnostic plots; mid-infrared
colours of our targets and the comparison of H$\alpha$ star formation rates
with integrated radio luminosity are also consistent with the VLBI
identifications. We reconstruct star formation histories in our galaxies using
optical and UV photometry, and find that these radio AGN are not triggered
promptly in the merger process, consistent with previous findings for non-VLBI
samples of radio AGN. This delay can significantly limit the efficiency of
feedback by radio AGN triggered in galaxy mergers. We find that radio AGN hosts
have lower star formation rates than non-AGN radio-selected galaxies at the
same starburst age. Conventional and VLBI radio imaging shows these AGN to be
compact on arcsecond scales. Our modeling suggests that the actual sizes of
AGN-inflated radio lobes may be much larger than this, but these are too faint
to be detected in existing observations. Deep radio imaging is required to map
out the true extent of the AGN, and to determine whether the low star formation
rates in radio AGN hosts are a result of the special conditions required for
radio jet triggering, or the effect of AGN feedback.
