Galaxy Interactions Trigger Rapid Black Hole Growth: an unprecedented
  view from the Hyper Suprime-Cam Survey
  Collisions and interactions between gas-rich galaxies are thought to be
pivotal stages in their formation and evolution, causing the rapid production
of new stars, and possibly serving as a mechanism for fueling supermassive
black holes (BH). Harnessing the exquisite spatial resolution (~0.5 arcsec)
afforded by the first ~170 deg^2 of the Hyper Suprime-Cam (HSC) Survey, we
present our new constraints on the importance of galaxy-galaxy major mergers
(1:4) in growing BHs throughout the last ~8 Gyrs. Utilizing mid-infrared
observations in the WISE All-Sky survey, we robustly select active galactic
nuclei (AGN) and mass-matched control galaxy samples, totaling ~140,000
spectroscopically confirmed systems at i<22 mag. We identify galaxy interaction
signatures using a novel machine-learning random forest decision tree technique
allowing us to select statistically significant samples of major-mergers,
minor-mergers/irregular-systems, and non-interacting galaxies. We use these
samples to show that galaxies undergoing mergers are a factor ~2-7 more likely
to contain luminous obscured AGN than non-interacting galaxies, and this is
independent of both stellar mass and redshift to z < 0.9. Furthermore, based on
our comparison of AGN fractions in mass-matched samples, we determine that the
most luminous AGN population (L_AGN > 10^45 erg/s) systematically reside in
merging systems over non-interacting galaxies. Our findings show that
galaxy-galaxy interactions do, on average, trigger luminous AGN activity
substantially more often than in secularly evolving non-interacting galaxies,
and we further suggest that the BH growth rate may be closely tied to the
dynamical time of the merger system.
