The Role of Mergers in Early-type Galaxy Evolution and Black Hole Growth
  Models of galaxy formation invoke the major merger of gas-rich progenitor
galaxies as the trigger for significant phases of black hole growth and the
associated feedback that suppresses star formation to create red spheroidal
remnants. However, the observational evidence for the connection between
mergers and active galactic nucleus (AGN) phases is not clear. We analyze a
sample of low-mass early-type galaxies known to be in the process of migrating
from the blue cloud to the red sequence via an AGN phase in the green valley.
Using deeper imaging from SDSS Stripe 82, we show that the fraction of objects
with major morphological disturbances is high during the early starburst phase,
but declines rapidly to the background level seen in quiescent early-type
galaxies by the time of substantial AGN radiation several hundred Myr after the
starburst. This observation empirically links the AGN activity in low-redshift
early-type galaxies to a significant merger event in the recent past. The large
time delay between the merger-driven starburst and the peak of AGN activity
allows for the merger features to decay to the background and hence may explain
the weak link between merger features and AGN activity in the literature.
