Galaxy Zoo: dust lane early-type galaxies are tracers of recent,
  gas-rich minor mergers
  We present the second of two papers concerning the origin and evolution of
local early-type galaxies exhibiting dust features. We use optical and radio
data to examine the nature of active galactic nucleus (AGN) activity in these
objects, and compare these with a carefully constructed control sample. We find
that dust lane early-type galaxies are much more likely to host emission-line
AGN than the control sample galaxies. Moreover, there is a strong correlation
between radio and emission-line AGN activity in dust lane early-types, but not
the control sample. Dust lane early-type galaxies show the same distribution of
AGN properties in rich and poor environments, suggesting a similar triggering
mechanism. By contrast, this is not the case for early-types with no dust
features. These findings strongly suggest that dust lane early-type galaxies
are starburst systems formed in gas-rich mergers. Further evidence in support
of this scenario is provided by enhanced star formation and black hole
accretion rates in these objects. Dust lane early-types therefore represent an
evolutionary stage between starbursting and quiescent galaxies. In these
objects, the AGN has already been triggered but has not as yet completely
destroyed the gas reservoir required for star formation.
