The role of mergers in driving morphological transformation over cosmic
  time
  Understanding the processes that trigger morphological transformation is
central to understanding how and why the Universe transitions from being
disc-dominated at early epochs to having the morphological mix that is observed
today. We use Horizon-AGN, a cosmological hydrodynamical simulation, to perform
a comprehensive study of the processes that drive morphological change in
massive (M > 10^10 MSun) galaxies over cosmic time. We show that (1)
essentially all the morphological evolution in galaxies that are spheroids at
z=0 is driven by mergers with mass ratios greater than 1:10, (2) major mergers
alone cannot produce today's spheroid population -- minor mergers are
responsible for a third of all morphological transformation over cosmic time
and are its dominant driver after z~1, (3) prograde mergers trigger milder
morphological transformation than retrograde mergers -- while both types of
events produce similar morphological changes at z>2, the average change due to
retrograde mergers is around twice that due to their prograde counterparts at
z~0, (4) remnant morphology depends strongly on the gas fraction of a merger,
with gas-rich mergers routinely re-growing discs, and (5) at a given stellar
mass, discs do not exhibit drastically different merger histories from
spheroids -- disc survival in mergers is driven by acquisition of cold gas (via
cosmological accretion and gas-rich interactions) and a preponderance of
prograde mergers in their merger histories.
