  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.
