  Galaxy merger histories correlate strongly with stellar mass, largely
regardless of morphology. Thus, at fixed stellar mass, spheroids and discs
share similar assembly histories, both in terms of the frequency of mergers and
the distribution of their mass ratios. Since mergers are the principal drivers
of disc-to-spheroid morphological transformation, and the most massive galaxies
typically have the richest merger histories, it is surprising that discs exist
at all at the highest stellar masses (e.g. beyond the knee of the mass
function). Using Horizon-AGN, a cosmological hydro-dynamical simulation, we
show that extremely massive (M*> 10^11.4 MSun) discs are created via two
channels. In the primary channel (accounting for ~70% of these systems and ~8%
of massive galaxies) the most recent, significant merger (stellar mass ratio >
1:10) between a massive spheroid and a gas-rich satellite `spins up' the
spheroid by creating a new rotational stellar component, leaving a massive disc
as the remnant. In the secondary channel (accounting for ~30% of these systems
and ~3% of massive galaxies), a system maintains a disc throughout its
lifetime, due to an anomalously quiet merger history. Not unexpectedly, the
fraction of massive discs is larger at higher redshift, due to the Universe
being more gas-rich. The morphological mix of galaxies at the highest stellar
masses is, therefore, a strong function of the gas fraction of the Universe.
Finally, these massive discs have similar black-hole masses and accretion rates
to massive spheroids, providing a natural explanation for why a minority of
powerful AGN are surprisingly found in disc galaxies.
