Massive spheroids can form in single minor mergers
  Understanding how rotationally-supported discs transform into
dispersion-dominated spheroids is central to our comprehension of galaxy
evolution. Morphological transformation is largely merger-driven. While major
mergers can efficiently create spheroids, recent work has highlighted the
significant role of other processes, like minor mergers, in driving
morphological change. Given their rich merger histories, spheroids typically
exhibit large fractions of `ex-situ' stellar mass, i.e. mass that is accreted,
via mergers, from external objects. This is particularly true for the most
massive galaxies, whose stellar masses typically cannot be attained without a
large number of mergers. Here, we explore an unusual population of extremely
massive (M* > 10^11 MSun) spheroids, in the Horizon-AGN simulation, which
exhibit anomalously low ex-situ mass fractions, indicating that they form
without recourse to significant merging. These systems form in a single
minor-merger event (with typical merger mass ratios of 0.11 - 0.33), with a
specific orbital configuration, where the satellite orbit is virtually
co-planar with the disc of the massive galaxy. The merger triggers a
catastrophic change in morphology, over only a few hundred Myrs, coupled with
strong in-situ star formation. While this channel produces a minority (~5 per
cent) of such galaxies, our study demonstrates that the formation of at least
some of the most massive spheroids need not involve major mergers -- or any
significant merging at all -- contrary to what is classically believed.
