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 can not be attained without a large number of mergers .
Here , we explore an unusual population of massive ( M _ { * } > 10 ^ { 11 } M _ { \odot } ) 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 ( \sim 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 .