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In our hierarchical structure-formation paradigm, the observed morphologicalevolution of massive galaxies -- from rotationally-supported discs todispersion-dominated spheroids -- is largely explained via galaxy merging.However, since mergers are likely to destroy discs, and the most massivegalaxies have the richest merger histories, it is surprising that any discsexist at all at the highest stellar masses. Recent theoretical work by ourgroup has used a cosmological, hydrodynamical simulation to suggest thatextremely massive (M* > 10^11.4 MSun) discs form primarily via minor mergersbetween spheroids and gas-rich satellites, which create new rotational stellarcomponents and leave discs as remnants. Here, we use UV-optical and HI data ofmassive galaxies, from the SDSS, GALEX, DECaLS and ALFALFA surveys, to testthese theoretical predictions. Observed massive discs account for ~13% ofmassive galaxies, in good agreement with theory (~11%). ~64% of the observedmassive discs exhibit tidal features, which are likely to indicate recent minormergers, in the deep DECaLS images (compared to ~60% in their simulatedcounterparts). The incidence of these features is at least four times higherthan in low-mass discs, suggesting that, as predicted, minor mergers play asignificant (and outsized) role in the formation of these systems. Theempirical star-formation rates agree well with theoretical predictions and, fora small galaxy sample with HI detections, the HI masses and fractions areconsistent with the range predicted by the simulation. The good agreementbetween theory and observations indicates that extremely massive discs areindeed remnants of recent minor mergers between spheroids and gas-richsatellites.

Extremely massive disc galaxies in the nearby Universe form through gas-rich minor mergers