Extremely massive disc galaxies in the nearby Universe form through
  gas-rich minor mergers
  In our hierarchical structure-formation paradigm, the observed morphological
evolution of massive galaxies -- from rotationally-supported discs to
dispersion-dominated spheroids -- is largely explained via galaxy merging.
However, since mergers are likely to destroy discs, and the most massive
galaxies have the richest merger histories, it is surprising that any discs
exist at all at the highest stellar masses. Recent theoretical work by our
group has used a cosmological, hydrodynamical simulation to suggest that
extremely massive (M* > 10^11.4 MSun) discs form primarily via minor mergers
between spheroids and gas-rich satellites, which create new rotational stellar
components and leave discs as remnants. Here, we use UV-optical and HI data of
massive galaxies, from the SDSS, GALEX, DECaLS and ALFALFA surveys, to test
these theoretical predictions. Observed massive discs account for ~13% of
massive galaxies, in good agreement with theory (~11%). ~64% of the observed
massive discs exhibit tidal features, which are likely to indicate recent minor
mergers, in the deep DECaLS images (compared to ~60% in their simulated
counterparts). The incidence of these features is at least four times higher
than in low-mass discs, suggesting that, as predicted, minor mergers play a
significant (and outsized) role in the formation of these systems. The
empirical star-formation rates agree well with theoretical predictions and, for
a small galaxy sample with HI detections, the HI masses and fractions are
consistent with the range predicted by the simulation. The good agreement
between theory and observations indicates that extremely massive discs are
indeed remnants of recent minor mergers between spheroids and gas-rich
satellites.
