  Contrary to many stereotypes about massive galaxies, observed brightest group
galaxies (BGGs) are diverse in their star formation rates, kinematic
properties, and morphologies. Studying how they evolve into and express such
diverse characteristics is an important piece of the galaxy formation puzzle.
We use a high-resolution cosmological suite of simulations Romulus and compare
simulated central galaxies in group-scale halos at $z=0$ to observed BGGs. The
comparison encompasses the stellar mass-halo mass relation, various kinematic
properties and scaling relations, morphologies, and the star formation rates.
Generally, we find that Romulus reproduces the full spectrum of diversity in
the properties of the BGGs very well, albeit with a tendency toward lower than
the observed fraction of quenched BGGs. We find both early-type S0 and
elliptical galaxies as well as late-type disk galaxies; we find Romulus
galaxies that are fast-rotators as well as slow-rotators; and we observe
galaxies transforming from late-type to early-type following strong dynamical
interactions with satellites. We also carry out case studies of selected
Romulus galaxies to explore the link between their properties, and the recent
evolution of the stellar system as well as the surrounding
intragroup/circumgalactic medium. In general, mergers/strong interactions
quench star-forming activity and disrupt the stellar disk structure. Sometimes,
however, such interactions can also trigger star-formation and galaxy
rejuvenation. Black hole feedback can also lead to a decline of the star
formation rate but by itself, it does not typically lead to complete quenching
of the star formation activity in the BGGs.
