The Formation of Ultra-Diffuse Galaxies in the RomulusC Galaxy Cluster
  Simulation
  We study the origins of 122 ultra-diffuse galaxies (UDGs) in the {\sc
RomulusC} zoom-in cosmological simulation of a galaxy cluster (M$_{200} =
1.15\times10^{14}$ M$_{\odot}$), one of the only such simulations capable of
resolving the evolution and structure of dwarf galaxies (M$_{\star} < 10^9$
M$_{\odot}$). We find broad agreement with observed cluster UDGs and predict
that they are not separate from the overall cluster dwarf population. UDGs in
cluster environments form primarily from dwarf galaxies that experienced early
cluster in-fall and subsequent quenching due to ram pressure. The ensuing
dimming of these dwarf galaxies due to passive stellar evolution results in a
population of very low surface brightness galaxies that are otherwise typical
dwarfs. UDGs and non-UDGs alike are affected by tidal interactions with the
cluster potential. Tidal stripping of dark matter, as well as mass loss from
stellar evolution, results in the adiabatic expansion of stars, particularly in
the lowest mass dwarfs. High mass dwarf galaxies show signatures of tidal
heating while low mass dwarfs that survive until $z=0$ typically have not
experienced such impulsive interactions. There is little difference between
UDGs and non-UDGs in terms of their dark matter halos, stellar morphology,
colors, and location within the cluster. In most respects cluster UDG and
non-UDGs alike are similar to isolated dwarf galaxies, except for the fact that
they are typically quenched.
