Formation of ultra-diffuse galaxies in the field and in galaxy groups
  We study ultra-diffuse galaxies (UDGs) in zoom in cosmological simulations,
seeking the origin of UDGs in the field versus galaxy groups. We find that
while field UDGs arise from dwarfs in a characteristic mass range by multiple
episodes of supernova feedback (Di Cintio et al. 2017), group UDGs may also
form by tidal puffing up and they become quiescent by ram-pressure stripping.
The field and group UDGs share similar properties, independent of distance from
the group centre. Their dark-matter haloes have ordinary spin parameters and
centrally dominant dark-matter cores. Their stellar components tend to have a
prolate shape with a Sersic index n~1 but no significant rotation. Ram pressure
removes the gas from the group UDGs when they are at pericentre, quenching star
formation in them and making them redder. This generates a
colour/star-formation-rate gradient with distance from the centre, as observed
in clusters. We find that ~20 per cent of the field UDGs that fall into a
massive halo survive as satellite UDGs. In addition, normal field dwarfs on
highly eccentric orbits can become UDGs near pericentre due to tidal puffing
up, contributing about half of the group-UDG population. We interpret our
findings using simple toy models, showing that gas stripping is mostly due to
ram pressure rather than tides. We estimate that the energy deposited by tides
in the bound component of a satellite over one orbit can cause significant
puffing up provided that the orbit is sufficiently eccentric.
