Origins of ultra-diffuse galaxies in the Coma cluster II. Constraints
  from their stellar populations
  In this second paper of the series, we study with new Keck/DEIMOS spectra the
stellar populations of seven spectroscopically confirmed ultra-diffuse galaxies
(UDGs) in the Coma cluster. We find intermediate to old ages (~ 7Gyr), low
metallicities ([Z/H] ~ -0.7dex) and mostly super-solar abundance patterns
([Mg/Fe] ~ 0.13dex). These properties are similar to those of low-luminosity
(dwarf) galaxies inhabiting the same area in the cluster and are most
consistent with being the continuity of the stellar mass scaling relations of
more massive galaxies. These UDGs' star formation histories imply a relatively
recent infall into the Coma cluster, consistent with the theoretical
predictions for a dwarf-like origin. However, considering the scatter in the
resulting properties and including other UDGs in Coma, together with the
results from the velocity phase-space study of the Paper I in this series, a
mixed-bag of origins is needed to explain the nature of all UDGs. Our results
thus reinforce a scenario in which many UDGs are field dwarfs that become
quenched through their later infall onto cluster environments, whereas some
UDGs could be genuine primordial galaxies that failed to develop due to an
early quenching phase. The unknown proportion of dwarf-like to primordial-like
UDGs leaves the enigma of the nature of UDGs still open.
