Internal dynamics and stellar content of nine ultra-diffuse galaxies in
  the Coma cluster prove their evolutionary link with dwarf early-type galaxies
  Ultra-diffuse galaxies (UDGs) are spatially extended, low surface brightness
stellar systems with regular elliptical-like morphology found in a wide range
of environments. Studies of the internal dynamics and dark matter content of
UDGs that would elucidate their formation and evolution have been hampered by
their low surface brightnesses. Here we present spatially resolved velocity
profiles, stellar velocity dispersions, ages and metallicities for 9 UDGs in
the Coma cluster. We use intermediate-resolution spectra obtained with
Binospec, the MMT's new high-throughput optical spectrograph. We derive dark
matter fractions between 50~\%\ and 90~\% within the half-light radius using
Jeans dynamical models. Three galaxies exhibit major axis rotation, two others
have highly anisotropic stellar orbits, and one shows signs of triaxiality. In
the Faber--Jackson and mass--metallicity relations, the 9 UDGs fill the gap
between cluster dwarf elliptical (dE) and fainter dwarf spheroidal (dSph)
galaxies. Overall, the observed properties of all 9 UDGs can be explained by a
combination of internal processes (supernovae feedback) and environmental
effects (ram-pressure stripping, interaction with neighbors). These
observations suggest that UDGs and dEs are members of the same galaxy
population.
