The HI content of isolated ultra-diffuse galaxies: A sign of multiple
  formation mechanisms?
  We report on the results of radio observations in the 21 cm emission line of
atomic hydrogen (HI) of four relatively isolated ultra-diffuse galaxies (UDGs):
DGSAT I, R-127-1, M-161-1, and SECCO-dI-2. Our Effelsberg observations resulted
in non-detections for the first three UDGs, and a clear detection for the last.
DGSAT I, R-127-1, and M-161-1 are quiescent galaxies with gas fractions that
are much lower than those of typical field galaxies of the same stellar mass.
On the other hand, SECCO-dI-2 is a star forming gas-rich dwarf, similar to two
other field UDGs that have literature HI data: SECCO-dI-1 and UGC 2162. This
group of three gas-rich UDGs have stellar and gaseous properties that are
compatible with a recently proposed theoretical mechanism for the formation of
UDGs, based on feedback-driven outflows. In contrast, the physical
characteristics of R-127-1 and M-161-1 are puzzling, given their isolated
nature. We interpret this dichotomy in the gaseous properties of field UDGs as
a sign of the existence of multiple mechanisms for their formation, with the
formation of the quiescent gas-poor UDGs remaining a mystery.
