On the nature of the unusual transient AT 2018cow from HI observations
  of its host galaxy
  Unusual stellar explosions represent an opportunity to learn about both
stellar and galaxy evolution. Mapping the atomic gas in host galaxies of such
transients can lead to an understanding of the conditions triggering them. We
provide resolved atomic gas observations of the host galaxy, CGCG137-068, of
the unusual, poorly-understood transient AT2018cow searching for clues to
understand its nature. We test whether it is consistent with a recent inflow of
atomic gas from the intergalactic medium, as suggested for host galaxies of
gamma-ray bursts (GRBs) and some supernovae (SNe). We observed the HI hyperfine
structure line of the AT2018cow host with the Giant Metrewave Radio Telescope.
There is no unusual atomic gas concentration near the position of AT2018cow.
The gas distribution is much more regular than those of GRB/SN hosts. The
AT2018cow host has an atomic gas mass lower by 0.24 dex than predicted from its
star formation rate (SFR) and is at the lower edge of the galaxy main sequence.
In the continuum we detected the emission of AT2018cow and of a star-forming
region in the north-eastern part of the bar (away from AT2018cow). This region
hosts a third of the galaxy's SFR. The absence of atomic gas concentration
close to AT2018cow, along with a normal SFR and regular HI velocity field, sets
CGCG137-068 apart from GRB/SN hosts studied in HI. The environment of AT2018cow
therefore suggests that its progenitor may not have been a massive star. Our
findings are consistent with an origin of the transient that does not require a
connection between its progenitor and gas concentration or inflow: an exploding
low-mass star, a tidal disruption event, a merger of white dwarfs, or a merger
between a neutron star and a giant star. We interpret the recently reported
atomic gas ring in CGCG137-068 as a result of internal processes connected with
gravitational resonances caused by the bar.
