ALMA observations of molecular gas in the host galaxy of AT2018cow
  We investigate the molecular gas in, and star-formation properties of, the
host galaxy (CGCG 137-068) of a mysterious transient, AT2018cow, at kpc and
larger scales, using archival band-3 data from the Atacama Large
Millimeter/submillimeter Array (ALMA). AT2018cow is the nearest Fast-Evolving
Luminous Transient (FELT), and this is the very first study unveiling
molecular-gas properties of FELTs. The achieved rms and beam size are 0.21 mJy
beam$^{-1}$ at a velocity resolution of $40$ km s$^{-1}$ and $3".66\times2".71$
($1.1~{\rm kpc} \times 0.8~{\rm kpc}$), respectively. CO($J$=1-0) emission is
successfully detected. The total molecular gas mass inferred from the CO data
is $(1.85\pm0.04)\times10^8$ M$_\odot$ with the Milky Way CO-to-H$_2$
conversion factor. The H$_2$ column density at the AT2018cow site is estimated
to be $8.6\times10^{20}$ cm$^{-2}$. The ALMA data reveal that (1) CGCG 137-068
is a normal star-forming (SF) dwarf galaxy in terms of its molecular gas and
star-formation properties and (2) AT2018cow is located between a CO peak and a
blue star cluster. These properties suggest on-going star formation and favor
the explosion of a massive star as the progenitor of AT2018cow. We also find
that CGCG 137-068 has a solar or super-solar metallicity. If the metallicity of
the other FELT hosts is not higher than average, then some property of SF dwarf
galaxies other than metallicity may be related to FELTs.
