2019ApJ...879L..13M

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); this is the first study unveiling molecular-gas properties of FELT hosts. The achieved rms and beam size are 0.21 mJy beam^–1 at a velocity resolution of 40 km s^–1 and 3.''66 x 2.''71 (1.1 kpc x 0.8 kpc), respectively. CO(J = 1-0) emission is successfully detected. The total molecular gas mass inferred from the CO data is (1.85 ± 0.04) x 10⁸ M_☉ with the Milky Way CO-to-H₂ conversion factor. The H₂ column density at the AT2018cow site is estimated to be 8.6 x 10²⁰ 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) that AT2018cow is located between a CO peak and a blue star cluster. These properties suggest ongoing 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 properties of SF dwarf galaxies other than metallicity may be related to FELTs.