ALMA observations of molecular gas in the host galaxy of AT2018cow

Abstract

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 $3".66\times 2".71$ ($1.1\mathrm{kpc}\times 0.8\mathrm{kpc}$), respectively. CO($J$=1-0) emission is successfully detected. The total molecular gas mass inferred from the CO data is $(1.85\pm 0.04)\times 10^8$ 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 $8.6\times 10^{20}$ 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.

8 pages, 3 figures, 2 tables, accepted for publication in ApJL