Large Molecular and Dust Reservoir of a Gravitationally-Lensed Submillimeter Galaxy Behind the Lupus-I Molecular Cloud

We report the Australian Telescope Compact Array and Nobeyama 45 m telescope detection of a remarkably bright (S1.1mm = 44 mJy) submillimeter galaxy MM J154506.4-344318 in emission lines at 48.5 and 97.0 GHz, respectively. We also identify part of an emission line at approximate to 218.3 GHz using the Atacama Large Millimeter/submillimeter Array (ALMA). Together with photometric redshift estimates and the ratio between the line and infrared luminosities, we conclude that the emission lines are most likely to be the J = 2-1, 4-3, and 9-8 transitions of 12CO at redshift z = 3.753 +/- 0.001. ALMA 1.3 mm continuum imaging reveals an arc and a spot separated by an angular distance of 1.'' 6 , indicative of a strongly lensed dusty star-forming galaxy with respective molecular and dust masses of logMmol/M circle dot approximate to 11.5 and logMdust/M circle dot approximate to 9.4 after being corrected for approximate to 6.6x gravitational magnification. The inferred dust-to-gas mass ratio is found to be high (approximate to 0.0083) among coeval dusty star-forming galaxies, implying the presence of a massive, chemically enriched reservoir of cool interstellar medium at z approximate to 4, or 1.6 Gyr after the Big Bang.