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 ( S _1.1mm  = 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 ≈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 ^12 CO 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 $1\mathop{.}\limits^{^{\prime\prime} }6$ , indicative of a strongly lensed dusty star-forming galaxy with respective molecular and dust masses of $\mathrm{log}{M}_{{\rm{mol}}}/{M}_{\odot }\approx 11.5$ and $\mathrm{log}{M}_{{\rm{dust}}}/{M}_{\odot }\approx 9.4$ after being corrected for ≈6.6× gravitational magnification. The inferred dust-to-gas mass ratio is found to be high (≈0.0083) among coeval dusty star-forming galaxies, implying the presence of a massive, chemically enriched reservoir of cool interstellar medium at z  ≈ 4, or 1.6 Gyr after the Big Bang.