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

Abstract 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>1</mml:mn> <mml:mover accent="true"> <mml:mrow> <mml:mi>.</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>″</mml:mi> </mml:mrow> </mml:mover> <mml:mn>6</mml:mn> </mml:math> , indicative of a strongly lensed dusty star-forming galaxy with respective molecular and dust masses of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">mol</mml:mi> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo>≈</mml:mo> <mml:mn>11.5</mml:mn> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">dust</mml:mi> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo>≈</mml:mo> <mml:mn>9.4</mml:mn> </mml:math> 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.