Tamura et al. report the ATCA and Nobeyama 45m telescope detection of a remarkably bright submillimeter galaxy, MM J154506.4-344318, in emission lines at 48.5 and 97.0 GHz, respectively. They also identify part of an emission line at near 218 GHz using ALMA. Together with photometric redshift estimates and the ratio between the line and infrared luminosities, they conclude that the emission lines are most likely to be transitions of <sup>12</sup>CO at a redshift of z=3.75. ALMA 1.3 mm continuum imaging reveals an arc and a spot, indicative of a strongly gravitationally-lensed dusty star-forming galaxy. The inferred dust-to-gas mass ratio is found to be high among dusty star-forming galaxies at similar redshifts, implying the presence of a massive, chemically-enriched reservoir of cool interstellar medium 1.6 billion years after the Big Bang.
The figure above shows the CO spectrum of MM J154506.4-344318. From top to bottom the ATCA, Nobeyama Radio Observatory (NRO) 45 m, and ALMA band 6 spectra are shown. The orange curve shows the best-fitting Gaussian. The inset in the bottom panel shows the 4 arcsecond × 4 arcsecond ALMA image of CO (9–8) overlaid on the 1.3 mm continuum.