ISM Properties of a Massive Dusty Star-forming Galaxy Discovered at z ∼ 7

We report the discovery and constrain the physical conditions of the interstellar medium of the highest-redshift millimeter-selected dusty star-forming galaxy to date, SPT-S J031132−5823.4 (hereafter SPT0311−58), at $z=6.900\pm 0.002$. SPT0311−58 was discovered via its 1.4 mm thermal dust continuum emission in the South Pole Telescope (SPT)-SZ survey. The spectroscopic redshift was determined through an Atacama Large Millimeter/submillimeter Array 3 mm frequency scan that detected CO(6–5), CO(7–6), and $\left[CI\right]$(2–1), and subsequently was confirmed by detections of CO(3–2) with the Australia Telescope Compact Array and $\left[CII\right]$ with APEX. We constrain the properties of the ISM in SPT0311−58 with a radiative transfer analysis of the dust continuum photometry and the CO and $\left[CI\right]$ line emission. This allows us to determine the gas content without ad hoc assumptions about gas mass scaling factors. SPT0311−58 is extremely massive, with an intrinsic gas mass of $M_{gas}=3.3\pm 1.9\times {10}^{11}{M}_{\odot }$. Its large mass and intense star formation is very rare for a source well into the epoch of reionization.