Spatially Resolved 12 CO(2–1)/ 12 CO(1–0) in the Starburst Galaxy NGC 253: Assessing Optical Depth to Constrain the Molecular Mass Outflow Rate

Abstract

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of 12 CO(1–0) and 12 CO(2–1) in the central 40″ (680 pc) of the nuclear starburst galaxy NGC 253, including its molecular outflow. We measure the ratio of brightness temperature for CO(2–1)/CO(1–0), r 21 , in the central starburst and outflow-related features. We discuss how r 21 can be used to constrain the optical depth of the CO emission, which impacts the inferred mass of the outflow and consequently the molecular mass outflow rate. We find r 21  ≲ 1 throughout, consistent with a majority of the CO emission being optically thick in the outflow, as it is in the starburst. This suggests that the molecular outflow mass is 3–6 times larger than the lower limit reported for optically thin CO emission from warm molecular gas. The implied molecular mass outflow rate is 25–50 M ⊙ yr −1 , assuming that the conversion factor for the outflowing gas is similar to our best estimates for the bulk of the starburst. This is a factor of 9–19 times larger than the star formation rate in NGC 253. We see tentative evidence for an extended, diffuse CO(2–1) component.