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We present Atacama Large Millimeter/submillimeter Array (ALMA) observationsof 12CO(1-0) and 12CO(2-1) in the central 40" (680 pc) of the nuclear starburstgalaxy NGC 253, including its molecular outflow. We measure the ratio ofbrightness temperature for CO(2-1)/CO(1-0), r_21, in the central starburst andoutflow-related features. We discuss how r_21 can be used to constrain theoptical depth of the CO emission, which impacts the inferred mass of theoutflow and consequently the molecular mass outflow rate. We find r_21 lessthan or equal to 1 throughout, consistent with a majority of the CO emissionbeing optically-thick in the outflow, as it is in the starburst. This suggeststhat the molecular outflow mass is 3-6 times larger than the lower limitreported for optically thin CO emission from warm molecular gas. The impliedmolecular mass outflow rate is 25-50 solar masses per year, assuming thatconversion factor for the outflowing gas is similar to our best estimates forthe bulk of the starburst. This is a factor of 9-19 times larger than the starformation rate in NGC 253. We see tentative evidence for an extended, diffuseCO(2-1) component.

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