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

TL;DR

This study uses ALMA observations of CO emission lines in NGC 253 to determine the optical depth of molecular gas, revealing that the outflow's mass and rate are significantly higher than previous estimates assuming optically thin emission.

Contribution

It introduces a method to constrain the optical depth of CO emission in galaxy outflows, improving estimates of molecular mass and outflow rates in starburst galaxies.

Findings

Molecular outflow mass is 3-6 times larger than previous estimates.

Outflow rate is 25-50 solar masses per year, much higher than star formation rate.

CO(2-1)/CO(1-0) ratio indicates optically thick emission in outflow.

Abstract

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of 12CO(1-0) and 12CO(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 less than or equal to 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 solar masses per year, assuming that 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.