New constraints on the (CO)-C-12(2-1)/(1-0) line ratio across nearby disc galaxies

Abstract

Both the CO(2-1) and CO(1-0) lines are used to trace the mass of molecular gas in galaxies. Translating the molecular gas mass estimates between studies using different lines requires a good understanding of the behaviour of the CO(2-1)-to-CO(1-0) ratio, R-21. We compare new, high-quality CO(1-0) data from the IRAM 30-m EMIR MultiLine Probe of the ISM Regulating Galaxy Evolution survey to the latest available CO(2-1) maps from HERA CO-Line Extragalactic Survey, Physics at High Angular resolution in Nearby Galaxies-ALMA, and a new IRAM 30-m M51 Large Program. This allows us to measure R-21 across the full star-forming disc of nine nearby, massive, star-forming spiral galaxies at 27 arcsec (similar to 1-2 kpc) resolution. We find an average R-21 = 0.64 +/- 0.09 when we take the luminosity-weighted mean of all individual galaxies. This result is consistent with the mean ratio for disc galaxies that we derive from single-pointing measurements in the literature, R-21,R- lit = 0.59(-0.09)(+0.18). The ratio shows weak radial variations compared to the point-to-point scatter in the data. In six out of nine targets, the central enhancement in R-21 with respect to the galaxy-wide mean is of order of similar to 10-20. We estimate an azimuthal scatter of similar to 20 percent in R-21 at fixed galactocentric radius but this measurement is limited by our comparatively coarse resolution of 1.5 kpc. We find mild correlations between R-21 and carbon monoxide (CO) brightness temperature, infrared (IR) intensity, 70-160 mu m ratio, and IR-to-CO ratio. All correlations indicate that R-21 increases with gas surface density, star formation rate surface density, and the interstellar radiation field.