New constraints on the 12CO(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 mass of molecular gas in galaxies. Translating estimates between studies using different requires a good understanding behaviour CO(2–1)-to-CO(1–0) ratio, R21. We compare new, high-quality data from IRAM 30-m EMIR MultiLine Probe ISM Regulating Galaxy Evolution survey latest available maps HERA CO-Line Extragalactic Survey, Physics at High Angular resolution Nearby Galaxies-ALMA, new M51 Large Program. This allows us measure R21 across full star-forming disc nine nearby, massive, spiral galaxies 27 arcsec (∼1–2 kpc) resolution. find an average = 0.64 ± 0.09 when we take luminosity-weighted mean all individual result is consistent with ratio for that derive single-pointing measurements literature, $R_{\rm 21, lit}~=~0.59^{+0.18}_{-0.09}$. The shows weak radial variations compared point-to-point scatter data. In six out targets, central enhancement respect galaxy-wide order ${\sim}10{-}20{{\ \rm per\ cent}}$. estimate azimuthal ∼20 per cent fixed galactocentric radius but this measurement limited by our comparatively coarse 1.5 kpc. mild correlations carbon monoxide (CO) brightness temperature, infrared (IR) intensity, 70–160 µm IR-to-CO ratio. All indicate increases surface density, star formation rate interstellar radiation field.