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We study the Carbon Monoxide (CO) excitation, mean molecular gas density andinterstellar radiation field (ISRF) intensity in a comprehensive sample of 76galaxies from local to high redshift (z~0-6), selected based on detections oftheir CO transitions J=2-1 and 5-4 and their optical/infrared/(sub-)millimeterspectral energy distributions (SEDs). We confirm the existence of a tightcorrelation between CO excitation as traced by the CO(5-4)/(2-1) line ratio(R52), and the mean ISRF intensity U as derived from infrared SED fitting usingdust SED templates. By modeling the molecular gas density probabilitydistribution function (PDF) in galaxies and predicting CO line ratios withlarge velocity gradient radiative transfer calculations, we present a frameworklinking global CO line ratios to the mean molecular hydrogen gas density nH2and kinetic temperature Tkin. Mapping in this way observed R52 ratios to nH2and Tkin probability distributions, we obtain positive U-nH2 and U-Tkincorrelations, which imply a scenario in which the ISRF in galaxies is mainlyregulated by Tkin and (non-linearly) by nH2. A small fraction of starburstgalaxies showing enhanced nH2 could be due to merger-driven compaction. Ourwork demonstrates that ISRF and CO excitation are tightly coupled, and thatdensity-PDF modeling is a promising tool for probing detailed ISM propertiesinside galaxies.

CO excitation, molecular gas density and interstellar radiation field in local and high-redshift galaxies