Abstract
Abstract We study the carbon monoxide (CO) excitation, mean molecular gas density, and interstellar radiation field (ISRF) intensity in a comprehensive sample of 76 galaxies from local to high redshift (z ∼ 0–6), selected based on detections of their CO transitions J = 2 → 1 and 5 → 4 and their optical/infrared/(sub)millimeter spectral energy distributions (SEDs). We confirm the existence of a tight correlation between CO excitation as traced by the CO (5–4)/(2–1) line ratio R 52 and the mean ISRF intensity U as derived from infrared SED fitting using dust SED templates. By modeling the molecular gas density probability distribution function (PDF) in galaxies and predicting CO line ratios with large velocity gradient radiative transfer calculations, we present a framework linking global CO line ratios to the mean molecular hydrogen gas density n H 2 and kinetic temperature T kin. Mapping in this way observed R 52 ratios to n H 2 and T kin probability distributions, we obtain positive U – n H 2 and U –T kin correlations, which imply a scenario in which the ISRF in galaxies is mainly regulated by T kin and (nonlinearly) by n H 2 . A small fraction of starburst galaxies showing enhanced n H 2 could be due to merger-driven compaction. Our work demonstrates that ISRF and CO excitation are tightly coupled and that density–PDF modeling is a promising tool for probing detailed ISM properties inside galaxies.
Highlights
Star formation in galaxies is regulated by their reservoir of molecular gas
For a given DL07 interstellar radiation field (ISRF) distribution powerlaw index (=−2) and Umax (=107; Draine et al 2014), áUñ is proportional to the ratio between LIR and Mdust, with a coefficient P0 ≈ 138 from this work, where P0 represents the power absorbed per unit dust mass in a radiation field U = 1: LIR,8–1000mm = P0 · áUñ · Mdust where P0 » 120–150
We compiled a comprehensive sample of galaxies from local to high redshift with carbon monoxide (CO) (2–1) and CO (5–4) detections and well-sampled IR spectral energy distributions (SEDs)
Summary
Star formation in galaxies is regulated by their reservoir of molecular gas. Globally, the star formation rate (SFR) correlates with the total amount of molecular gas mass via the Kennicutt–. Through the study of both CO excitation and dust SED traced mean ISRF intensity (áUñ) in about 20 galaxies, Daddi et al (2015) found that the CO (5–4)/(2–1) line ratio, R52, is tightly correlated with áUñ. This indicates that CO excitation, or its related ISM properties, is sensitive to the ISRF. 17 A PYTHON package (co-excitation-gas-modeling) is provided with this paper for the calculation: https://pypi.org/project/co-excitation-gasmodeling It fits an input line ratio with error to our model grid and determines the probable ranges of ánH2ñ and Tkin.
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