CO isotopologue-derived molecular gas conditions and CO-to-H${}_2$ conversion factors in M51

Abstract

Over the past decade, several millimeter interferometer programs have mapped the nearby star-forming galaxy M51 at a spatial resolution of $\le 170$ pc. This study combines observations from three major programs: the PdBI Arcsecond Whirlpool Survey (PAWS), the SMA M51 large program (SMA-PAWS), and the Surveying the Whirlpool at Arcseconds with NOEMA (SWAN). The dataset includes the (1-0) and (2-1) rotational transitions of ${}^{12}$CO, ${}^{13}$CO, and C${}^{18}$O isotopologues. The observations cover the $r<3\mathrm{kpc}$ region including center and part of the disk, thereby ensuring strong detections of the weaker ${}^{13}$CO and C${}^{18}$O lines. All observations are convolved in this analysis to an angular resolution of 4${}^{\prime \prime }$, corresponding to a physical scale of $\sim$170 pc. We investigate empirical line ratio relations and quantitatively evaluate molecular gas conditions such as temperature, density, and the CO-to-H${}_2$ conversion factor ($\hat{I}{\pm }_{\mathrm{CO}}$). We employ two approaches to study the molecular gas conditions: (i) assuming local thermal equilibrium (LTE) to analytically determine the CO column density and $\hat{I}{\pm }_{\mathrm{CO}}$, and (ii) using non-LTE modeling with RADEX to fit physical conditions to observed CO isotopologue intensities. We find that the $\hat{I}{\pm }_{\mathrm{CO}}$ values {in the center and along the inner spiral arm} are $\sim$0.5 dex (LTE) and $\sim$0.1 dex (non-LTE) below the Milky Way inner disk value. The average non-LTE $\hat{I}{\pm }_{\mathrm{CO}}$ is $2.4\pm 0.5$ M${}_{\odot }$ pc${}^{-2}$ (K km s${}^{-1}$)${}^{-1}$. While both methods show dispersion due to underlying assumptions, the scatter is larger for LTE-derived values. This study underscores the necessity for robust CO line modeling to accurately constrain the molecular ISM's physical and chemical conditions in nearby galaxies.

accepted for publication in AJ; 31 pages, 16 figures, 7 tables