2025AJ....169...18D

Over the past decade, several millimeter interferometer programs have mapped the nearby star-forming galaxy M51 at a spatial resolution of ≤170 pc. This study combines observations from three major programs: the PdBI Arcsecond Whirlpool Survey, the SMA M51 large program, and the Surveying the Whirlpool at Arcseconds with NOEMA. The data set includes the (1-0) and (2-1) rotational transitions of ¹²CO, ¹³CO, and C¹⁸O isotopologues. The observations cover the r < 3 kpc region, including the center and part of the disk, thereby ensuring strong detections of the weaker ¹³CO and C¹⁸O lines. All observations are convolved in this analysis to an angular resolution of 4″, corresponding to a physical scale of 170 pc. We investigate empirical line ratio relations and quantitatively evaluate molecular gas conditions such as temperature, density, and the CO-to-H₂ conversion factor (α_CO). We employ two approaches to study the molecular gas conditions: (i) assuming local thermodynamic equilibrium (LTE) to analytically determine the CO column density and α_CO, and (ii) using non-LTE modeling with RADEX to fit physical conditions to observed CO isotopologue intensities. We find that the α_CO values in the center and along the inner spiral arm are ∼0.5 dex (LTE) and 0.1 dex (non-LTE) below the Milky Way inner disk value. The average non-LTE α_CO is 2.4 ± 0.5 M_⊙ 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 interstellar medium's physical and chemical conditions in nearby galaxies.