CO and its isotopologues are crucial tracers of bulk molecular gas. While these isotopologues have been extensively studied at high resolution within the Milky Way, beyond our Galaxy they have traditionally been limited to galaxy centers, starburst galaxies, or (ultra-)luminous infrared galaxies. Our team addressed this gap through new NOEMA (Northern Extended Millimetre Array) observations of the nearby, face-on grand design spiral galaxy M51. This data constitutes part of SWAN (Surveying the Whirlpool at Arcseconds with NOEMA) survey, mapping 13CO, C18O and multiple dense gas emission lines in the inner 5 x 7 kpc^2 at the unprecedented resolution (~125 pc) for normal star-forming galaxies. Using SWAN, we present an analysis of the influence of opacity and molecular abundance on the spatial distribution of 13CO(1-0)/C18O(1-0) in M51. Our analysis encompasses a wide range of environments within M51, including the nuclear bar, molecular ring, northern and southern spiral arms, and interarm regions. We find a moderate positive correlation between the ratio and galactocentric radius, as well as a moderate negative correlation with star formation rate (SFR) surface density. While the overall trends for the galaxy align with previous kiloparsec-scale studies—suggesting that the physical and chemical processes governing these emission lines operate similarly across both scales—the observed correlations vary across different environments. This variation indicates that localized processes may play a more significant role than previously expected. To further pin-down the impact of stellar populations and feedback processes on the interstellar medium, we attempt to capture the star-formation process in its entirety by combining SWAN dense gas observations with JWST-FEAST (Feedback in Emerging extrAgalactic Star clusTers) SFR tracers and the subsequent emerging young stellar cluster catalogue produced by JWST and LEGUS (Legacy ExtraGalactic UV Survey).
