2025MNRAS.541.4031D

In this work, we study CO isotopologue emission in the largest cluster galaxy sample to date: 48 VERTICO spiral galaxies in Virgo. We show for the first time in a significant sample that the physical conditions within the molecular gas appear to change as a galaxy's interstellar medium (ISM) is affected by environmental processes. $^{13}$CO is detected across the sample, both directly and via stacking, while C$^{18}$O is detected in a smaller number of systems. We use these data to study trends with global and radial galaxy properties. We show that the $^{12}$CO/$^{13}$CO line ratio changes systematically with a variety of galaxy properties, including mean gas surface density, H I-deficiency, and galaxy morphology. $^{13}$CO/C$^{18}$O line ratios vary significantly, both radially and between galaxies, suggesting real variations in abundances are present. Such abundance changes may be due to star formation history differences, or speculatively even stellar initial mass function variations. We present a model where the optical depth of the molecular gas appears to change as a galaxy's ISM is affected by environmental processes. The molecular gas appears to become more transparent as the molecular medium is stripped, and then more opaque as the tightly bound remnant gas settles deep in the galaxy core. This explains the variations we see, and also helps explain similar observations in cluster early-type galaxies. Next generation simulations and dedicated observations of additional isotopologues could thus provide a powerful tool to help us understand the impact of environment on the ISM, and thus the quenching of galaxies.