2022ApJ...940..176V

We measure the molecular-to-atomic gas ratio, R_mol, and the star formation rate (SFR) per unit molecular gas mass, SFE_mol, in 38 nearby galaxies selected from the Virgo Environment Traced in CO (VERTICO) survey. We stack ALMA ¹²CO (J = 2-1) spectra coherently using H I velocities from the VIVA survey to detect faint CO emission out to galactocentric radii r_gal ∼ 1.2 r₂₅. We determine the scale lengths for the molecular and stellar components, finding a ∼3:5 relation compared to ∼1:1 in field galaxies, indicating that the CO emission is more centrally concentrated than the stars. We compute R_mol as a function of different physical quantities. While the spatially resolved R_mol on average decreases with increasing radius, we find that the mean molecular-to-atomic gas ratio within the stellar effective radius R_e, R_mol(r < R_e), shows a systematic increase with the level of H I, truncation and/or asymmetry (H_I perturbation). Analysis of the molecular- and the atomic-to-stellar mass ratios within R_e, ${R}_{\star }^{\mathrm{mol}}(r\lt {R}_{e})$ and ${R}_{\star }^{\mathrm{atom}}(r\lt {R}_{e})$, shows that VERTICO galaxies have increasingly lower ${R}_{\star }^{\mathrm{atom}}(r\lt {R}_{e})$ for larger levels of H_I perturbation (compared to field galaxies matched in stellar mass), but no significant change in ${R}_{\star }^{{\rm{m}}{\rm{o}}{\rm{l}}}(r\lt {R}_{e})$. We also measure a clear systematic decrease of the SFE_mol within R_e, SFE_mol(r < R_e), with increasingly perturbed H I. Therefore, compared to field galaxies from the field, VERTICO galaxies are more compact in CO emission in relation to their stellar distribution, but increasingly perturbed atomic gas increases their R_mol and decreases the efficiency with which their molecular gas forms stars.