The Physical Drivers and Observational Tracers of CO-to-H2 Conversion Factor Variations in Nearby Barred Galaxy Centers

Abstract

The CO-to-H$_2$ conversion factor (\alpha_\rm{CO}) is central to measuring the amount and properties of molecular gas. It is known to vary with environmental conditions, and previous studies have revealed lower \alpha_\rm{CO} in the centers of some barred galaxies on kpc scales. To unveil the physical drivers of such variations, we obtained ALMA Band 3, 6, and 7 observations toward the inner 2 kpc of NGC 3627 and NGC 4321 tracing $^{12}$CO, $^{13}$CO, and C$^{18}$O lines on 100 pc scales. Our multi-line modeling and Bayesian likelihood analysis of these datasets reveal variations of molecular gas density, temperature, optical depth, and velocity dispersion, which are among the key drivers of \alpha_\rm{CO}. The central 300 pc nuclei in both galaxies show strong enhancement of temperature T_\rm{k}>100 K and density n_\rm{H_2}>10^3 cm$^{-3}$. Assuming a CO-to-H$_2$ abundance of $3\times10^{-4}$, we derive 4-15 times lower \alpha_\rm{CO} than the Galactic value across our maps, which agrees well with previous kpc-scale measurements. Combining the results with our previous work on NGC 3351, we find a strong correlation of \alpha_\rm{CO} with low-J $^{12}$CO optical depths (\tau_\rm{CO}), as well as an anti-correlation with T_\rm{k}. The \tau_\rm{CO} correlation explains most of the \alpha_\rm{CO} variation in the three galaxy centers, whereas changes in T_\rm{k} influence \alpha_\rm{CO} to second order. Overall, the observed line width and $^{12}$CO/$^{13}$CO 2-1 line ratio correlate with \tau_\rm{CO} variation in these centers, and thus they are useful observational indicators for \alpha_\rm{CO} variation. We also test current simulation-based \alpha_\rm{CO} prescriptions and find a systematic overprediction, which likely originates from the mismatch of gas conditions between our data and the simulations.Comment: Accepted for publication in ApJ; 30 pages of main text + 3 appendice