2023ApJ...950..119T

The CO-to-H₂ conversion factor (α_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 α_CO in the centers of some barred galaxies on kiloparsec scales. To unveil the physical drivers of such variations, we obtained Atacama Large Millimeter/submillimeter Array bands (3), (6), and (7) observations toward the inner ∼2 kpc of NGC 3627 and NGC 4321 tracing ¹²CO, ¹³CO, and C¹⁸O lines on ∼100 pc scales. Our multiline modeling and Bayesian likelihood analysis of these data sets reveal variations of molecular gas density, temperature, optical depth, and velocity dispersion, which are among the key drivers of α_CO. The central 300 pc nuclei in both galaxies show strong enhancement of temperature T_k ≳ 100 K and density ${n}_{{{\rm{H}}}_{2}}\gt {10}^{3}$ cm^–3. Assuming a CO-to-H₂ abundance of 3 × 10^–4, we derive 4-15 times lower α_CO than the Galactic value across our maps, which agrees well with previous kiloparsec-scale measurements. Combining the results with our previous work on NGC 3351, we find a strong correlation of α_CO with low-J¹²CO optical depths (τ_CO), as well as an anticorrelation with T_k. The τ_CO correlation explains most of the α_CO variation in the three galaxy centers, whereas changes in T_k influence α_CO to second order. Overall, the observed line width and ¹²CO/¹³CO 2-1 line ratio correlate with τ_CO variation in these centers, and thus they are useful observational indicators for α_CO variation. We also test current simulation-based α_CO prescriptions and find a systematic overprediction, which likely originates from the mismatch of gas conditions between our data and the simulations.