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

The CO-to-H _2 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 ^12 CO, ^13 CO, and C ^18 O 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-H _2 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- J ^12 CO 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 ^12 CO/ ^13 CO 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.