Investigating the Drivers of CO-to-H2 Conversion Factor Variations in Nearby Galaxy Centers

The CO-to-H₂ conversion factor (α_CO) is crucial for accurate estimation of the amount and properties of molecular gas. However, α_CO is known to vary with environmental conditions, and previous kpc-scale studies have revealed lower α_CO in the centers of some barred galaxies, including NGC 3351, 3627, and 4321. We present ALMA Band 3, 6, and 7 observations toward the inner ∼2 kpc of these galaxies tracing ¹²CO, ¹³CO, and C¹⁸O lines at ∼100 pc resolution. We show that dynamical effects resulting from turbulence/shear can lead to substantially lower α_CO in the bar-driven inflows of NGC 3351 due to lower optical depth. A clear, positive correlation between α_CO and ¹²CO optical depth is seen in all three galaxy centers. We also find that the CO/¹³CO(2-1) ratio mainly traces the ¹²CO optical depth, and thus it may be a useful observable in predicting α_CO variation in galaxy centers.