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

Abstract

The CO-to-H2 conversion factor (αCO) 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 αCO 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 αCO. The central 300 pc nuclei in both galaxies show strong enhancement of temperature Tk ≳ 100 K and density cm−3. Assuming a CO-to-H2 abundance of 3 × 10−4, we derive 4–15 times lower αCO 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 αCO with low-J12CO optical depths (τCO), as well as an anticorrelation with Tk. The τCO correlation explains most of the αCO variation in the three galaxy centers, whereas changes in Tk influence αCO to second order. Overall, the observed line width and 12CO/13CO 2–1 line ratio correlate with τCO variation in these centers, and thus they are useful observational indicators for αCO variation. We also test current simulation-based αCO prescriptions and find a systematic overprediction, which likely originates from the mismatch of gas conditions between our data and the simulations. Show more