Papers › The Physical Drivers and Observational Tracers of CO-to-H2 Conversion Factor...
The Physical Drivers and Observational Tracers of CO-to-H2 Conversion Factor Variations in Nearby Barred Galaxy Centers
Yu-Hsuan Teng, Karin M. Sandstrom, Jiayi Sun, Munan Gong, Alberto D. Bolatto, I-Da Chiang, Adam K. Leroy, Antonio Usero, Simon C. O. Glover, Ralf S. Klessen, Daizhong Liu, Miguel Querejeta, Eva Schinnerer, Frank Bigiel, Yixian Cao, Melanie Chevance, Cosima Eibensteiner, Kathryn Grasha, Frank P. Israel, Eric J. Murphy, Lukas Neumann, Hsi-An Pan, Francesca Pinna, Mattia C. Sormani, J. D. T. Smith, Fabian Walter, Thomas G. Williams
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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 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 ¹²CO, ¹³CO, and C¹⁸O 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 α_(CO). The central 300 pc nuclei in both galaxies show strong enhancement of temperature Tₖ>100 K and density n_(H₂)>10³ 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 kpc-scale measurements. Combining the results with our previous work on NGC 3351, we find a strong correlation of α_(CO) with low-J ¹²CO optical depths (τ_(CO)), as well as an anti-correlation with Tₖ. The τ_(CO) correlation explains most of the α_(CO) variation in the three galaxy centers, whereas changes in Tₖ 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.
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