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

**Authors:** 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  
**arXiv:** [2304.04732](https://arxiv.org/abs/2304.04732)  
**Date:** 2023-04-10  
**Summary page:** https://www.summarizepaper.com/en/paper/2304.04732v1/

## Key points

- Researchers used ALMA Band 3, 6, and 7 observations to study CO-to-H$_2$ conversion factor ($\alpha_\rm{CO}$) variations in NGC 3627 and NGC 4321.
- Molecular gas density, temperature, optical depth, and velocity dispersion were identified as key drivers of $\alpha_\rm{CO}$.
- Central nuclei within both galaxies showed strong enhancements in temperature and density, leading to lower $\alpha_\rm{CO}$ values across their maps.
- There was a strong correlation between $\alpha_\rm{CO}$ and low-J $^{12}$CO optical depths ($\tau_\rm{CO}$), as well as an anti-correlation with temperature.
- The correlation between $\tau_\rm{CO}$ explained most of the variation observed in the three galaxy centers while changes in temperature influenced $\alpha_\rm{CO}$ to a lesser extent.

## Summary

A team of researchers obtained ALMA Band 3, 6, and 7 observations towards the inner 2 kpc of NGC 3627 and NGC 4321 to better understand the physical drivers of CO-to-H$_2$ conversion factor ($\alpha_\rm{CO}$) variations. Their multi-line modeling and Bayesian likelihood analysis revealed variations in molecular gas density, temperature, optical depth, and velocity dispersion as key drivers of $\alpha_\rm{CO}$. The central nuclei within both galaxies showed strong enhancements in temperature ($T_\rm{k}>100$ K) and density ($n_\rm{H_2}>10^3$ cm$^{-3}$), leading to deriving $\alpha_\rm{CO}$ values that were between four to fifteen times lower than the Galactic value across their maps. These results agreed well with previous kpc-scale measurements. Combining these findings with their previous work on NGC 3351, the researchers found a strong correlation between $\alpha_\rm{CO}$ and low-J $^{12}$CO optical depths ($\tau_\rm{CO}$), as well as an anti-correlation with temperature. The correlation between $\tau_\rm{CO}$ explained most of the variation observed in the three galaxy centers while changes in temperature influenced $\alpha_\rm{CO}$ to a lesser extent.