2024ApJ...971..176H

CO emission has been widely used as a tracer of molecular gas mass. However, it is a long-standing issue to accurately constrain the CO-to-H₂ conversion factor (α_CO) that converts CO luminosity to molecular gas mass, especially in starburst galaxies. We present the first resolved α_CO modeling results with multiple Atacama Large Millimeter/submillimeter Array CO and ¹³CO transition observations at both giant molecular cloud (GMC) scale at 150 pc and kiloparsec scale for one of the closest starburst mergers, the Antennae. By combining our CO modeling results and measurements of 350 GHz dust continuum, we find that most GMCs in the Antennae have α_CO values approximately four times smaller than the commonly adopted Milky Way value (4.3). We find that α_CO at GMC scales shows a strong dependence on CO intensity, ¹³CO/CO ratio, and GMC velocity dispersion, which is consistent with various theoretical and simulation predictions. Specifically, we suggest that the ¹³CO/CO line ratio and the velocity dispersion can be used to infer α_CO in starburst regions. By applying our modeled α_CO in GMC analyses, we find that GMCs in the Antennae are less gravitationally bound than in normal spiral galaxies, which is more consistent with what is predicted by merger simulations. At kiloparsec scale, we find that our modeled α_CO values are smaller than the modeled α_CO at GMC scale by 40%, which can be due to inclusion of a diffuse gas component with lower α_CO values. We find a similar correlation of α_CO and CO intensity at kiloparsec scales to that at GMC scales.