2022ApJ...925...72T

The CO-to-H₂ conversion factor (α _CO) is critical to studying molecular gas and star formation in galaxies. The value of α _CO has been found to vary within and between galaxies, but the specific environmental conditions that cause these variations are not fully understood. Previous observations on ∼kiloparsec scales revealed low values of α _CO in the centers of some barred spiral galaxies, including NGC 3351. We present new Atacama Large Millimeter/submillimeter Array Band 3, 6, and 7 observations of ¹²CO, ¹³CO, and C¹⁸O lines on 100 pc scales in the inner ∼2 kpc of NGC 3351. Using multiline radiative transfer modeling and a Bayesian likelihood analysis, we infer the H₂ density, kinetic temperature, CO column density per line width, and CO isotopologue abundances on a pixel-by-pixel basis. Our modeling implies the existence of a dominant gas component with a density of 2-3 x 10³ cm^–3 in the central ∼1 kpc and a high temperature of 30-60 K near the nucleus and near the contact points that connect to the bar-driven inflows. Assuming a CO/H₂ abundance of 3 x 10^–4, our analysis yields α _CO ∼ 0.5-2.0 M _☉ _(K km s^–1 pc²)^–1 with a decreasing trend with galactocentric radius in the central ∼1 kpc. The inflows show a substantially lower α _CO <= 0.1 M _☉ _(K km s^–1 pc²)^–1, likely due to lower optical depths caused by turbulence or shear in the inflows. Over the whole region, this gives an intensity-weighted α _CO of ∼1.5 M _☉ _(K km s^–1 pc²)^–1, which is similar to previous dust-modeling-based results at kiloparsec scales. This suggests that low α _CO on kiloparsec scales in the centers of some barred galaxies may be due to the contribution of low-optical-depth CO emission in bar-driven inflows.