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Abstract

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 × 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 × 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.