paper

Molecular Gas Properties and CO-to-H2 Conversion Factors in the Central Kiloparsec of NGC 3351

arXiv:2111.05844 · doi:10.3847/1538-4357/ac382f

Abstract

The CO-to-H conversion factor ($α_\rm{CO}$) is critical to studying molecular gas and star formation in galaxies. The value of $α_\rm{CO}$ 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 kpc scales revealed low values of $α_\rm{CO}$ in the centers of some barred spiral galaxies, including NGC 3351. We present new ALMA Band 3, 6, and 7 observations of CO, CO, and CO lines on 100 pc scales in the inner 2 kpc of NGC 3351. Using multi-line radiative transfer modeling and a Bayesian likelihood analysis, we infer the H 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 in the central 1 kpc and a high temperature of 3060 K near the nucleus and near the contact points that connect to the bar-driven inflows. Assuming a CO/H abundance of , our analysis yields $α_\rm{CO}{\sim}0.5{-}2.0$ with a decreasing trend with galactocentric radius in the central 1 kpc. The inflows show a substantially lower $α_\rm{CO} < 0.1$ , likely due to lower optical depths caused by turbulence or shear in the inflows. Over the whole region, this gives an intensity-weighted $α_\rm{CO}$ of , which is similar to previous dust modeling based results at kpc scales. This suggests that low $α_\rm{CO}$ on kpc scales in the centers of some barred galaxies may be due to the contribution of low optical depth CO emission in bar-driven inflows.

arXiv comment31 pages, 17 figures; accepted for publication in ApJ; font size of text and figures adjusted

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