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

Abstract

The CO-to-H${}_2$ 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 $\sim$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 ${}^{12}$CO, ${}^{13}$CO, and C${}^{18}$O lines on 100 pc scales in the inner $\sim$2 kpc of NGC 3351. Using multi-line radiative transfer modeling and a Bayesian likelihood analysis, we infer the H${}_2$ 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 $2-3\times 10^3$ $\mathrm{c}{\mathrm{m}}^{-3}$ in the central $\sim$1 kpc and a high temperature of 30$-$60 K near the nucleus and near the contact points that connect to the bar-driven inflows. Assuming a CO/H${}_2$ abundance of $3\times 10^{-4}$, our analysis yields $α_\rm{CO}{\sim}0.5{-}2.0$ ${\mathrm{M}}_{\odot }(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}\mathrm{p}{\mathrm{c}}^2{)}^{-1}$ with a decreasing trend with galactocentric radius in the central $\sim$1 kpc. The inflows show a substantially lower $α_\rm{CO} < 0.1$ ${\mathrm{M}}_{\odot }(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}\mathrm{p}{\mathrm{c}}^2{)}^{-1}$, 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 $\sim 1.5$ ${\mathrm{M}}_{\odot }(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}\mathrm{p}{\mathrm{c}}^2{)}^{-1}$, 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.

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