Papers › Molecular Gas Properties and CO-to-H2 Conversion Factors in the Central Kiloparsec of NGC 3351
Molecular Gas Properties and CO-to-H2 Conversion Factors in the Central Kiloparsec of NGC 3351
Yu-Hsuan Teng, Karin M. Sandstrom, Jiayi Sun, Adam K. Leroy, L. Clifton Johnson, Alberto D. Bolatto, J. M. Diederik Kruijssen, Andreas Schruba, Antonio Usero, Ashley T. Barnes, Frank Bigiel, Guillermo A. Blanc, Brent Groves, Frank P. Israel, Daizhong Liu, Erik Rosolowsky, Eva Schinnerer, J. D. Smith, Fabian Walter
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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 ∼kpc scales revealed low values of α_(CO) in the centers of some barred spiral galaxies, including NGC 3351. We present new ALMA 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 multi-line 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 kpc scales. This suggests that low α_(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.
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