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

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.

Predicting HCN, HCO + , multi-transition CO, and dust emission of star-forming galaxies: Constraining the properties of resolved gas and dust disks of local spiral galaxies

“…weighted conversion factor is five times higher than that of Sandstrom et al (2013). Likewise, our model CO conversion Galaxy CO(2-1) factor in the inner kpc of NGC 3351 is about a factor of five higher than that of Teng et al (2022) based on 100 pc resolution 12 CO, 13 CO, and C 18 O observations and assuming a CO abundance of 3 × 10 −4 . A lower CO abundance caused by a low metallicity as suggested by Díaz et al (2007) decreases the difference between the Teng et al (2022) and our conversion factors.…”

“…Likewise, our model CO conversion Galaxy CO(2-1) factor in the inner kpc of NGC 3351 is about a factor of five higher than that of Teng et al (2022) based on 100 pc resolution 12 CO, 13 CO, and C 18 O observations and assuming a CO abundance of 3 × 10 −4 . A lower CO abundance caused by a low metallicity as suggested by Díaz et al (2007) decreases the difference between the Teng et al (2022) and our conversion factors. The models suggest that the conversion factors are slightly sub-Galactic in the center and exceed the Galactic value by up to a factor of 2 at the edge of the CO-emitting disk.…”

Down but Not Out: Properties of the Molecular Gas in the Stripped Virgo Cluster Early-type Galaxy NGC 4526

“…As a check on the plausibility of those conditions, we note that the density n H 2 , the column density N( 12 CO), and a typical CO abundance of 10 −4 give a line-ofsight thickness of 12 pc. That value is reasonable for a giant molecular cloud; additional discussion can be found in Teng et al (2021). Finally, in the outer disk (Figure 21) we infer n H 2 , T kin , and [ 12 CO/ 13 CO] values similar to those in the ring, with [ 12 CO/ 13 CO] = -+ 6.5 , In short, we find strong evidence for a radial gradient in the [ 13 CO/C 18 O] abundance ratio, increasing to larger values at larger radii; the change is a factor of 1.6 over 1 kpc.…”

“…6.5 1.3 3.0 . For comparison, estimated [ 12 C/ 13 C] abundances in nearby spirals and the Milky Way tend to range from 20 to 60 (e.g., Romano et al 2019;Tang et al 2019;Martín et al 2019Martín et al , 2021Teng et al 2021). In this regard, NGC 4526ʼs status as a longtime, virialized resident of the Virgo Cluster is particularly relevant.…”

“…The grid also spans [ 12 CO/ 13 CO] abundance ratios of 3.4 to 400 and [ 13 CO/C 18 O] of 2.0 to 27, both in steps of 0.067 dex. This grid strategy is very similar to that employed by Teng et al (2021), though independently coded.…”