CO-to-H$_2$ conversion factor and grain size distribution through the analysis of $α_\mathrm{CO}$-$q_\mathrm{PAH}$ relation
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| Format: | Preprint |
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2024
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| _version_ | 1866913602034728960 |
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| author | Chiang, I-Da Hirashita, Hiroyuki Chastenet, Jeremy Sandstrom, Karin M. Koch, Eric W. Leroy, Adam K. Teng, Yu-Hsuan Williams, Thomas G. |
| author_facet | Chiang, I-Da Hirashita, Hiroyuki Chastenet, Jeremy Sandstrom, Karin M. Koch, Eric W. Leroy, Adam K. Teng, Yu-Hsuan Williams, Thomas G. |
| contents | The CO-to-H$_2$ conversion factor ($α_\mathrm{CO}$) is expected to vary with dust abundance and grain size distribution through the efficiency of shielding gas from CO-dissociation radiation. We present a comprehensive analysis of $α_\mathrm{CO}$ and grain size distribution for nearby galaxies, using the PAH fraction ($q_\mathrm{PAH}$) as an observable proxy of grain size distribution. We adopt the resolved observations at 2-kpc resolution in 42 nearby galaxies, where $α_\mathrm{CO}$ is derived from measured metallicity and surface densities of dust and HI assuming a fixed dust-to-metals ratio. We use an analytical model for the evolution of H$_2$ and CO, in which the evolution of grain size distribution is controlled by the dense gas fraction ($η$). We find that the observed level of $q_\mathrm{PAH}$ is consistent with the diffuse-gas-dominated model ($η=0.2$) where dust shattering is more efficient. Meanwhile, the slight decreasing trend of observed $q_\mathrm{PAH}$ with metallicity is more consistent with high-$η$ predictions, likely due to the more efficient loss of PAHs by coagulation. We discuss how grain size distribution (indicated by $q_\mathrm{PAH}$) and metallicity impact $α_\mathrm{CO}$; we however did not obtain conclusive evidence that the grain size distribution affects $α_\mathrm{CO}$. Observations and model predictions show similar anti-correlation between $α_\mathrm{CO}$ and 12+log(O/H). Meanwhile, there is a considerable difference in how resolved $α_\mathrm{CO}$ behaves with $q_\mathrm{PAH}$. The observed $α_\mathrm{CO}$ has a positive correlation with $q_\mathrm{PAH}$, while the model-predicted $α_\mathrm{CO}$ does not have a definite correlation with $q_\mathrm{PAH}$. This difference is likely due to the limitation of one-zone treatment in the model. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2412_03954 |
| institution | arXiv |
| publishDate | 2024 |
| record_format | arxiv |
| spellingShingle | CO-to-H$_2$ conversion factor and grain size distribution through the analysis of $α_\mathrm{CO}$-$q_\mathrm{PAH}$ relation Chiang, I-Da Hirashita, Hiroyuki Chastenet, Jeremy Sandstrom, Karin M. Koch, Eric W. Leroy, Adam K. Teng, Yu-Hsuan Williams, Thomas G. Astrophysics of Galaxies The CO-to-H$_2$ conversion factor ($α_\mathrm{CO}$) is expected to vary with dust abundance and grain size distribution through the efficiency of shielding gas from CO-dissociation radiation. We present a comprehensive analysis of $α_\mathrm{CO}$ and grain size distribution for nearby galaxies, using the PAH fraction ($q_\mathrm{PAH}$) as an observable proxy of grain size distribution. We adopt the resolved observations at 2-kpc resolution in 42 nearby galaxies, where $α_\mathrm{CO}$ is derived from measured metallicity and surface densities of dust and HI assuming a fixed dust-to-metals ratio. We use an analytical model for the evolution of H$_2$ and CO, in which the evolution of grain size distribution is controlled by the dense gas fraction ($η$). We find that the observed level of $q_\mathrm{PAH}$ is consistent with the diffuse-gas-dominated model ($η=0.2$) where dust shattering is more efficient. Meanwhile, the slight decreasing trend of observed $q_\mathrm{PAH}$ with metallicity is more consistent with high-$η$ predictions, likely due to the more efficient loss of PAHs by coagulation. We discuss how grain size distribution (indicated by $q_\mathrm{PAH}$) and metallicity impact $α_\mathrm{CO}$; we however did not obtain conclusive evidence that the grain size distribution affects $α_\mathrm{CO}$. Observations and model predictions show similar anti-correlation between $α_\mathrm{CO}$ and 12+log(O/H). Meanwhile, there is a considerable difference in how resolved $α_\mathrm{CO}$ behaves with $q_\mathrm{PAH}$. The observed $α_\mathrm{CO}$ has a positive correlation with $q_\mathrm{PAH}$, while the model-predicted $α_\mathrm{CO}$ does not have a definite correlation with $q_\mathrm{PAH}$. This difference is likely due to the limitation of one-zone treatment in the model. |
| title | CO-to-H$_2$ conversion factor and grain size distribution through the analysis of $α_\mathrm{CO}$-$q_\mathrm{PAH}$ relation |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2412.03954 |