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| Natura: | Preprint |
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2025
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| Accesso online: | https://arxiv.org/abs/2503.07931 |
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| author | Patra, Sudeshna Evans II, Neal J. Kim, Kee-Tae Heyer, Mark Giannetti, Andrea Elia, Davide Jose, Jessy Kauffmann, Jens Samal, Manash R. Karska, Agata Das, Swagat R. Lee, Gyuho Park, Geumsook |
| author_facet | Patra, Sudeshna Evans II, Neal J. Kim, Kee-Tae Heyer, Mark Giannetti, Andrea Elia, Davide Jose, Jessy Kauffmann, Jens Samal, Manash R. Karska, Agata Das, Swagat R. Lee, Gyuho Park, Geumsook |
| contents | HCN and HCO$^+$ are the most common dense gas tracers used both in the Milky Way and external galaxies. The luminosity of HCN and HCO$^+$ $J = 1-0$ lines are converted to a dense gas mass by the conversion factor, $α_{Q}$. Traditionally, this $α_{Q}$ has been considered constant throughout the Galaxy and in other galaxies, regardless of the environment. We analyzed 17 outer Galaxy clouds and 5 inner Galaxy clouds with metallicities ranging from 0.38 Z$_{\odot}$ to 1.29 Z$_{\odot}$. Our analysis indicates that $α_{Q}$ is not constant; instead, it varies with metallicity. The metallicity-corrected $α_{Q}$ derived from the HCN luminosity of the entire cloud is almost three times higher in the outer Galaxy than in the inner galaxy. In contrast, HCO$^+$ seems less sensitive to metallicity. We recommend using the metallicity-corrected dense gas conversion factors $α^{'}_{\rm tot, Gas}(\rm HCN) = 19.5^{+5.6}_{-4.4} Z^{(-1.53 \pm 0.59)}$ and $α^{'}_{\rm tot, Gas}(\rm HCO^{+}) = 21.4^{+5.5}_{-4.4} Z^{(-1.32\pm0.55)}$ for extragalactic studies. Radiation from nearby stars has an effect on the conversion factor of similar magnitude as that of the metallicity. If we extend the metallicity-corrected scaling relation for HCN to the Central Molecular Zone, the value of $α(\rm HCN)$ becomes $1/3$ to $1/2$ of the local values. This effect could partially account for the low star formation rate per dense gas mass observed in the CMZ. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2503_07931 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Variation of Dense Gas Mass-Luminosity conversion factor with metallicity in the Milky Way Patra, Sudeshna Evans II, Neal J. Kim, Kee-Tae Heyer, Mark Giannetti, Andrea Elia, Davide Jose, Jessy Kauffmann, Jens Samal, Manash R. Karska, Agata Das, Swagat R. Lee, Gyuho Park, Geumsook Astrophysics of Galaxies HCN and HCO$^+$ are the most common dense gas tracers used both in the Milky Way and external galaxies. The luminosity of HCN and HCO$^+$ $J = 1-0$ lines are converted to a dense gas mass by the conversion factor, $α_{Q}$. Traditionally, this $α_{Q}$ has been considered constant throughout the Galaxy and in other galaxies, regardless of the environment. We analyzed 17 outer Galaxy clouds and 5 inner Galaxy clouds with metallicities ranging from 0.38 Z$_{\odot}$ to 1.29 Z$_{\odot}$. Our analysis indicates that $α_{Q}$ is not constant; instead, it varies with metallicity. The metallicity-corrected $α_{Q}$ derived from the HCN luminosity of the entire cloud is almost three times higher in the outer Galaxy than in the inner galaxy. In contrast, HCO$^+$ seems less sensitive to metallicity. We recommend using the metallicity-corrected dense gas conversion factors $α^{'}_{\rm tot, Gas}(\rm HCN) = 19.5^{+5.6}_{-4.4} Z^{(-1.53 \pm 0.59)}$ and $α^{'}_{\rm tot, Gas}(\rm HCO^{+}) = 21.4^{+5.5}_{-4.4} Z^{(-1.32\pm0.55)}$ for extragalactic studies. Radiation from nearby stars has an effect on the conversion factor of similar magnitude as that of the metallicity. If we extend the metallicity-corrected scaling relation for HCN to the Central Molecular Zone, the value of $α(\rm HCN)$ becomes $1/3$ to $1/2$ of the local values. This effect could partially account for the low star formation rate per dense gas mass observed in the CMZ. |
| title | Variation of Dense Gas Mass-Luminosity conversion factor with metallicity in the Milky Way |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2503.07931 |