Variation of Dense Gas Mass–Luminosity Conversion Factor with Metallicity in the Milky Way
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...
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2025-01-01
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| Series: | The Astrophysical Journal |
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| Online Access: | https://doi.org/10.3847/1538-4357/adbf8d |
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| author | Sudeshna Patra Neal J. Evans Kee-Tae Kim Mark Heyer Andrea Giannetti Davide Elia Jessy Jose Jens Kauffmann Manash R. Samal Agata Karska Swagat R. Das Gyuho Lee Geumsook Park |
| author_facet | Sudeshna Patra Neal J. Evans Kee-Tae Kim Mark Heyer Andrea Giannetti Davide Elia Jessy Jose Jens Kauffmann Manash R. Samal Agata Karska Swagat R. Das Gyuho Lee Geumsook Park |
| author_sort | Sudeshna Patra |
| collection | DOAJ |
| description | 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 five inner Galaxy clouds with metallicities ranging from 0.38 Z _⊙ to 1.29 Z _⊙ . 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 ${\alpha }_{\mathrm{tot,}\mathrm{gas}}^{{\prime} }(\mathrm{HCN})=19.{5}_{-4.4}^{+5.6}{Z}^{(-1.53\pm 0.59)}$ and ${\alpha }_{\mathrm{tot,}\mathrm{gas}}^{{\prime} }({\mathrm{HCO}}^{+})=21.{4}_{-4.4}^{+5.5}{Z}^{(-1.32\pm 0.55)}$ for extragalactic studies. Radiation from nearby stars has an effect on the conversion factor of a similar magnitude as that of the metallicity. If we extend the metallicity-corrected scaling relation for HCN to the Central Molecular Zone (CMZ), the value of $\alpha ({\rm{HCN}})$ becomes one-third to one-half of the local values. This effect could partially account for the low star formation rate per dense gas mass observed in the CMZ. |
| format | Article |
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| institution | OA Journals |
| issn | 1538-4357 |
| language | English |
| publishDate | 2025-01-01 |
| publisher | IOP Publishing |
| record_format | Article |
| series | The Astrophysical Journal |
| spelling | doaj-art-b880e8b3ff244276a1eb54257bedea3c2025-08-20T02:12:23ZengIOP PublishingThe Astrophysical Journal1538-43572025-01-01983213310.3847/1538-4357/adbf8dVariation of Dense Gas Mass–Luminosity Conversion Factor with Metallicity in the Milky WaySudeshna Patra0https://orcid.org/0000-0002-3577-6488Neal J. Evans1https://orcid.org/0000-0001-5175-1777Kee-Tae Kim2https://orcid.org/0000-0003-2412-7092Mark Heyer3https://orcid.org/0000-0002-3871-010XAndrea Giannetti4https://orcid.org/0000-0003-3869-6501Davide Elia5https://orcid.org/0000-0002-9120-5890Jessy Jose6https://orcid.org/0000-0003-4908-4404Jens Kauffmann7https://orcid.org/0000-0002-5094-6393Manash R. Samal8https://orcid.org/0000-0002-9431-6297Agata Karska9https://orcid.org/0000-0001-8913-925XSwagat R. Das10https://orcid.org/0000-0001-7151-0882Gyuho Lee11Geumsook Park12https://orcid.org/0000-0001-8467-3736Department of Physics, Indian Institute of Science Education and Research Tirupati , Yerpedu, Tirupati—517619, Andhra Pradesh, India ; inspire.sudeshna@gmail.comDepartment of Astronomy, The University of Texas at Austin , 2515 Speedway, Stop C1400 Austin, TX 78712-1205, USA ; nje@astro.as.utexas.eduKorea Astronomy and Space Science Institute , 776 Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Republic of Korea; University of Science and Technology , Korea (UST), 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of KoreaDepartment of Astronomy, University of Massachusetts , Amherst, MA 01003, USAINAF—Istituto di Radioastronomia , Via P. Gobetti 101, I-40129 Bologna, ItalyINAF—IAPS , Via del Fosso del Cavaliere 100, I-00133 Rome, ItalyDepartment of Physics, Indian Institute of Science Education and Research Tirupati , Yerpedu, Tirupati—517619, Andhra Pradesh, India ; inspire.sudeshna@gmail.comHaystack Observatory MIT , 99 Milstone Rd, Westford, MA 01886, USAAstronomy & Astrophysics Division , Physical Research Laboratory, Ahmedabad, 380009, Gujarat, IndiaMax Planck Institute for Radio Astronomy , Auf dem Hügel 69, 53121 Bonn, Germany; Argelander-Institut fur Astronomie, Universität Bonn , Auf dem Hügel 71, 53121 Bonn, Germany; Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University , ul. Grudziadzka 5, 87-100 Toruń, PolandDepartamento de Astronomia, Universidad de Chile , Las Condes, 7591245 Santiago, ChileKorea Astronomy and Space Science Institute , 776 Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Republic of Korea; University of Science and Technology , Korea (UST), 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of KoreaKorea Astronomy and Space Science Institute , 776 Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Republic of Korea; Telepix Co., Ltd. , 17, Techno 4-ro, Yuseong-gu, Daejeon 34013, Republic of Korea; Research Institute of Natural Sciences , Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, Republic of KoreaHCN 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 five inner Galaxy clouds with metallicities ranging from 0.38 Z _⊙ to 1.29 Z _⊙ . 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 ${\alpha }_{\mathrm{tot,}\mathrm{gas}}^{{\prime} }(\mathrm{HCN})=19.{5}_{-4.4}^{+5.6}{Z}^{(-1.53\pm 0.59)}$ and ${\alpha }_{\mathrm{tot,}\mathrm{gas}}^{{\prime} }({\mathrm{HCO}}^{+})=21.{4}_{-4.4}^{+5.5}{Z}^{(-1.32\pm 0.55)}$ for extragalactic studies. Radiation from nearby stars has an effect on the conversion factor of a similar magnitude as that of the metallicity. If we extend the metallicity-corrected scaling relation for HCN to the Central Molecular Zone (CMZ), the value of $\alpha ({\rm{HCN}})$ becomes one-third to one-half of the local values. This effect could partially account for the low star formation rate per dense gas mass observed in the CMZ.https://doi.org/10.3847/1538-4357/adbf8dMolecular dataMetallicityInterstellar mediumGas-to-dust ratioExtinctionStar forming regions |
| spellingShingle | Sudeshna Patra Neal J. Evans Kee-Tae Kim Mark Heyer Andrea Giannetti Davide Elia Jessy Jose Jens Kauffmann Manash R. Samal Agata Karska Swagat R. Das Gyuho Lee Geumsook Park Variation of Dense Gas Mass–Luminosity Conversion Factor with Metallicity in the Milky Way The Astrophysical Journal Molecular data Metallicity Interstellar medium Gas-to-dust ratio Extinction Star forming regions |
| title | Variation of Dense Gas Mass–Luminosity Conversion Factor with Metallicity in the Milky Way |
| title_full | Variation of Dense Gas Mass–Luminosity Conversion Factor with Metallicity in the Milky Way |
| title_fullStr | Variation of Dense Gas Mass–Luminosity Conversion Factor with Metallicity in the Milky Way |
| title_full_unstemmed | Variation of Dense Gas Mass–Luminosity Conversion Factor with Metallicity in the Milky Way |
| title_short | Variation of Dense Gas Mass–Luminosity Conversion Factor with Metallicity in the Milky Way |
| title_sort | variation of dense gas mass luminosity conversion factor with metallicity in the milky way |
| topic | Molecular data Metallicity Interstellar medium Gas-to-dust ratio Extinction Star forming regions |
| url | https://doi.org/10.3847/1538-4357/adbf8d |
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