Tracing the total molecular gas in galaxies

• Verfasst von: Madden, Suzanne [VerfasserIn]
• Verfasst von: Cormier, Diane [VerfasserIn]
• Verfasst von: Hony, Sacha [VerfasserIn]
• Verfasst von: Lebouteiller, Vianney [VerfasserIn]
• Verfasst von: Abel, N. [VerfasserIn]
• Verfasst von: Galametz, Maud [VerfasserIn]
• Verfasst von: Looze, I. De [VerfasserIn]
• Verfasst von: Chevance, Mélanie [VerfasserIn]
• Verfasst von: Polles, Fiorella L. [VerfasserIn]
• Verfasst von: Lee, Min-Young [VerfasserIn]
• Verfasst von: Galliano, F. [VerfasserIn]
• Verfasst von: Lambert-Huyghe, A. [VerfasserIn]
• Verfasst von: Hu, D. [VerfasserIn]
• Verfasst von: Ramambason, L. [VerfasserIn]
• Titel: Tracing the total molecular gas in galaxies
• Titelzusatz: [CII] and the CO-dark gas
• Verf.angabe: S.C. Madden, D. Cormier, S. Hony, V. Lebouteiller, N. Abel, M. Galametz, I. De Looze, M. Chevance, F.L. Polles, M.-Y. Lee, F. Galliano, A. Lambert-Huyghe, D. Hu, and L. Ramambason
• E-Jahr: 2020
• Jahr: 17 November 2020
• Umfang: 21 S.
• Fussnoten: Gesehen am 07.01.2021
• Titel Quelle: Enthalten in: Astronomy and astrophysics
• Ort Quelle: Les Ulis : EDP Sciences, 1969
• Jahr Quelle: 2020
• Band/Heft Quelle: 643(2020) Artikel-Nummer A141, 21 Seiten
• ISSN Quelle: 1432-0746
• DOI: doi:10.1051/0004-6361/202038860
• Datenträger: Online-Ressource
• Sprache: eng
• K10plus-PPN: 1744146861

Abstract

<i>Context.<i/> Molecular gas is a necessary fuel for star formation. The CO (1−0) transition is often used to deduce the total molecular hydrogen but is challenging to detect in low-metallicity galaxies in spite of the star formation taking place. In contrast, the [C II]<i>λ<i/>158 <i>μ<i/>m is relatively bright, highlighting a potentially important reservoir of H<sub>2<sub/> that is not traced by CO (1−0) but is residing in the C<sup>+<sup/>-emitting regions.<i>Aims.<i/> Here we aim to explore a method to quantify the total H<sub>2<sub/> mass (<i>M<i/><sub>H<sub>2<sub/><sub/>) in galaxies and to decipher what parameters control the CO-dark reservoir.<i>Methods.<i/> We present Cloudy grids of density, radiation field, and metallicity in terms of observed quantities, such as [O I], [C I], CO (1−0), [C II], <i>L<i/><sub>TIR<sub/>, and the total <i>M<i/><sub>H<sub>2<sub/><sub/>. We provide recipes based on these models to derive total <i>M<i/><sub>H<sub>2<sub/><sub/> mass estimates from observations. We apply the models to the <i>Herschel<i/> Dwarf Galaxy Survey, extracting the total <i>M<i/><sub>H<sub>2<sub/><sub/> for each galaxy, and compare this to the H<sub>2<sub/> determined from the observed CO (1−0) line. This allows us to quantify the reservoir of H<sub>2<sub/> that is CO-dark and traced by the [C II]<i>λ<i/>158 <i>μ<i/>m.<i>Results.<i/> We demonstrate that while the H<sub>2<sub/> traced by CO (1−0) can be negligible, the [C II]<i>λ<i/>158 <i>μ<i/>m can trace the total H<sub>2<sub/>. We find 70 to 100% of the total H<sub>2<sub/> mass is not traced by CO (1−0) in the dwarf galaxies, but is well-traced by [C II]<i>λ<i/>158 <i>μ<i/>m. The CO-dark gas mass fraction correlates with the observed <i>L<i/><sub>[C II]<sub/>/<i>L<i/><sub>CO(1−0)<sub/> ratio. A conversion factor for [C II]<i>λ<i/>158 <i>μ<i/>m to total H<sub>2<sub/> and a new CO-to-total-<i>M<i/><sub>H<sub>2<sub/><sub/> conversion factor as a function of metallicity are presented.<i>Conclusions.<i/> While low-metallicity galaxies may have a feeble molecular reservoir as surmised from CO observations, the presence of an important reservoir of molecular gas that is not detected by CO can exist. We suggest a general recipe to quantify the total mass of H<sub>2<sub/> in galaxies, taking into account the CO and [C II] observations. Accounting for this CO-dark H<sub>2<sub/> gas, we find that the star-forming dwarf galaxies now fall on the Schmidt-Kennicutt relation. Their star-forming efficiency is rather normal because the reservoir from which they form stars is now more massive when introducing the [C II] measures of the total H<sub>2<sub/> compared to the small amount of H<sub>2<sub/> in the CO-emitting region.