J/A+A/643/A141ivo://CDS.VizieR/J/A+A/643/A141doi:10.26093/cds/vizier.36430141CDSMadden S.C.Cormier D.Hony S.Lebouteiller V.Abel N.Galametz M.,De Looze I.Chevance M.Polles F.L.Lee M.-Y.Galliano F.,Lambert-Huyghe A.Hu D.Ramambason L.2021-02-02T08:54:45Z2020-11-17T05:56:53ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frgalaxiesinfrared-sourcesinterstellar-mediummolecular-cloudsMolecular 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 [CII] 158um is relatively bright, highlighting a potentially important reservoir of H_2_ that is not traced by CO (1-0), but residing in the [CII] - emitting regions.Here we aim to explore a method to quantify the total H_2_ mass (MH_2_) in galaxies and learn what parameters control the CO-dark reservoir. We present Cloudy grids of density, radiation field and metallicity in terms of observed quantities, such as [OI], [CI], CO (1-0), [CII] and L_TIR_ and the total MH_2_. We provide recipes based on these models to derive total MH_2_ mass estimates from observations. We apply the models to the Herschel Dwarf Galaxy Survey, extracting the total MH_2_ for each galaxy and compare this to the H_2_ determined from the observed CO (1-0) line. This allows us to quantify the reservoir of H_2_ that is CO-dark and traced by the [CII]158um. We demonstrate that while the H2 traced by CO(1-0) can be negligible, the [CII] 518um can trace the total H2. We find 70% to 100 % of the total H2 mass is not traced by CO (1-0) in the dwarf galaxies, but is well-traced by [CII] 158um. The CO-dark gas mass fraction correlates with the observed L[CII]/LCO(1-0) ratio. A conversion factor for [CII] 158um to total H_2_ and a new CO-to-total-MH_2_ as a function of metallicity, is presented. While low metallicity galaxies may have a feeble molecular reservoir as surmised from CO observations, the presence of an important reservoir of molecular gas, not detected by CO, can exist. We suggest a general recipe to quantify the total mass of H2 in galaxies, taking into account the CO and [CII] observations. Accounting for this CO-dark H_2_ gas, we find that the star forming dwarf galaxies now fall on the Schmidt-Kennicuttrelation. Their star-forming efficiency is rather normal, since the reservoir from which they form stars is now more massive when introducing the [CII] measures of the total H_2_, compared to the little amount of H_2_ in the CO-emitting region.2020A&A...643A.141Mhttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/643/A141CatalogResearchIsServedByTAP VizieR generic servicehttps://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/643/A141https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/A+A/643/A141http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/A+A/643/A141https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/A+A/643/A141https://vizier.iucaa.in/viz-bin/votable?-source=J/A+A/643/A141http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/A+A/643/A141GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapInfrareddefaultJ/A+A/643/A141/tableCloudy model parameters and predictions, as plotted in Figure 6recnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintZModel metallicity, in Z_{sun}_ unitlog(Sun)meta.unit;phys.abund.Z;meta.modelledfloatlognHModel densitylog(cm**-3)phys.density;meta.modelledfloatlogRinModel inner radiuslog(cm)phys.size.radius;meta.modelledfloatlogGo? Model radiation field in terms of the Habing Radiation Fieldphys.absorption;meta.modelledfloatnullableAV? Model visual extinctionmagphys.absorption;meta.modelledfloatnullabletauCO? Model CO(1-0) optical depthphys.absorption.opticalDepth;meta.modelledfloatnullablelogM(H2)? Predicted H_2_ masslog(solMass)phys.massfloatnullablelogL(CII157)? Predicted [CII] 157um luminositylog(solLum)phys.luminosityfloatnullablelogL(CO1-0)? Predicted CO(1-0) luminositylog(solLum)phys.luminosityfloatnullablen_logL(CO1-0)[I] I for -infinitymeta.notecharlogL(CI610)? Predicted CI 610um luminositylog(solLum)phys.luminosityfloatnullablelogL(OI63)? Predicted OI 63um luminositylog(solLum)phys.luminosityfloatnullablelogL(OI145)? Predicted OI 145um luminositylog(solLum)phys.luminosityfloatnullable