J/A+A/643/A141      Tracing total molecular gas in galaxies      (Madden+, 2020)

Tracing the total molecular gas in galaxies: [CII] and the CO-dark gas. Madden 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. <Astron. Astrophys. 643, A141 (2020)> =2020A&A...643A.141M 2020A&A...643A.141M (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies, IR ; Interstellar medium ; Molecular clouds Keywords: photon-dominated region (PDR) - galaxies: ISM - galaxies: dwarf - HII regions - infrared: ISM Abstract: 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 [CII] 158um is relatively bright, highlighting a potentially important reservoir of H2 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 H2 mass (MH2) 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 LTIR and the total MH2. We provide recipes based on these models to derive total MH2 mass estimates from observations. We apply the models to the Herschel Dwarf Galaxy Survey, extracting the total MH2 for each galaxy and compare this to the H2 determined from the observed CO (1-0) line. This allows us to quantify the reservoir of H2 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 H2 and a new CO-to-total-MH2 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 H2 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 H2, compared to the little amount of H2 in the CO-emitting region. Description: We have run Cloudy grids to solve for the total molecular hydrogen gas mass particularly for low metallicity environments such as the Dwarf Galaxy Survey. The grids were computed by varying the initial density at the illuminated face of the HII region, nH, and the distance from the source to the edge of the illuminated HII region, the inner radius, rin. The ionization parameter (U) is deduced in the model, based on the input ionizing source, rin and nH. The models are calculated for 5 metallicity bins: Z=0.05, 0.1, 0.25, 0.5 and 1.0 Z, and nH ranging from 10 to 104cm-3. The rin values range from log(rincm)=20.0 to 21.3, in steps of 0.3dex, which, for the various models, covers a range of G0=17 to 11481 in terms of the Habing radiation field. AV and tauCO are calculated for the models. From these models the user can obtain the predicted MH2, [CI], CO (1-0), [CI] and [OI] luminosities from the model output. This table follows the plots of Figure 6 "spaghetti plots". File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . This file table.dat 96 3400 Cloudy model parameters and predictions, as plotted in Figure 6 -------------------------------------------------------------------------------- Byte-by-byte Description of file: table.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 5- 8 F4.2 [Sun] Z Model metallicity, in Z unit 13- 16 F4.2 [cm-3] lognH Model density 19- 24 F6.3 [cm] logRin Model inner radius 28- 32 F5.3 [-] logGo ? Model radiation field in terms of the Habing Radiation Field 35- 40 F6.3 mag AV ? Model visual extinction 43- 48 F6.3 --- tauCO ? Model CO(1-0) optical depth 51- 56 F6.3 [Msun] logM(H2) ? Predicted H2 mass 59- 64 F6.3 [Lsun] logL(CII157) ? Predicted [CII] 157um luminosity 66- 72 F7.3 [Lsun] logL(CO1-0) ? Predicted CO(1-0) luminosity 73 A1 --- n_logL(CO1-0) [I] I for -infinity 75- 80 F6.3 [Lsun] logL(CI610) ? Predicted CI 610um luminosity 83- 88 F6.3 [Lsun] logL(OI63) ? Predicted OI 63um luminosity 91- 96 F6.3 [Lsun] logL(OI145) ? Predicted OI 145um luminosity -------------------------------------------------------------------------------- Acknowledgements: Suzanne Madden, suzanne.madden(at)cea.fr
(End) Suzanne Madden [CEA], Patricia Vannier [CDS] 23-Oct-2020
The document above follows the rules of the Standard Description for Astronomical Catalogues; from this documentation it is possible to generate f77 program to load files into arrays or line by line