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:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table.dat 96 3400 Cloudy model parameters and predictions,
as plotted in Figure 6
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Byte-by-byte Description of file: table.dat
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Bytes Format Units Label Explanations
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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
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Acknowledgements:
Suzanne Madden, suzanne.madden(at)cea.fr
(End) Suzanne Madden [CEA], Patricia Vannier [CDS] 23-Oct-2020