Issue |
A&A
Volume 473, Number 3, October III 2007
|
|
---|---|---|
Page(s) | 863 - 870 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20077277 | |
Published online | 13 August 2007 |
Molecular gas in high-velocity clouds: revisited scenario*
1
Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland e-mail: miroslava.dessauges@obs.unige.ch
2
Observatoire de Paris, LERMA, 61 Av. de l'Observatoire, 75014 Paris, France
Received:
10
February
2007
Accepted:
4
July
2007
We report a new search for 12CO(1–0) emission in high-velocity
clouds (HVCs) performed with the IRAM 30 m millimeter-wave telescope. This
search was motivated by the recent detection of cold dust emission ( K) in the HVCs of Complex C, implying a total gas column density 5 times
larger than the column density measured in
and suggesting undetected
gas, presumably in molecular form. Despite a spatial resolution which is three
times better and sensitivity twice as good compared to previous studies, no CO emission is detected in the HVCs of Complex C down to a best 5σ limit
of 0.16 K km s-1 at a
resolution. The non-detection of both the
12CO(1–0) emission and of the diffuse H2 absorption with the Far
Ultraviolet Spectroscopic Explorer does not provide any evidence in favor of
large amounts of molecular gas in these HVCs and hence in favor of the infrared
findings. We discuss different configurations which, however, allow us to
reconcile the negative CO result with the presence of molecular gas and cold
dust emission. H2 column densities higher than our detection limit,
cm-2, are expected to be confined in very
small and dense clumps with 20 times smaller sizes than the 0.5 pc clumps
resolved in our observations according to the results obtained in cirrus
clouds, and might thus still be highly diluted. As a consequence, the
inter-clump gas at the 1 pc scale, as resolved in our data, has a volume
density lower than 20 cm-3 and already appears as too diffuse to excite
the CO molecules. The observed physical conditions in the HVCs of Complex C
also play an important role against CO emission detection. The sub-solar
metallicity of
dex affects the H2 formation rate onto dust grains,
and it has been shown that the CO-to-H2 conversion factor in low metallicity
media is 60 times higher than at the solar metallicity, leading for a given
H2 column density to a 60 times weaker integrated CO intensity. And the very
low dust temperature estimated in these HVCs implies the possible presence of
gas cold enough (<20 K) to cause CO condensation onto dust grains under
interstellar medium pressure conditions and thus CO depletion in gas-phase
observations.
Key words: Galaxy: halo / ISM: clouds / ISM: molecules / radio lines: ISM
© ESO, 2007
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