J/A+A/471/103 SIMBA observations of SMC (Bot+, 2007)
Millimeter dust continuum emission revealing the true mass of giant molecular
clouds in the Small Magellanic Cloud
Bot C., Boulanger F., Rubio M., Rantakyro F.
<Astron. Astrophys. 471, 103 (2007)>
=2007A&A...471..103B 2007A&A...471..103B
ADC_Keywords: Magellanic Clouds ; Molecular clouds ; Millimetric/submm sources
Keywords: clouds - submillimeter - ISM: molecules - Magellanic Clouds -
ISM: magnetic fields
Abstract:
CO observations have been the best way so far to trace molecular gas
in external galaxies, but in low metallicity environments the gas mass
deduced could be largely underestimated due to enhanced
photodissociation of the CO molecule. Large envelopes of H2 could
therefore be missed by CO observations. At present, the kinematic
information of CO data cubes are used to estimate virial masses and
trace the total mass of the molecular clouds. Millimeter dust emission
can also be used as a dense gas tracer and could unveil H2 envelopes
lacking CO. These different tracers must be compared in different
environments. This study compares virial masses to masses deduced from
millimeter emission, in two GMC samples: the local molecular clouds in
our Galaxy (104-105M☉), and their equivalents in the Small
Magellanic Cloud (SMC), one of the nearest low metallicity dwarf
galaxies. In our Galaxy, mass estimates deduced from millimeter
(FIRAS) emission are consistent with masses deduced from gamma ray
analysis and therefore trace the total mass of the clouds. Virial
masses are systematically larger (twice on average) than mass
estimates from millimeter dust emission. This difference decreases
toward high masses and has been reported in previous studies. This is
not the case for SMC giant molecular clouds: molecular cloud masses
deduced from SIMBA millimeter observations are systematically higher
(twice on average for conservative values of the dust to gas ratio and
dust emissivity) than the virial masses from SEST CO observations. The
observed excess cannot be accounted for by any plausible change of
dust properties. Taking a general form for the virial theorem, we show
that a magnetic field strength of ∼15µG in SMC clouds could
provide additional support for the clouds and explain the difference
observed. We conclude that masses of SMC molecular clouds have so far
been underestimated. Magnetic pressure may contribute significantly to
their support.
Objects:
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RA (2000) DE Designation(s) (File)
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00 46 40 -73 06.2 SMC = NGC 292 (smcsimba.fit)
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
smcsimba.fit 2880 157 SIMBA data (1.2mm) for the SMC
(centered at J004640-73062, size 45x46')
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Acknowledgements:
Caroline Bot, bot(at)caltech.edu
(End) Patricia Vannier [CDS] 09-Aug-2007