Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005jrasc..99q.129g&link_type=abstract
Journal of the Royal Astronomical Society of Canada, Vol. 99, No. 4, p.129
Astronomy and Astrophysics
Astronomy
Scientific paper
Cold atomic gas contains some 30% of the mass of the interstellar medium (ISM) in clouds occupying only a few percent of the ISM volume. The arcminute resolution of the International Galactic Plane Survey (IGPS) allows these clouds to be imaged over a large area as 21cm-line H i self-absorption (HISA) shadows against warmer background H i emission. The IGPS HISA features have several characteristics in common with molecular clouds: (1) rich, intricate structure over many size scales; (2) concentration at radial velocities expected for spiral arms; and (3) gas temperatures often in the few tens of Kelvin range. Despite these similarities, many HISA features lack CO emission. Equilibrium ISM models require molecular gas to maintain the cold H i temperatures seen in both HISAand continuum absorption studies. But what if the HISAclouds are not in equilibrium? We suggest that the IGPS HISA may trace cooling atomic gas in transition to the molecular phase, following passage through a spiral arm shock. Such ISM evolution is a natural precursor to massive star formation in spiral arms.
Brunt Chris M.
Dewdney Peter E.
Gibson Steven J.
Higgs Lloyd A.
Stil Jeroen M.
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