Molecular Clouds in the Extreme Outer Galaxy

Details Molecular Clouds in the Extreme Outer Galaxy Molecular Clouds in the Extreme Outer Galaxy

May 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...180.5306d&link_type=abstract

American Astronomical Society, 180th AAS Meeting, #53.06; Bulletin of the American Astronomical Society, Vol. 24, p.817

Astronomy and Astrophysics

Astronomy

2

Scientific paper

Six large molecular clouds have been observed at Galactocentric distances of 18-22 kpc which are comparable in size ( ~ 100 pc) and virial mass (5-10 x 10(4) M_&sun;) to the largest molecular clouds in the solar neighborhood, but are less luminous in CO by a factor of 10-20 and show no evidence of massive star formation. These clouds, discovered with the CfA 1.2 m telescope, are the most removed from the Galactic center of any yet studied. They are nearly coincident in position and velocity with extended H I emission, and therefore may represent the cores of more massive H I associations. At R = 18-22 kpc the interstellar gas is largely atomic and the average surface density is below Kennicutt's (Ap J 1989, 344, 685) critical value for star formation. Our observations in the outer Milky Way are consistent with this limit - although molecular clouds have formed, the IRAS 100 microns survey reveals no embedded star-forming regions. The significance of the faintness of CO and its status as a tracer of H2 in these clouds is uncertain. The H2 masses of the clouds may actually be comparable to the virial masses, with the gas deficient in CO emission owing to a decreased metallicity or kinetic temperature; alternately, the CO line widths may largely be determined by the pressure of the atomic gas.

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