Astronomy and Astrophysics – Astronomy
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008aas...212.8602d&link_type=abstract
American Astronomical Society, AAS Meeting #212, #86.02; Bulletin of the American Astronomical Society, Vol. 40, p.266
Astronomy and Astrophysics
Astronomy
Scientific paper
Molecular clouds are perhaps the best overall tracers of galactic spiral arms. The broad sweeps of the Carina and Perseus arms stand out clearly in CO longitude-velocity diagrams of the Galaxy, and sensitive, high-resolution observations of external spirals such as M31 and M51 reveal similar high contrast CO spiral arms. These observations are entirely consistent with the prevailing view that molecular clouds are relatively short-lived galactic objects which produce all of the young stars and HII regions that are the defining constituents of galactic spiral arms.
Although the Galactic molecular gas shows very complex structure on many spatial scales, most of the molecular mass is contained in a fairly small number of large clouds such as those associated with W51, W44, and Cas A. Tracing the Galactic spiral arms is best done with these GMCs because they are generally well defined, they can be readily detected and resolved throughout the Galaxy in CO even with a small telescope, and they contain so much gas and dust, and so many young objects, that they are conspicuous in every major wavelength band from gamma rays to long-wavelength radio.
This talk will review the progress that has been made to date in tracing the Galactic spiral arms with GMCs, and the prospects for overcoming the considerable challenges that remain. Key among these are difficulties in identifying individual GMCs in the crowded molecular ring region of the inner Galaxy and uncertainties imposed on kinematically-derived distances by spiral density wave streaming and shocks, the Galactic bar, local disturbances, and random cloud motions.
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