Evidence for a Thick Molecular Disk in the Milky Way

Details Evidence for a Thick Molecular Disk in the Milky Way Evidence for a Thick Molecular Disk in the Milky Way

May 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...180.5304d&link_type=abstract

American Astronomical Society, 180th AAS Meeting, #53.04; Bulletin of the American Astronomical Society, Vol. 24, p.817

Astronomy and Astrophysics

Astrophysics

Scientific paper

Over the past two decades, sensitive 21 cm surveys of the Milky Way have revealed a very complicated vertical (z) structure for atomic gas, with many large sheets, filaments, and shells extending more than 1 kpc above the central approximately gaussian layer ~ 220 pc thick at half intensity. In contrast, the observed distribution of molecular gas with z has remained essentially simple --- a single thin layer ~ 120 pc thick --- if only because CO surveys with latitude coverage and sensitivity adequate to detect possible molecular counterparts to the high-z atomic structures have yet to be completed. We are currently using the 1.2 m telescope at the Center for Astrophysics to carry out the first sensitive and systematic search for Galactic molecular gas more than 100 pc from the plane. Four pilot regions at Galactic longitudes of 25(deg) , 30(deg) , 40(deg) , and 50(deg) , each ~ 1(deg) wide in l and extending to +/-4(deg) in b, are being surveyed in the J = 1->0 line of CO. With an rms sensitivity per point of 0.1 K at Delta v = 1.3 km s(-1) and a sampling interval of 3'.75 (0.43 beamwidth), this survey is 3-10 times more sensitive per unit solid angle than existing CO surveys of the Galactic plane. The observations have so far revealed two discrete molecular clouds as far from the plane as any previously known (z >= 200 pc), as well as general diffuse emission extending from the plane to heights of 200-300 pc. This diffuse CO, detected in all four of the regions, is evidence for a faint, thick molecular disk in the Milky Way 2-3 times as wide as that of the dense molecular layer previously studied, and comparable in width to the central H I layer. A still unresolved question is how this diffuse CO emission is related to the local molecular gas associated with the IRAS cirrus.

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