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Session 50 - Interstellar Medium I.
Display session, Thursday, January 08
Exhibit Hall,
Initial analysis of the Midcourse Space Experiment (MSX) Galactic plane survey data within b=\pm 5 degrees reveals an apparently new class of objects in the interstellar medium. In the data analyzed, covering l=269 to l=91, we have discovered approximately 2000 small clouds, one-half to a few arcminutes in diameter, seen in silhouette, against the bright emission of the Galactic plane. The clouds are seen in absorption at all MSX wavebands, covering the spectral region from 7 to 25 microns. We have examined the IRAS data for the largest clouds, and find no emission from these objects at 60 and 100 microns. The clouds near the Galactic center have been observed in emission at 800 microns by Lis amp; Carlstrom (1994). In the mid-IR data, there is also no obvious emission from an associated envelope. We believe the objects to be a heretofore undetected population of cold, dense, isolated clouds. To better determine the physical properties, 2mm H_2CO line observations of the 10 largest clouds are scheduled for November 1997. We shall present the spatial and size distribution of the clouds, and examine their radiative and morphological properties. Finally, we will discuss the origin, properties, and evolution of the clouds.
