IRAS OBSERVATIONS OF THE OUTER GALAXY. Ⅰ. DISCRETE SOURCES AND LARGE-SCALE (DIFFUSE) EMISSION

摘要

The infrared emission in two fields of the outer Galaxy is analyzed using the IRAS image data. We present a census of discrete (but extended) IRAS sources, identified from the 60 μm images. This sample can be used to characterize the properties of typical star-forming regions. We distinguish class A sources (Galactic star-forming regions) from other Galactic objects on the basis of their flux density distributions. However, our class A sources have different infrared colors than other star formation groups (young stellar objects [YSOs] and ultracompact H Ⅱ regions), as measured by the IRAS Point Source Catalogue. The class A colors, in particular the high 12 μm/25 μm ratio, are similar to the color sequence found by Boulanger et al. for the O-star-excited California Nebula. They are also similar to the colors measured, on IRAS image products, for larger "classical" H n regions embedded in large molecular clouds. The similar infrared colors suggest that most class A sources are larger, lower density regions that sample a variety of local radiation field strengths. The infrared colors of class A sources are also found to match the colors of normal spiral galaxies. This suggests that a close correspondence exists between the physical conditions in outer Galaxy star-forming regions and normal spiral galaxies. The discrete sources have been carefully measured on IRAS Coadd images, and detailed comparisons are made with the results from other IRAS data products, especially with the Point Source Catalog (PSC). Great care has been taken to investigate the uncertainties associated with these measurements, and these results may be useful for other studies that make use of IRAS image products. Information from these measurements is combined with larger scale IRAS images to investigate the relative contributions of different components of the outer Galaxy to the total mid- and far-infrared emission. At 60 and 100 μm, the diffuse emission (i.e., infrared cirrus) dominates the emission from the discrete sources, which include all possible sites of star formation.