IRAS OBSERVATIONS OF THE OUTER GALAXY .1. DISCRETE SOURCES AND LARGE-SCALE (DIFFUSE) EMISSION

The infrared emission in two fields of the outer Galaxy is analyzed us ing the IRAS image data. We present a census of discrete (but extended ) IRAS sources, identified from the 60 mu m 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 ot her Galactic objects on the basis of their flux density distributions. However, our class A sources have different infrared colors than othe r star formation groups (young stellar objects [YSOs] and ultracompact H II regions), as measured by the IRAS Point Source Catalogue. The cl ass A colors, in particular the high 12 mu m/25 mu m ratio, are simila r to the color sequence found by Boulanger et al. for the O-star-excit ed California Nebula. They are also similar to the colors measured, on IRAS image products, for larger ''classical'' H II regions embedded i n large molecular clouds. The similar infrared colors suggest that mos t class A sources are larger, lower density regions that sample a vari ety of local radiation held 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 gala xies. 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). Gr eat care has been taken to investigate the uncertainties associated wi th these measurements, and these results may be useful for other studi es that make use of IRAS image products. Information from these measur ements is combined with larger scale IRAS images to investigate the re lative contributions of different components of the outer Galaxy to th e total mid- and far-infrared emission. At 60 and 100 mu m, the diffus e emission (i.e., infrared cirrus) dominates the emission from the dis crete sources, which include all possible sites of star formation.