An infrared survey of the diffuse emission within 5 ̊of the galactic plane

The AFGL Infrared Sky Survey data have been processed to deconvolve the low frequency attenuation, restoring the signals from extended sources. The 11 and 20-micrometer scans which cover three quarters of the galactic plane have been analyzed, with lesser area covered at 4 and 27 micrometers. A detailed description is given of the instrumentation, the conduct of the experiment, and the data reduction procedures. The measured extended emission in the 3- to 30-micrometer region may be divided into several components. A number of discrete, extended sources are observed within 5 deg of the galactic plane, the majority of which are associated with HII regions. About 25 percent of these sources are not in the AFGL catalog. A large scale diffuse emission is centered on the galactic plane at longitudes less than 90 deg from the galactic center. The 11 to 20-micrometer color ratio of this emission is distinctly smaller in the direction of the Perseus external arm and the Sagittarius-Carina spiral than closer to the center (1 <65 deg). Most of this background is probably due to thermal radiation from HII regions along the line of sight. Interior to approx. 65 deg, the 4-micrometer measurements can be understood in terms of the 2.4-micrometer balloon-borne survey observation and interpreting the background at both wavelengths as being due to late type stars. The 20-micrometer values over these longitudes are probably due to HII regions. Additional 11-micrometer sources are required to explain the measured values.