Large‐scale physical conditions in the interstellar medium from DIRBE observations

Observations from the COBE Diffuse Infrared Background Experiment (DIRBE) of the 140 and 240 μm emission from the Galactic plane region (‖b‖<10°) are analyzed. Assuming an isothermal dust distribution along each line of sight, maps of dust temperature, optical depth, and total far‐infrared (FIR) brightness are derived. These quantities are combined with available 12CO and HI data to produce maps of the gas‐to‐dust mass ratio and the FIR luminosity per hydrogen mass. The results differ from those of a similar analysis using IRAS 60 and 100 μm data (Sodroski et al. 1987), at least in part because emission from small, transiently heated grains does not contribute significantly at 140 or 240 μm.A linear correlation analysis is used to decompose the 140 and 240 μm maps into components associated with the neutral atomic (HI), molecular (H2), and extended low‐density ionized (HII) gas phases of the interstellar medium. From the resulting emission components the large‐scale physical conditions within each gas pha...