SUBMILLIMETER OBSERVATIONS OF THE GALACTIC PLANE BY IRTS

We report on observations of interstellar dust emission at \b\ less th an or equal to 3.degrees 5 and l similar to 46 degrees with the Far In frared Photometer (FIRP) on board the Infrared Telescope in Space (IRT S). The FIRP is a four-channel photometer with bands centered at 150, 250, 400, and 700 mu m and with a spatial resolution of 0.degrees 5 (F WHM). The 150-250 mu m and the 250-400 mu m brightness ratios of the i nterstellar dust emission show a slight decrease with increasing galac tic latitude at \b\ less than or equal to 3.degrees 5, consistent with the 140-240 mu m brightness ratio observed by COBE/DIRBE. The 250-700 mu m brightness ratio shows a much stronger dependence on the galacti c latitude. The effect may be due to a single component of warm (T sim ilar to 20 K) dust with an emissivity index of n < 2, that decreases w ith galactic latitude. Alternatively, the effect may be due to the exi stence of two components of dust, a warm (T similar to 20 K) component and a cold (T similar to 4-7 K) component, each with an emissivity in dex of n = 2. In the latter case, either the temperature or the optica l depth of the cold component increases as a function of the galactic latitude. In either case, the physical properties of the interstellar dust must be assumed to vary significantly over the range \b\ less tha n or equal to 3.degrees 5.