PRELIMINARY SPECTRAL OBSERVATIONS OF THE GALAXY WITH A 7-DEGREES BEAM BY THE COSMIC BACKGROUND EXPLORER (COBE)

The far-infrared absolute spectrophotometer (FIRAS) on the Cosmic Background Explorer (COBE) has carried out the first all-sky spectral line survey in the far-infrared region, as well as mapping spectra of the Galactic dust distribution at lambda > 100-mu-m. Lines of [C I], [C II], [N II], and CO are all clearly detected, [C II] (158-mu-m) and [N II] (205.3-mu-m) with sufficient strength to be mapped, and the wavelength of the [N II] line at 205.3-mu-m is determined by observation for the first time. The mean line intensities are interpreted in terms of the heating and cooling of the multiple phases of the interstellar gas. In addition, an average spectrum of the galaxy is constructed and searched for weak lines. The spectrum of the galaxy observed by FIRAS has two major components: a continuous spectrum due to interstellar dust heated by starlight with a total luminosity of (1.8 +/- 0.6)(R0/8.5 kpc)2 x 10(10) L. within the solar circle; and a line spectrum dominated by the strong 158-mu-m line from singly ionized carbon, with a spatial distribution similar to the dust distribution, and a luminosity of 0.3% of the dust luminosity. There are in addition moderately strong 122 and 205.3-mu-m lines, identified as coming from singly ionized nitrogen, which contribute 0.04% and 0.03% of the total dust luminosity. The much weaker lines of neutral carbon at 370 and 609-mu-m are seen, as are the 2-1 through 5-4 CO lines. These low-J CO lines contribute 0.003% of the total dust luminosity. Maps of the emission by dust, [C II], and [N II] are presented.