LARGE-SCALE [C-II] 158 MICRON EMISSION FROM THE GALAXY

A diffuse far-infrared [C II] emission line has been detected in an extensive region (30-degrees less-than-or-equal-to l less-than-or-equal-to 51-degrees) along the Galactic plane. The [C II] line is bright and extended far from discrete luminous H II regions. Latitudinal and longitudinal profiles of the [C II] intensity distribution are quite similar to those of (CO)-C-12(J = 1-0) and the 100-mu-m continuum, but are completely different from those of H I 21 cm. The diffuse [C II] emission probably comes from the photodissociated C+ regions enveloping giant molecular clouds exposed to the general interstellar UV radiation field. The extended low-density ionized gas might also contribute to the diffuse [C II] emission. The mass of the C+ regions may be as much as 35%-50% of that of CO molecular gas. The total luminosity of the [C II] line emitted from the inner Galactic disk amounts to 2.8(+/-1.1) x 10(7) L., or about 0.36(-0.17)+0.28% of the total far-infrared luminosity of the same region.