Far-Infrared [C II] Line Survey Observations of the Galactic Plane

We present results of our survey observations of the [C II] 158 micron line emission from the Galactic plane using the Balloon-borne Infrared Carbon Explorer (BICE). Our survey covers a wide area (350 deg < l < 25 deg, |b| < 3 deg) with a spatial resolution of 15'. We employed a new observing method called the ``fast spectral scanning'' to make large-scale observations efficiently. Strong [C II] line emission was detected from almost all areas we observed. In the general Galactic plane, the spatial distribution of the [C II] line emission correlates very well with that of far-infrared continuum emission, but diffuse components are more prominent in the [C II] line emission; the I_[CII]/I_FIR ratio is ~0.6 % for diffuse components but is ~0.2 % for compact sources such as active star-forming regions. In the Galactic center region, on the other hand, the distribution of the [C II] line emission is quite different from that of the far-infrared continuum emission, and the I_[CII]/I_FIR ratio is systematically lower there. The FWHM velocity resolution of our instrument is 175 km/s, but we determined the central velocity of the line at each observed point very precisely with statistical errors as small as +- 6 km/s. The longitudinal distribution of the central velocity clearly shows the differential rotation pattern of the Galactic disk and also violent velocity fields around the Galactic center.