Deficit of Far-Infrared [C II] Line Emission toward the Galactic Center

We have observed the [C II] 158 μm line emission from the Galactic plane (-10° < l < 25°, |b| ≤ 3°) with the Balloon-borne Infrared Carbon Explorer (BICE). The observed longitudinal distribution of the [C II] line emission is clearly different from that of the far-infrared continuum emission; the Galactic center is not the dominant peak in the [C II] emission. Indeed, the ratio of the [C II] line emission to that of the far-infrared continuum (I[C II]/IFIR) is systematically low within the central several hundred parsecs of the Galaxy. The observational results indicate that the abundance of the C+ ions themselves is low in the Galactic center. We attribute this low abundance mainly to soft UV radiation with fewer C-ionizing photons. This soft radiation field, together with the pervasively high molecular gas density, makes the molecular self-shielding more effective in the Galactic center. The self-shielding further reduces the abundance of C+ ions, and raises the temperature of molecular gas at the C+/C/CO transition zone.