FAR-INFRARED, SUBMILLIMETER, AND MILLIMETER SPECTROSCOPY OF THE GALACTIC-CENTER - RADIO ARC AND +20/+50 KILOMETER PER 2ND CLOUDS

We present far-infrared, submillimeter, and millimeter spectroscopic observations of the radio arc and the + 20/+50 km s-1 molecular clouds in the Galactic center. The spatial distributions of [C II] 158 μm and molecular line radiation and of the thermal radio continuum emission in the arched filaments of the radio arc are very similar. About 2 × 104 M⊙, or 10% of the total gas mass in the radio arc, are contained in C+ regions. The H+/C+ regions are probably located at the surfaces of the dense molecular clouds in the arc. Profiles, fluxes, and spatial distributions of the [C II] fine structure and CO rotational lines reported here, together with published intensities of the 63 μm [O I] line and of the midinfrared, far-infrared, and radio continua, are very well matched by theoretical models for molecular clouds that are photoionized and photodissociated by stellar UV radiation. The line and continuum intensities do not fit available models in which the neutral interstellar clouds in the arc are ionized by shocks or by magnetohydrodynamic phenomena. The most likely interpretations are that OB stars have recently formed within and/or near the set of molecular clouds of the arc or that UV radiation reaches the arc from the Galactic center itself. This still leaves open the puzzle of the remarkable uniformity of the arc's ionization over scales of tens of parsec. [C II] emission from the +50 km s-1 cloud (M-0.02-0.07) is substantially stronger than from the + 20 km s-1 cloud (M-0.13-0.08). CO 7 → 6 and C18O 2 → 1 emission in the 50 km s-1 cloud comes from a narrow (3 pc) ridge at the eastern edge of the Sgr A radio source complex. Detection of blueshifted high-velocity C18O line emission toward the center of Sgr A East gives direct kinematic evidence for expanding gas motions associated with this nonthermal radio shell source. The energy of the explosion resulting in Sgr A East is estimated to be at least 8 × 1051 ergs. We conclude that the +20 and +50 km s-1 clouds are within 15 pc of the Galactic center. The +50 km s-1 cloud is compressed and accelerated by the expanding Sgr A East shell and may be exposed to the intense UV radiation from Sgr A West. CO 7 → 6 emission from the clouds near the Galactic center and in the radio arc is relatively weak. Either the high temperatures and densities derived from NH3 and CS observations may not be characteristic of the bulk of the molecular gas, or the high filling factor low-J 12CO emission does not come from the bulk of the clouds, but from warm, low-density interclump gas.