APERTURE SYNTHESIS IMAGING OF PARTIALLY-IONIZED GAS IN W48

We present the results of VLA observations of cool gas surrounding the H II region W48 at the H168alpha and C168alpha transition frequencies . Previous single-dish observations of W48 showed that the 168alpha re combination spectrum was composed of a broad line from ionized hydroge n (H+) within the H II region, as well as strong, narrow lines (H-0, C +, and S+) from gas outside the Stromgren sphere. High spatial resolut ion imaging of W48 reveals that the strong C+ and S+ lines arise from the stimulated amplification of the background H II region. Like the C + and S+ lines, the narrow line from partially ionized hydrogen (H-0) is strongest near the continuum peak of the H II region and suggests s timulated amplification of the line. The width of the H-0 line constra ins the temperature of the partially ionized medium to be less than 20 0 K. However, the gas in which the H-0 line originates is found not to be spatially coincident with the carbon-emitting region. We suggest t hat the H-0 emission arises in the advancing ionization front of the H II region, while the C+ and S+ lines originate in a more extended reg ion of the ambient molecular cloud partially ionized by photons with l ambda > 982 angstrom escaping the boundary of the Stromgren sphere.