FAR OUTER GALAXY H-II REGIONS

We have made a multiwavelength (6, 3.6, and 2 cm), high-resolution (3 ''-6 '') radio continuum survey of IRAS-selected sources to search for and study H II regions in the far-outer Galaxy. We identified 31 sour ces with R greater than or similar to 15 kpc and L(FIR) greater than o r similar to 10(4) L., indicating the presence of high-mass star forma tion. We have observed 11 of these sources with the Very Large Array ( VLA). The 6 and 2 cm observations were made using ''scaled arrays,'' m aking possible a direct and reliable determination of spectral indices . Of the 39 sources we detected at 6 cm, 10 have spectral indices cons istent with optically thin free-free emission from H II regions and ar e within 45 '' of the associated IRAS source. Combining our data with previous VLA observations by other investigators we analyzed a sample of 15 of the most remote H II regions found in our Galaxy, located at R = 15-18.2 kpc. The sizes of the H II regions range from less than or similar to 0.10 to 2.3 pc. Using the measured flux densities and size s, we determined their electron densities, emission measures, and exci tation parameters, as well as their Lyman continuum fluxes needed to k eep the nebulae ionized. The sizes and electron densities indicate tha t most of the sources are (ultra)compact H II regions. Seven of the 15 H II regions have sizes less than or similar to 0.20 pc. The large nu mber of compact H II regions suggests that the time these regions spen d in a compact phase must be much longer than their dynamical expansio n times. Five of the 15 H II regions have cometary shapes; the remaind er are spherical or unresolved. Comparison of our data with molecular line maps suggests that the cometary shape of the two H II regions in S127 may be due to pressure confinement of the expanding ionized gas, as in the ''blister'' or ''champagne flow'' models of H II regions. Co mparison of our data with the IRAS data indicates that the five most l uminous H II regions are consistent with a single zero-age main sequen ce O or B star exciting a dust-free H II region.