The Radial Extent and Warp of the Ionized Galactic Disk. I. A VLBA Survey of Extragalactic Sources toward the Anticenter

We report multifrequency Very Long Baseline Array observations of twelveactive galactic nuclei seen toward the Galactic anticenter. All of the sourcesare at |b| < 10 degrees and seven have |b| < 0.5 degrees. Our VLBA observationscan detect an enhancement in the angular broadening of these sources due to anextended H II disk, if the orientation of the H II disk in the outer Galaxy issimilar to that of the H I disk. Such an extended H II disk is suggested by theC IV absorption in a quasar's spectrum, the appearance of H I disks of nearbyspiral galaxies, and models of Ly-alpha cloud absorbers and the Galacticfountain. We detect eleven of the twelve sources at one or more frequencies;nine of the sources are compact and suitable for an angular broadeninganalysis. A preliminary analysis of the observed angular diameters suggeststhat the H II disk does not display considerable warping or flaring and doesnot extend to large Galactocentric distances (R >~ 100 kpc). A companion paper(Lazio & Cordes 1997) combines these observations with those in the literatureand presents a more comprehensive analysis.