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 12 a ctive galactic nuclei seen toward the Galactic anticenter. All of the sources are at \b\ < 10 degrees, and seven have \b\ < 0 degrees 5. Our VLBA observations can detect an enhancement in the angular broadening of these sources due to an extended H II disk, if the orientation of the H II disk in the outer Galaxy is similar to that of the H I disk. Such an extended H II disk is suggested by the C Iv absorption in a qu asar's spectrum, the appearance of H I disks of nearby spiral galaxies , and models of Ly alpha cloud absorbers and the Galactic fountain. We detect 11 of the 12 sources at one or more frequencies; nine of the s ources are compact and suitable for an angular broadening analysis. A preliminary analysis of the observed angular diameters suggests that t he H 11 disk does not display considerable warping or flaring and does not extend to large Galactocentric distances (R greater than or simil ar to 100 kpc). A companion paper (Lazio & Cordes) combines these obse rvations with those in the literature and presents a more comprehensiv e analysis.