First results from the Arecibo Galactic H I survey: The disk/halo interface region in the outer Galaxy

The consortium for Galactic studies with the Arecibo L-band Feed Array (ALFA) is conducting a neutral hydrogen (HI) survey of the whole Arecibo sky (declination range from -1 degrees to 38 degrees), with high angular (3.'5) and velocity resolution (0.2 km s(-1)). The precursor observations with ALFA of a region in the Galactic anticenter reveal numerous isolated, small (a few parsecs in size), and cold (T-k < 400 K) H I clouds at low negative velocities, distinctly separated from the H I disk emission ('low-velocity clouds'' [ LVCs]). These clouds are most likely located in the transition region between the Galactic disk and halo (at scale heights of 60-900 pc), yet they have properties of typical cold neutral clouds. LVCs are colder and, most likely, smaller and less massive than Lockman's clouds in the disk/halo interface region of the inner Galaxy. Our observations demonstrate that the cloudy structure of the interface region is most likely a general phenomenon, not restricted to the inner Galaxy. LVCs have sizes and radial velocities in agreement with the expectations for clouds formed in low-temperature fountain flows, although we measure a factor of 10 higher H I column densities. Alternatively, LVCs could represent the final stages of the infalling intergalactic material in the ongoing construction of the Galaxy. In the same data set at higher negative velocities, we have discovered a "companion'' H I cloud located 50' southwest of HVC186+19-114. HVC 186+19-114 is a typical compact high-velocity cloud (HVC) with a well-defined core/envelope structure. The companion cloud has a diameter of only 70; 90 and is one of the smallest HVCs known, most likely stripped from the main cloud through the interactions with the halo medium.