First Results from the Arecibo Galactic HiSurvey: 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 to 38 degrees), with high angular (3.5') andvelocity resolution (0.2 km/s). The precursor observations with ALFA of aregion in the Galactic anti-center reveal numerous isolated, small (a few pc insize), and cold (T_k<400 K) HI clouds at low negative velocities, distinctlyseparated from the HI disk emission (`low-velocity clouds', LVCs). These cloudsare most likely located in the transition region between the Galactic disk andhalo (at scale heights of 60--900 pc), yet they have properties of typical coldneutral clouds. LVCs are colder and, most likely, smaller and less massive thanLockman's clouds in the disk/halo interface region of the inner Galaxy. Ourobservations demonstrate that the cloudy structure of the interface region ismost likely a general phenomenon, not restricted to the inner Galaxy. LVCs havesizes and radial velocities in agreement with the expectations for cloudsformed in low-temperature fountain flows, although we measure a factor of tenhigher HI column densities. Alternatively, LVCs could represent the finalstages of the infalling intergalactic material in the on-going construction ofthe Galaxy. In the same dataset at higher negative velocities, we have discovered a`companion' HI cloud located 50' southwest of CHVC186+19-114. CHVC186+19-114 isa typical compact high velocity cloud (HVC) with a well-defined core/envelopestructure. The companion cloud has a diameter of only 7'x9', and is one of thesmallest HVCs known, most likely stripped from the main cloud through theinteractions with the halo medium.Comment: Accepted by Astrophysical Journa