Faint H I 21 cm emission line wings at forbidden velocities

We present the results of a search for faint Hi 21 cm emission linewings at velocities forbidden by Galactic rotation in the Galactic plane using the Leiden/Dwingeloo H I Survey data and the H I Southern Galactic Plane Survey data. These "forbidden-velocity wings'' (FVWs) appear as protruding excessive emission in comparison with their surroundings in limited (less than or similar to 2 degrees ) spatial regions over velocity extents of more than similar to 20 km s(-1) in large-scale (l, v) diagrams. Their high velocities imply that there should be some dynamical phenomena associated. We have identified 87 FVWs. We present their catalog and discuss their distribution and statistical properties. We found that 85% of FVWs are not coincident with known supernova remnants (SNRs), galaxies, or high-velocity clouds. Their natures are currently unknown. We suspect that many of them are fast-moving H I shells and filaments associated with the oldest SNRs that are essentially invisible except via their H I line emission. We discuss other possible origins.