Faint HI 21-cm Emission Line Wings at Forbidden-Velocities

TL;DR

This paper reports the discovery and cataloging of 87 faint HI 21-cm emission features at forbidden velocities, suggesting they may be linked to old supernova remnants or other dynamical phenomena in the Galaxy.

Contribution

It introduces a new catalog of forbidden-velocity HI features and analyzes their distribution, proposing potential origins related to supernova remnants and galactic dynamics.

Findings

87 FVWs identified and cataloged.

85% of FVWs are not associated with known objects.

Many FVWs may be old SNRs or related to galactic dynamical phenomena.

Abstract

We present the results of a search for faint HI 21-cm emission line wings at velocities forbidden by Galactic rotation in the Galactic plane using the Leiden/Dwingeloo HI Survey data and the HI Southern Galactic Plane Survey data. These ``forbidden-velocity wings (FVWs)'' appear as protruding excessive emission in comparison with their surroundings in limited (< 2 deg) spatial regions over velocity extent more than ~20 km/s 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 HI shells and filaments associated with the oldest SNRs that are essentially invisible except via their HI line emission. We discuss other possible origins.