Interstellar HI Shells Identified in the SETHI Survey

TL;DR

This paper reports the discovery of 74 new interstellar HI shells using the high-resolution SETHI survey data, expanding the catalog of Galactic shells and analyzing their properties and distribution.

Contribution

The study provides a new catalog of 74 previously unidentified HI shells from the SETHI survey, including their properties and spatial distribution, with an emphasis on unbiased detection.

Findings

Most shells are large and old, likely formed by multiple supernovae.

The shells show no preferential alignment with the Galactic plane.

The catalog is not complete due to data limitations and search biases.

Abstract

Galactic HI (neutral hydrogen) shells are central to our understanding of the interstellar medium (ISM), which plays a key role in the development and evolution of galaxies, including our own. Several models involving supernovae and stellar winds have contributed to our broad understanding, but a complete, detailed picture remains elusive. To extend existing Galactic shell catalogs, we visually examined the SETHI (Search for Extraterrestrial HI) database to identify shell-like structures. This high-sensitivity 21-cm radio survey covering the Arecibo sky uniquely provides high-resolution data on shells at a wide range of Galactic latitudes. We present basic information (location, radial velocity, angular size, shape) for 74 previously unidentified HI shells. Due to limitations of coverage and data quality, and the biases inherent in search techniques, our catalog is not a complete sample of Galactic shells. We discuss the catalog completeness, and comment on the new shells' relationship with known interstellar structure as warranted. Unlike many previous catalogs, this sample is not biased towards expanding shells. Where possible we also estimate the kinematic distances, physical sizes, expansion velocities, and energies of these shells. Overall, they are relatively large and old, each the result of multiple supernovae. Unlike previous surveys, we do not find that the shells in our sample are preferentially aligned relative to the Galactic plane.