Hi Shells and Supershells in the I-GALFA Hi 21-cm Line Survey: I. Fast-Expanding Hi Shells Associated with Supernova Remnants

TL;DR

This study uses high-resolution 21-cm line data from the I-GALFA survey to identify and analyze fast-expanding Hi shells associated with four Galactic supernova remnants, revealing new details about their dynamics and physical conditions.

Contribution

First detection of both sides of an expanding Hi shell in an SNR, providing new insights into SNR-ISM interactions and the physical characteristics of these remnants.

Findings

Detected Hi shells in four SNRs, including the first dual-velocity detection in W44.

Identified a highly symmetric expanding Hi shell in G54.4-0.3.

SNRs likely originate from core-collapse supernovae in dense environments.

Abstract

We search for fast-expanding Hi shells associated with Galactic supernova remnants (SNRs) in the longitude range l \approx 32\arcdeg to 77\arcdeg using 21-cm line data from the Inner-Galaxy Arecibo L-band Feed Array (I-GALFA) Hi survey. Among the 39 known Galactic SNRs in this region, we find such Hi shells in four SNRs: W44, G54.4-0.3, W51C, and CTB 80. All four were previously identified in low-resolution surveys, and three of those (excluding G54.4-0.3) were previously studied with the Arecibo telescope. A remarkable new result, however, is the detection of Hi emission at both very high positive and negative velocities in W44 from the receding and approaching parts of the Hi expanding shell, respectively. This is the first detection of both sides of an expanding shell associated with an SNR in Hi 21-cm emission. The high-resolution I-GALFA survey data also reveal a prominent expanding Hi shell with high circular symmetry associated with G54.4-0.3. We explore the physical characteristics of four SNRs and discuss what differentiates them from other SNRs in the survey area. We conclude that these four SNRs are likely the remnants of core-collapse supernovae interacting with a relatively dense (>~1 cm^-3) ambient medium, and we discuss the visibility of SNRs in the Hi 21-cm line.