# Near-infrared [Fe II] and H$_{2}$ Emission-line Study of Galactic Supernova Remnants in the First Quadrant

## TL;DR

This study uses near-infrared imaging to detect [Fe II] and H₂ emission lines in Galactic supernova remnants, revealing emission characteristics, spatial distributions, and the brightest sources, contributing to understanding SNRs' impact on the interstellar medium.

## Contribution

First comprehensive NIR emission-line survey of Galactic SNRs in the first quadrant, identifying emission features and spatial patterns, and analyzing their origins and luminosities.

## Key findings

- 24% detection rate for [Fe II] and H₂ emissions among surveyed SNRs.
- Identification of an '[Fe II]-H₂ reversal' in some SNRs indicating complex shock interactions.
- W49B is the brightest SNR in both [Fe II] and H₂ emissions, dominating the luminosity contributions.

## Abstract

We report the detection of near-infrared (NIR) [Fe II] (1.644 $\mu$m) and H$_{2}$ 1-0 S(1) (2.122 $\mu$m) line features associated with Galactic supernova remnants (SNRs) in the first quadrant using two narrowband imaging surveys, UWIFE and UWISH2. Among the total of 79 SNRs fully covered by both surveys, we found 19 [Fe II]-emitting and 19 H$_{2}$-emitting SNRs, giving a detection rate of 24% for each. Eleven SNRs show both emission features. The detection rate of [Fe II] and H$_{2}$ peaks at the Galactic longitude ($l$) of $40^{\circ}$-$50^{\circ}$ and $30^{\circ}$-$40^{\circ}$, respectively, and gradually decreases toward smaller/larger $l$. Five out of the eleven SNRs emitting both emission lines clearly show an "[Fe II]-H$_{2}$ reversal," where H$_{2}$ emission features are found outside the SNR boundary in [Fe II] emission. Our NIR spectroscopy shows that the H$_{2}$ emission originates from collisionally excited H$_{2}$ gas. The brightest SNR in both [Fe II] and H$_{2}$ emissions is W49B, contributing more than 70% and 50% of the total [Fe II] 1.644 $\mu$m ($2.0 \times 10^4$ L$_{\odot}$) and H$_{2}$ 2.122 $\mu$m ($1.2 \times 10^3$ L$_{\odot}$) luminosities of the detected SNRs. The total [Fe II] 1.644 $\mu$m luminosity of our Galaxy is a few times smaller than that expected from the SN rate using the correlation found in nearby starburst galaxies. We discuss possible explanations for this.