# The host galaxy of GRB 980425 / SN1998bw: a collisional ring galaxy

## TL;DR

This study presents multi-wavelength observations of the host galaxy of GRB 980425/SN 1998bw, revealing a collisional ring galaxy caused by interaction with a dwarf companion, which likely triggered star formation and the gamma-ray burst.

## Contribution

It provides the first detailed evidence of a collisional ring galaxy hosting a gamma-ray burst, supported by observations and simulations of dwarf galaxy interaction.

## Key findings

- Detection of a gas-rich dwarf companion galaxy at 13 kpc.
- Identification of a high-density HI ring passing through star-forming regions.
- Simulation results confirming a head-on collision scenario.

## Abstract

We report Giant Metrewave Radio Telescope (GMRT) , Very Large Telescope (VLT) and Spitzer Space Telescope observations of ESO 184$-$G82, the host galaxy of GRB 980425/SN 1998bw, that yield evidence of a companion dwarf galaxy at a projected distance of 13 kpc. The companion, hereafter GALJ193510-524947, is a gas-rich, star-forming galaxy with a star formation rate of $\rm0.004\,M_{\odot}\, yr^{-1}$, a gas mass of $10^{7.1\pm0.1} M_{\odot}$, and a stellar mass of $10^{7.0\pm0.3} M_{\odot}$. The interaction between ESO 184$-$G82 and GALJ193510-524947 is evident from the extended gaseous structure between the two galaxies in the GMRT HI 21 cm map. We find a ring of high column density HI gas, passing through the actively star forming regions of ESO 184$-$G82 and the GRB location. This ring lends support to the picture in which ESO 184$-$G82 is interacting with GALJ193510-524947. The massive stars in GALJ193510-524947 have similar ages to those in star-forming regions in ESO 184$-$G82, also suggesting that the interaction may have triggered star formation in both galaxies. The gas and star formation properties of ESO 184$-$G82 favour a head-on collision with GALJ193510-524947 rather than a classical tidal interaction. We perform state-of-the art simulations of dwarf--dwarf mergers and confirm that the observed properties of ESO 184$-$G82 can be reproduced by collision with a small companion galaxy. This is a very clear case of interaction in a gamma ray burst host galaxy, and of interaction-driven star formation giving rise to a gamma ray burst in a dense environment.