First measurement of HI 21cm emission from a GRB host galaxy indicates a post-merger system

TL;DR

This study presents the first detection of HI 21cm emission from a GRB host galaxy, revealing a disturbed gas disk likely caused by a recent merger that triggered intense star formation and the gamma ray burst.

Contribution

It provides the first emission detection of HI in a GRB host galaxy and links recent merger activity to GRB progenitor environments.

Findings

HI mass is twice the stellar mass of the galaxy.

The galaxy shows a disturbed rotating gas disk.

GRB and Wolf-Rayet region are in high HI density areas.

Abstract

We report the detection and mapping of atomic hydrogen in HI 21cm emission from ESO 184-G82, the host galaxy of the gamma ray burst 980425. This is the first instance where HI in emission has been detected from a galaxy hosting a gamma ray burst. ESO 184-G82 is an isolated galaxy and contains a Wolf-Rayet region close to the location of the gamma ray burst and the associated supernova, SN 1998bw. This is one of the most luminous HII regions identified in the local Universe, with a very high inferred density of star formation. The HI 21cm observations reveal a high HI mass for the galaxy, twice as large as the stellar mass. The spatial and velocity distribution of the HI 21cm emission reveals a disturbed rotating gas disk, which suggests that the galaxy has undergone a recent minor merger that disrupted its rotation. We find that the Wolf-Rayet region and the gamma ray burst are both located in the highest HI column density region of the galaxy. We speculate that the merger event has resulted in shock compression of the gas, triggering extreme star formation activity, and resulting in the formation of both the Wolf-Rayet region and the gamma ray burst. The high HI column density environment of the GRB is consistent with the high HI column densities seen in absorption in the host galaxies of high redshift gamma ray bursts.