The 2175 A dust feature in a Gamma Ray Burst afterglow at redshift 2.45

TL;DR

This study reports the detection of the 2175 Å dust extinction feature in a high-redshift gamma-ray burst afterglow, indicating the presence of Milky Way-like dust in its host galaxy at z=2.45.

Contribution

First clear detection of the 2175 Å dust feature at high redshift, revealing early universe dust composition similar to the Milky Way.

Findings

Detection of the 2175 Å bump in a GRB host galaxy at z=2.45

Evidence for carbon-rich dust environment at high redshift

Implications for dust formation in early galaxies

Abstract

We present optical and near-infrared photometry of the afterglow of the long Gamma-Ray Burst GRB 070802 at redshift 2.45 obtained with the ESO/MPI 2.2 m telescope equipped with the multi-channel imager GROND. Follow-up observations in g'r'i'z' and JHK_S bands started ~17 min and extended up to 28 h post burst. We find an increase in brightness of the afterglow at early times, which can be explained by the superposition of reverse and forward shock (FS) emission or the onset of the afterglow FS. Additionally, we detect a strong broad-band absorption feature in the i' band, which we interpret as extinction from the redshifted 2175 A bump in the GRB host galaxy. This is one of the first and clearest detections of the 2175 A feature at high redshift. It is strong evidence for a carbon rich environment, indicating that Milky Way or Large Magellanic Cloud like dust was already formed in substantial amounts in a galaxy at z=2.45.