Dust Extinction in High-z Galaxies with GRB Afterglow Spectroscopy - The 2175{\AA} Feature at z=2.45

TL;DR

This study reports the first detection of the 2175Å dust extinction feature at z=2.45 in a GRB host galaxy, revealing insights into dust properties and metallicity at high redshift.

Contribution

It presents the highest redshift detection of the 2175Å bump in a GRB host and explores its correlation with environmental factors like CI absorption and metallicity.

Findings

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

The extinction curve best fits an LMC-type law.

Presence of CI absorption correlates with the bump.

Abstract

We report the clear detection of the 2175A dust absorption feature in the optical afterglow spectrum of the gamma-ray burst (GRB) GRB070802 at a redshift of z=2.45. This is the highest redshift for a detected 2175A dust bump to date, and it is the first clear detection of the 2175A bump in a GRB host galaxy, while several tens of optical afterglow spectra without the bump have been recorded in the past decade. The derived extinction curve gives A_V=0.8-1.5 depending on the assumed intrinsic slope. Of the three local extinction laws, an LMC type extinction gives the best fit to the extinction curve of the host of GRB070802. Besides the 2175A bump we find that the spectrum of GRB070802 is characterized by unusually strong low-ionization metal lines and possibly a high metallicity for a GRB sightline ([Si/H]=-0.46+/-0.38, [Zn/H]=-0.50+/-0.68). In particular, the spectrum of GRB070802 is unique for a GRB spectrum in that it shows clear CI absorption features, leading us to propose a correlation between the presence of the bump and CI. The gas to dust ratio for the host galaxy is found to be significantly lower than that of other GRB hosts with N(HI)/A_V=(2.4+/-1.0)x10^21 cm^-2 mag^-1, which lies between typical MW and LMC values. Our results are in agreement with the tentative conclusion reached by Gordon et al. 2003 that the shape of the extinction curve, in particular the presence of the bump, is affected by the UV flux density in the environment of the dust.