The 2175 \AA\ extinction feature in the optical afterglow spectrum of GRB 180325A at z=2.25

TL;DR

This study reports the first unambiguous detection of the 2175 Å extinction feature in a high-redshift GRB afterglow, revealing details about dust properties and host galaxy characteristics at z=2.25.

Contribution

It provides the first clear detection of the 2175 Å bump in a GRB afterglow at high redshift, with detailed spectral analysis and host galaxy characterization.

Findings

Detection of the 2175 Å bump at z=2.25 in a GRB afterglow.

Extinction law with R_V~4.4 and A_V~1.5, shallower than Galactic.

Host galaxy has a star-formation rate of ~46 M_sun/yr and stellar mass log M*/M_sun~9.3.

Abstract

The UV extinction feature at 2175 \AA\ is ubiquitously observed in the Galaxy but is rarely detected at high redshifts. Here we report the spectroscopic detection of the 2175 \AA\ bump on the sightline to the \gamma-ray burst (GRB) afterglow GRB 180325A at z=2.2486, the only unambiguous detection over the past ten years of GRB follow-up, at four different epochs with the Nordic Optical Telescope (NOT) and the Very Large Telescope (VLT)/X-shooter. Additional photometric observations of the afterglow are obtained with the Gamma-Ray burst Optical and Near-Infrared Detector (GROND). We construct the near-infrared to X-ray spectral energy distributions (SEDs) at four spectroscopic epochs. The SEDs are well-described by a single power-law and an extinction law with R_V~4.4, A_V~1.5, and the 2175 \AA\ extinction feature. The bump strength and extinction curve are shallower than the average Galactic extinction curve. We determine a metallicity of [Zn/H]>-0.98 from the VLT/X-shooter spectrum. We detect strong neutral carbon associated with the GRB with an equivalent width of Wr(\lambda 1656) = 0.85+/-0.05. We also detect optical emission lines from the host galaxy. Based on the H\alpha emission line flux, the derived dust-corrected star-formation rate is ~46+/-4 M_sun/yr and the predicted stellar mass is log M*/M_sun~9.3+/-0.4, suggesting the host galaxy is amongst the main-sequence star-forming galaxies.