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2175 angstrom Extinction Feature in the Optical Afterglow Spectrum of

The UV extinction feature at 2175 ˚A is ubiquitously observed in the Galaxy but is rarely detected at high redshifts. Here we report the spectroscopic detection of the 2175 ˚A bump on the sightline to the γ -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 ˚A 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 equivalent width of W r ( λ 1656) = 0 . 85 ± 0 . 05. We also detect optical emission lines from the host galaxy. Based on the H α emission line ﬂux, the derived dust-corrected star-formation rate is ∼ 46 ± 4 M (cid:12) yr − 1 and the predicted stellar mass is log M ∗ /M (cid:12) ∼ 9 . 3 ± 0 . 4, suggesting the host galaxy is amongst the main-sequence star-forming galaxies.