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