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We report the clear detection of the 2175A dust absorption feature in theoptical afterglow spectrum of the gamma-ray burst (GRB) GRB070802 at a redshiftof 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 beenrecorded in the past decade. The derived extinction curve gives A_V=0.8-1.5depending 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 hostof GRB070802. Besides the 2175A bump we find that the spectrum of GRB070802 ischaracterized by unusually strong low-ionization metal lines and possibly ahigh 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 aGRB spectrum in that it shows clear CI absorption features, leading us topropose a correlation between the presence of the bump and CI. The gas to dustratio for the host galaxy is found to be significantly lower than that of otherGRB hosts with N(HI)/A_V=(2.4+/-1.0)x10^21 cm^-2 mag^-1, which lies betweentypical MW and LMC values. Our results are in agreement with the tentativeconclusion reached by Gordon et al. 2003 that the shape of the extinctioncurve, in particular the presence of the bump, is affected by the UV fluxdensity in the environment of the dust.

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