Three intervening galaxy absorbers towards GRB 060418: faint and dusty?

We present an analysis of three strong, intervening Mg II absorption systems (zabs= 0.603, 0.656, 1.107) towards the optical afterglow of gamma-ray burst (GRB) 060418. From high-resolution Ultraviolet and Visual Echelle Spectrograph (UVES) spectra we measure metal column densities and find that the highest redshift absorber exhibits a large amount of dust depletion compared with damped Lyman absorbers (DLAs) seen in quasi-stellar object (QSO) spectra. The intervening zabs= 1.107 absorber is also unusual in exhibiting a clear 2175-A bump, the first time this feature has been definitively detected in a GRB spectrum. The GRB afterglow spectrum is best fitted with a two-component extinction curve: a Small Magellanic Cloud (SMC) extinction law at z= 1.49 (the redshift of the host) with E(B−V) = 0.07 ± 0.01 and a Galactic extinction curve at z∼ 1.1 with E(B−V) = 0.08 ± 0.01. We also present a moderately deep New Technology Telescope (NTT) R-band image of the GRB 060418 field and spectroscopy of four galaxies within 1 arcmin. None of these objects has a redshift that matches any of the intervening absorbers, and we conclude that the galaxies responsible for the two intervening Mg ii absorbers at z∼ 0.6 have luminosities .