Three intervening galaxy absorbers towards GRB060418: faint and dusty?

Abstract

We present an analysis of three strong, intervening Mg II absorption systems (z_abs = 0.603, 0.656, 1.107) towards the optical afterglow of gamma-ray burst (GRB) 060418. From high resolution UVES spectra we measure metal column densities and find that the highest redshift absorber exhibits a large amount of dust depletion compared with DLAs seen in QSO spectra. The intervening z_abs = 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 fit with a two component extinction curve: an 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 NTT R-band image of the GRB060418 field and spectroscopy of four galaxies within 1 arcminute. 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 ~<0.3 L^star.