2011MNRAS.414.2793W

In this paper we show how a self-consistent treatment of hydrogen and helium emission line fluxes of the hosts of long gamma-ray bursts (GRBs) can result in improved understanding of the dust properties in these galaxies. In particular, we find that even with modest signal-to-noise ratio spectroscopy we can differentiate different values for R_V, the ratio of total to selective extinction. The inclusion of Paschen and Brackett lines, even at low signal-to-noise ratio, greatly increase the accuracy of the derived reddening. This method is often associated with strong systematic errors, caused by the need for multiple instruments to cover the wide wavelength range, the requirement to separate stellar hydrogen absorption from the nebular emission and because of the dependency of the predicted line fluxes on the electron temperature. We show how these three systematic errors can be negated, by using suitable instrumentation [in particular X-shooter on the Very Large Telescope (VLT)] and wide wavelength coverage. We demonstrate this method using an extensive optical and near-infrared spectroscopic campaign of the host galaxy of GRB 060218 (SN 2006aj), obtained with Focal Reducer/low dispersion Spectrograph, UV–Visual Echelle Spectrograph and Infrared Spectrometer and Array Camera on the VLT, covering a broad wavelength range with both high and low spectral resolution. We contrast our findings of this source with X-shooter data of a star-forming region in the host of GRB 100316D, and show the improvement over existing published fluxes of long GRB hosts.

Based on observations made with ESO Telescopes at the Paranal Observatory under programme ID 381.D-0723.