The extinction properties of long gamma-ray burst host galaxies from H and Hei recombination lines

Abstract

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 RV, 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 GRB060218 (SN2006aj), 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 GRB100316D, and show the improvement over existing published fluxes of long GRB hosts. © 2011 The Author Monthly Notices of the Royal Astronomical Society © 2011 RAS.