The extinction properties of long GRB host galaxies from H and He I
  recombination lines
  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 can result in
improved understanding of the dust properties in these galaxies. In particular,
we find that even with modest signal to noise 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, 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 dependancy 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) and wide wavelength coverage.
We demonstrate this method using an extensive optical and near-infrared
spectroscopic campaign of the host galaxy of gamma-ray burst 060218 (SN
2006aj), obtained with FORS1, UVES and ISAAC 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.
