The composition and amount of interstellar dust within gamma-ray burst ( GRB ) host galaxies is of key importance when addressing selection effects in the GRB redshift distribution , and when studying the properties of their host galaxies .
As well as the implications for GRB research , probing the dust within the high-z hosts of GRBs also contributes to our understanding of the conditions of the interstellar medium and star-formation in the distant Universe .
Nevertheless , the physical properties of dust within GRB host galaxies continues to be a highly contended issue .
In this paper we explore the mean extinction properties of dust within the host galaxies of a sample of 17 GRBs with total host galaxy visual extinction A _ { V } < 1 ( \langle A _ { V } \rangle =0.4 ) , covering a redshift range z = 0.7 - 3.1 .
We find the average host extinction curve to have an ultraviolet slope comparable to that of the LMC , but with little evidence of a 2175Ã Â dust extinction feature as observed along Milky Way and LMC sightlines .
We can not at present rule out the presence of a 2175Ã Â feature , and both the standard SMC and LMC extinction curves also provide good fits to our data .
However , we can reject an extinction curve that has a UV slope as flat as the mean Milky Way extinction curve , whilst also having a 2175Ã Â feature as prominent as seen in the mean Milky Way extinction curve .
This is in contrast to the clear detection of a 2175Ã Â bump and the flatter extinction curves of some more heavily extinguished GRBs ( A _ { V } > 1 ) , which may be indicative of there being a dependence between dust abundance and the wavelength dependence of dust extinction , as has been previously speculated .