Investigation of dust attenuation and star formation activity in galaxies hosting GRBs.

The gamma-ray bursts hosts (GRBHs) are excellent targets to study the extinctionproperties of dust and its effects on the global emission of distant galaxies. The dust extinctioncurve is measured along the GRB afterglowline of sight and the analysis of the spectral energy distribution( SED) of the host galaxy gives access to the global dust attenuation of the stellar light. We selected a sample of 30 GRBs for which the extinctioncurve along the GRB afterglowline-of-sight (l.o.s) is measured in the rest-frameultraviolet (UV) up to optical and we analysed the properties of the extinctioncurve as a function of the host galaxy properties. From these 30 GRBs, we selected seven GRBHs with a good rest-frameUV to near infra-red spectral coverage for the host. The attenuation curve was derived by fitting the SEDsof the GRBH sample with the CIGALE SEDfitting code. Different star formationhistories were studied to recover the star formationrates (SFR) derived using H$_{\ alpha}$ luminosities. The most extinguishedGRBs are preferentially found in the more massive hosts and the UV bump is preferentially found in the most extinguishedGRB l.o.s. Five out of seven hosts are best fitted with a recent burst of star formation, leading to lower stellar massestimates than previously found. The average attenuation in the host galaxies is about 70% of the amount of extinctionalong the GRB l.o.s. We find a great variety in the derived attenuation curves of GRBHs, the UV slope can be similar, flatter or even steeper than the extinctioncurve slope. We find that the flatter (steeper) attenuation curves are found in galaxies with the highest (lowest) SFR and stellar masses. The comparison of our results with radiative transfer simulations leads to a uniform distribution of dust and stars in a very clumpy ISM for half the GRBHs and various dust-stars geometries for the second half of the sample.