GOODS-HERSCHEL: evidence for a UV extinction bump in galaxies at z > 1
  Dust attenuation curves in external galaxies are useful to study their dust
properties as well as to interpret their intrinsic spectral energy
distributions. In particular the presence or absence of a UV bump at 2175 A
remains an open issue which has consequences on the interpretation of broad
band colours of distant galaxies. We study the dust attenuation curve in the UV
range at z >1. In particular we search for the presence of a UV bump. We use
deep photometric data of the CDFS obtained with intermediate and broad band
filters by the MUSYC project to sample the UV rest-frame of galaxies with 1<z
<2. Herschel/PACS and Spitzer/MIPS data are used to measure the dust emission.
30 galaxies were selected with high S/N in all bands. Their SEDs from the UV to
the far-IR are fitted using the CIGALE code and the characteristics of the dust
attenuation curve are obtained. The mean dust attenuation curve we derive
exhibits a UV bump at 2175A whose amplitude corresponds to 35 % (76%) that of
the MW (LMC2 supershell) extinction curve. An analytical expression of the
average attenuation curve is given, it is found slightly steeper than the
Calzetti et al. one, although at a 1 sigma level. Our galaxy sample is used to
study the derivation of the slopes of the UV continuum from broad band colours,
including the GALEX FUV-NUV colour. Systematic errors induced by the presence
of the bump are quantified. We compare dust attenuation factors measured with
CIGALE to the slope of the UV continuum and find that there is a large scatter
around the relation valid for local starbursts (0.7 mag). The uncertainties on
the determination of the UV slope lead to an extra systematic error of the
order of 0.3 to 0.7 mag on dust attenuation when a filter overlaps the UV bump.
