Dust properties of UV bright galaxies at z∼2

We investigate the properties of the extinction curve in the rest-frame UV for a sample of 34 UV-luminous galaxies at 2 < z < 2.5, selected from the FORS Deep Field (FDF) spectroscopic survey. A new parametric description of the rest-frame UV spectral energy distribution is adopted; its sensitivity to properties of the stellar populations or of dust attenuation is established with the use of models. The latter are computed by combining composite stellar population models and calculations of radiative transfer of the stellar and scattered radiation through the dusty interstellar medium (ISM) for a dust/stars configuration describing dust attenuation in local starbursts. In the favoured configuration the stars are enveloped by a shell with a two-phase, clumpy, dusty ISM. The distribution of the z similar to 2 UV-luminous FDF galaxies in several diagnostic diagrams shows that their extinction curves range between those typical of the Small and Large Magellanic Clouds (SMC and LMC, respectively). For the majority of strongly reddened objects having a UV continuum slope beta > -0.4 a significant 2175 angstrom absorption feature (or "UV bump") is inferred, indicating an LMC-like extinction curve. On the other hand, the UV continua of the least reddened objects are mostly consistent with SMC-like extinction curves, lacking a significant UV bump, as for the sample of local starbursts investigated by Calzetti and collaborators. Furthermore, the most opaque (beta similar to 0) and, thus (for our models), dustiest UV-luminous FDF galaxies tend to be among the most metal-rich, most massive, and largest systems at z similar to 2, indicating < Z > similar to 0.5-1 Z(circle dot), < M(stars)> similar to 6 x 10(10) M(circle dot), and < R(eff)> similar to 4 kpc, respectively. The presence of the UV bump does not seem to depend on the total metallicity, as given by the equivalent width (EW) of the C IV doublet. Conversely, it seems to be associated with a large average EW of the six most prominent interstellar low-ionisation absorption lines falling in the FORS spectra. The average EW of these saturated lines offers a proxy for the ISM topology. We interpret these results as the evidence for a difference in the properties of the dusty ISM among the most evolved UV-luminous, massive galaxies at z similar to 2.