2009A&A...499...69N

The properties of dust attenuation at rest-frame UV wavelengths are inferred from very high-quality FORS2 spectra of 78 galaxies from the GMASS survey at redshift 1<z<2.5. These objects complement a previously investigated sample of 108 UV-luminous, intermediate-mass (i.e., with stellar masses ∼10¹⁰-10¹¹M_☉) galaxies at similar redshifts, selected from the FDF spectroscopic survey, the K20 survey, and the GDDS. Detection of the broad absorption feature centred on about 2175Å (``UV bump'') implies that the average UV extinction curve of a galaxy more closely resembles that of the Milky Way (MW) or the Large Magellanic Cloud (LMC), and differs from that of the Small Magellanic Cloud (SMC). The shape of the UV extinction curve is constrained by a parametric description of the rest-frame UV continuum with the support of a suite of models combining radiative transfer and stellar population synthesis. The UV bump is further characterised by fitting Lorentzian-like profiles. Spectra exhibit a significant 2175Å feature in at least 30% of the cases, especially those suffering from substantial reddening. If attenuation is dominated by dust ejected from the galaxy main body via galactic winds or more localised superwinds, UV extinction curves in-between those of the SMC and LMC characterise UV-luminous, intermediate-mass galaxies at 1<z<2.5. The fraction of galaxies with extinction curves differing from the SMC one increases, if more dust resides in the galactic plane or dust attenuation depends on stellar age. On average, the width of the manifested UV bumps is about 60% of the values typically measured along sightlines in the LMC and MW. This suggests the presence of dust similar to that found in the ``supergiant'' shell of ionised filaments LMC2, close to 30Dor. The presence of the carriers of the UV bump (probably organic carbon and amorphous silicates) at 1 < z < 2.5 argues for outflows from asymptotic giant branch (AGB) stars being copious then. Consistent with their higher star-formation rates, the GMASS galaxies with a manifested UV bump are more luminous at rest-frame 8µm, where the emission is dominated by polycyclic aromatic hydrocarbons (also products of AGB stars). In addition, they exhibit larger equivalent widths for prominent UV (metal) absorption features, mostly of interstellar origin, which indicates overall more evolved stellar populations. We conclude that diversification of the small-size dust component (responsible for the differential extinction at UV wavelengths and the emission at mid-IR wavelengths) has already started in the most evolved star-forming systems at 1<z<2.5.