摘要：Context. The analysis of the UV slopeβof Lyman-break galaxies (LBG) at different luminosities and redshifts is fundamental for understanding their physical properties, and in particular, their dust extinction.Aims. We analyse a unique sample of 517 bright (L > L*) LBGs at redshiftz ∼ 3 in order to characterise the distribution of their UV slopesβand infer their dust extinction under standard assumptions.Methods. We exploited multi-band observations over 750 arcmin2of the COSMOS field that were acquired with three different ground-based facilities: the Large Binocular Camera (LBC) on the Large Binocular Telescope (LBT), the Suprime-Cam on the SUBARU telescope, and the VIRCAM on the VISTA telescope (ULTRAVISTA DR2). Our multi-band photometric catalogue is based on a new method that is designed to maximise the signal-to-noise ratio in the estimate of accurate galaxy colours from images with different point spread functions (PSF). We adopted an improved selection criterion based on deepY-band data to isolate a sample of galaxies atz ∼ 3 to minimise selection biases. We measured the UV slopes (β) of the objects in our sample and then recovered the intrinsic probability density function ofβvalues (PDF(β)), taking into account the effect of observational uncertainties through detailed simulations.Results.The galaxies in our sample are characterised by mildly red UV slopes with ⟨β⟩≃ − 1.70 throughout the enitre luminosity range that is probed by our data (−24 ≲ M1600 ≲ −21). The resulting dust-corrected star formation rate density (SFRD) is log(SFRD)≃ − 1.6 M⊙ yr−1 Mpc−3, corresponding to a contribution of about 25% to the total SFRD atz ∼ 3 under standard assumptions.Conclusions.Ultra-bright LBGs atz ∼ 3 match the known trends, with UV slopes being redder at decreasing redshifts, and brighter galaxies being more highly dust extinct and more frequently star-forming than fainter galaxies.
