2019A&A...626A..45P

Context. The analysis of the UV slope β of 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 redshift z∼3 in order to characterise the distribution of their UV slopes β and infer their dust extinction under standard assumptions.
Methods. We exploited multi-band observations over 750 arcmin² of 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 deep Y-band data to isolate a sample of galaxies at z∼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 of β values (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)~=-1.6M_☉/yr/Mpc³, corresponding to a contribution of about 25% to the total SFRD at z∼3 under standard assumptions.
Conclusions. Ultra-bright LBGs at z∼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.