UV slope of z ∼ 3 bright (L > L*) Lyman-break galaxies in the COSMOS field

Abstract

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)≃ − 1.6 M⊙ yr−1 Mpc−3, 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.