The blue UV slopes of z ∼ 4 Lyman break galaxies: Implications for the corrected star formation rate density

Abstract

Context. Study of the dust extinction in high-redshift galaxies is fundamental to obtaining an estimate of the corrected star formation rate density (SFRD) and to put constraints on galaxy evolution models. Aims. We plan to analyse dust extinction in Lyman break galaxies (LBGs) by introducing a new and more reliable approach to their selection and to the characterization of their distribution of UV slopes β, using deep IR images from HST. We fully discuss the methodology and the results, focusing on a robust sample of z ∼ 4 LBGs. Methods. We exploit deep WFC3 IR observations of the ERS and HUDF fields over GOODS-South, combined with HST-ACS optical data, to select z ∼ 4 LBGs through a new (B-V) vs. (V-H) colour diagram. The UV slope of the selected galaxies is robustly determined by a linear fit over their observed I,Z,Y,J magnitudes, coherently with the original definition of β. The same fit is used to determine their rest-frame UV magnitudes M 1600 through a simple interpolation. We estimate the effect of observational uncertainties with detailed simulations that we also exploit, under a parametric maximum-likelihood approach, to constrain the probability density function of UV slopes PDF(β) as a function of rest-frame magnitude. Results. We find 142 robust LBGs in the ERS and 25 in the HUDF field, limiting our sample to S/N(H) > 10 objects. Our newly defined criteria improve the selection of z ∼ 4 LBGs and allow us to exclude red interlopers at lower redshifts, especially z ∼ 3-3.5 objects. We show that using a linear fit to estimate β and accurately characterizing observational effects are required in this kind of analysis of flux-limited samples. We find that z ∼ 4 LBGs are characterized by blue UV slopes, suggesting a low dust extinction: all L