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The blue UV slopes of z ~ 4 Lyman break galaxies: implications for the corrected star formation rate density
2012
Context.The study of the dust extinction in high-redshift galaxies is fundamental to obtain 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!, 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!. The same fit is used to determine their rest-frame UV magnitudesM 1600 through a simple interpolation. We estimate the e!ect 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(!) as a function of rest-frame magnitude. Results.We find 142 and 25 robust LBGs in the ERS and HUDF fields respectively, limiting our sample to S/N(H)>10 objects. Our newly defined criteria improve the selection ofz!4 LBGs and allow us to exclude red interlopers at lower redshift, especially z!3-3.5 objects. We show that the use of a linear fit to estimate!and an accurate characterization of observational e!ects are required in such analysis of flux limited samples. We find thatz!4 LBGs are characterized by blue UV slopes, suggesting a low dust extinction: allL
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