The VIMOS Ultra-Deep Survey (VUDS): fast increase in the fraction of strong Lyman-α emitters from z = 2 to z = 6^⋆

¹ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
e-mail: paolo.cassata@lam.fr
² Instituto de Fisica y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Gran Bretaña 1111, Playa Ancha, Valparaso, Chile
³ INAF–IASF, via Bassini 15, 20133 Milano, Italy
⁴ INAF–Osservatorio Astronomico di Bologna, via Ranzani,1, 40127 Bologna, Italy
⁵ INAF–Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
⁶ University of Bologna, Department of Physics and Astronomy (DIFA), V.le Berti Pichat, 6/2, 40127 Bologna, Italy
⁷ INAF–IASF Bologna, via Gobetti 101, 40129 Bologna, Italy
⁸ Institut d’Astrophysique de Paris, UMR 7095 CNRS, Université Pierre et Marie Curie, 98 bis Boulevard Arago, 75014 Paris, France
⁹ Institut de Recherche en Astrophysique et Planétologie, IRAP, CNRS, Université de Toulouse, UPS-OMP, 14 avenue E. Belin, 31400 Toulouse, France
¹⁰ Department of Astronomy, University of Geneva ch. d’Écogia 16, 1290 Versoix, Switzerland
¹¹ Geneva Observatory, University of Geneva, ch. des Maillettes 51, 1290 Versoix, Switzerland
¹² Centro de Estudios de Física del Cosmos de Aragón, Teruel, Spain
¹³ Department of Astronomy, California Institute of Technology, 1200 E. California Blvd., MC 249-17, Pasadena, CA 91125, USA
¹⁴ Astronomy Department, University of Massachusetts, Amherst, MA 01003, USA
¹⁵ Max-Planck-Institut für Extraterrestrische Physik, Postfach 1312, 85741 Garching bei München, Germany
¹⁶ Research Center for Space and Cosmic Evolution, Ehime University, Bunkyo-cho 2-5, 790-8577 Matsuyama, Japan
¹⁷ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

Received: 17 March 2014
Accepted: 15 September 2014

Abstract

Aims. The aim of this work is to constrain the evolution of the fraction of strong Lyα emitters among UV selected star-forming galaxies at 2 <z< 6, and to measure the stellar escape fraction of Lyα photons over the same redshift range.

Methods. We exploit the ultradeep spectroscopic observations with VIMOS on the VLT collected by the VIMOS Ultra-Deep Survey (VUDS) to build an unique, complete, and unbiased sample of ~4000 spectroscopically confirmed star-forming galaxies at 2 <z< 6. Our galaxy sample includes UV luminosities brighter than M^*_FUV at 2 <z< 6, and luminosities down to one magnitude fainter than M^*_FUV at 2 <z< 3.5.

Results. We find that 80% of the star-forming galaxies in our sample have EW₀(Lyα) < 10 Å, and correspondingly f_esc(Lyα) < 1%. By comparing these results with the literature, we conclude that the bulk of the Lyα luminosity at 2 <z< 6 comes from galaxies that are fainter in the UV than those we sample in this work. The strong Lyα emitters constitute, at each redshift, the tail of the distribution of the galaxies with extreme EW₀(Lyα) and f_esc(Lyα). This tail of large EW₀(Lyα) and f_esc(Lyα) becomes more important as the redshift increases, and causes the fraction of strong Lyα with EW₀(Lyα) > 25 Å to increase from ~5% at z ~ 2 to ~30% at z ~ 6, with the increase being stronger beyond z ~ 4. We observe no difference, for the narrow range of UV luminosities explored in this work, between the fraction of strong Lyα emitters among galaxies fainter or brighter than M^*_FUV, although the fraction for the faint galaxies evolves faster, at 2 <z< 3.5, than for the bright ones. We do observe an anticorrelation between E(B − V) and f_esc(Lyα): generally galaxies with high f_esc(Lyα) also have small amounts of dust (and vice versa). However, when the dust content is low (E(B − V) < 0.05) we observe a very broad range of f_esc(Lyα), ranging from 10^-3 to 1. This implies that the dust alone is not the only regulator of the amount of escaping Lyα photons.