The VIMOS VLT Deep Survey: star formation rate density of Lyα emitters from a sample of 217 galaxies with spectroscopic redshifts 2  ≤  z  ≤  6.6^⋆

¹ Department of AstronomyUniversity of Massachusetts, Amherst, MA 01003, USA
e-mail: paolo@astro.umass.edu
² Laboratoire d’Astrophysique de Marseille, UMR6110, CNRS-Université de Provence Aix-Marseille I, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
³ IASF-INAF – via Bassini 15, 20133 Milano, Italy
⁴ INAF - Osservatorio Astronomico di Bologna – via Ranzani 1, 40127 Bologna, Italy
⁵ Laboratoire d’Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 Av. E. Belin, 31400 Toulouse, France
⁶ Institut d’Astrophysique de Paris, UMR7095 CNRS, Université Pierre et Marie Curie, 98bis boulevard Arago, 75014 Paris, France
⁷ The Andrzej Soltan Institute for Nuclear Studies, ul. Hoza 69, 00-681 Warszawa, Poland
⁸ IRA-INAF, via Gobetti, 101, 40129 Bologna, Italy

Received: 11 March 2010
Accepted: 18 October 2010

Abstract

Aims. The aim of this work is to study the contribution of the Lyα emitters to the star formation rate density (SFRD) of the Universe in the interval 2 < z < 6.6.

Methods. We assembled a sample of 217 Lyα emitters (LAE) from the Vimos-VLT Deep Survey (VVDS) with secure spectroscopic redshifts in the redshift range 2 < z < 6.62 and fluxes down to F ~ 1.5  ×  10^-18   erg/s/cm². Of those Lyα emitters, 133 are serendipitous identifications in the 22 arcmin² total slit area surveyed with the VVDS-Deep and the 3.3 arcmin² from the VVDS Ultra-Deep survey, and 84 are targeted identifications in the 0.62 deg² surveyed with the VVDS-DEEP and 0.16 deg² from the Ultra-Deep survey. Among the serendipitous targets we estimate that 90% of the emission lines are most probably Lyα, while the remaining 10% could be either [OII]3727 or Lyα. We computed the luminosity function (LF) and derived the star-formation rate density using this sample of LAE.

Results. The VVDS-LAE sample reaches faint line fluxes F(Lyα) = 1.5  ×  10^-18   erg/s/cm² (corresponding to L(Lyα) ~ 10⁴¹   erg/s at z ~ 3), allows the faint-end slope of the luminosity function to be constrained to α ~ −1.6  ±  0.12 at redshift z ~ 2.5 and to at redshift  ~4, placing trends found in previous LAE studies on firm statistical grounds, and indicating that sub-L _∗  LAE (L_Ly − α ≲ 10^42.5   erg/s) contribute significantly to the SFRD. The projected number density and volume density of faint LAE in 2 ≤ z ≤ 6.6 with F > 1.5  ×  10^-18   erg/s/cm² are 33 galaxies/arcmin² and  ~4  ×  10^-2 Mpc^-3, respectively. We find that the observed luminosity function (LF) of LAEs does not evolve from z = 2 to z = 6. This implies that, after correction for the redshift-dependent IGM absorption, the intrinsic luminosity function must have evolved significantly over 3 Gyr. The SFRD from LAE contributes around 20% of the SFRD at z = 2−3, while the LAE appear to be the dominant source of star formation producing ionizing photons in the early universe z ~  > 5−6, equivalent to Lyman Break galaxies.