The Lyman continuum escape fraction of galaxies at z = 3.3 in the VUDS-LBC/COSMOS field^⋆

¹ INAF–Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone, Italy
e-mail: andrea.grazian@oa-roma.inaf.it
² Dipartimento di Fisica, Universitá di Roma La Sapienza, P.le A. Moro 2, 00185 Roma, Italy
³ Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
⁴ INAF–Osservatorio Astronomico di Bologna, via Ranzani,1, 40127 Bologna, Italy
⁵ Instituto de Fisica y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Av. Gran Bretaña 1111, Playa Ancha, Valparaíso, Chile
⁶ INAF–IASF, via Bassini 15, 20133 Milano, Italy
⁷ University of Bologna, Department of Physics and Astronomy (DIFA), V.le Berti Pichat, 6/2 - 40127 Bologna, Italy
⁸ Astronomy Department, University of Massachusetts, Amherst, MA 01003, USA
⁹ Department of Astronomy, University of Geneva, Ch. d’Écogia 16, 1290 Versoix, Switzerland
¹⁰ Geneva Observatory, University of Geneva, Ch. des Maillettes 51, 1290 Versoix, Switzerland
¹¹ Institut de Recherche en Astrophysique et Planétologie – IRAP, CNRS, Université de Toulouse, UPS-OMP, 14 avenue E. Belin, 31400 Toulouse, France
¹² Department of Astronomy, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
¹³ Institut d’Astrophysique de Paris, UMR7095 CNRS, Université Pierre et Marie Curie, 98bis boulevard Arago, 75014 Paris, France
¹⁴ SUPA, Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, EH9 3HJ, UK
¹⁵ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁶ Centro de Estudios de Física del Cosmos de Aragón, 44001 Teruel, Spain
¹⁷ Research Center for Space and Cosmic Evolution, Ehime University, Bunkyo-cho 2-5, 790-8577 Matsuyama, Japan

Received: 23 April 2015
Accepted: 2 August 2015

Abstract

Context. The ionizing Lyman continuum flux escaping from high-redshift galaxies into the intergalactic medium is a fundamental quantity to understand the physical processes involved in the reionization epoch. However, from an observational point of view, direct detections of HI ionizing photons at high redshifts are feasible for galaxies mainly in the interval z ~ 3−4.

Aims. We have investigated a sample of star-forming galaxies at z ~ 3.3 to search for possible detections of Lyman continuum ionizing photons escaping from galaxy halos.

Methods. We used deep ultraviolet (UV) imaging in the COSMOS field, obtained with the prime focus camera LBC at the LBT telescope, along with a catalogue of spectroscopic redshifts obtained by the VIMOS Ultra Deep Survey (VUDS) to build a sample of 45 galaxies at z ~ 3.3 with L> 0.5 L^∗. We obtained deep LBC images of galaxies with spectroscopic redshifts in the interval 3.27 <z< 3.40 both in the R- and deep U-bands (magnitude limit U ~ 29.7 at S/N = 1). At these redshifts, the R-band samples the non-ionizing 1500 Å rest-frame luminosity and the U-band samples the rest-frame spectral region just short-ward of the Lyman edge at 912 Å. Their flux ratio is related to the ionizing escape fraction after statistical removal of the absorption by the intergalactic medium along the line of sight.

Results. A subsample of ten galaxies apparently shows escape fractions >28%, but a detailed analysis of their properties reveals that, with the exception of two marginal detections (S/N ~ 2) in the U-band, all the other eight galaxies are most likely contaminated by the UV flux of low-redshift interlopers located close (in angular position) to the high-z targets. The average escape fraction derived from the stacking of the cleaned sample was constrained to f_esc^rel < 2%. The implied hydrogen photoionization rate is a factor two lower than that needed to keep the intergalactic medium ionized at z ~ 3, as observed in the Lyman-α forest of high-z quasar spectra or by the proximity effect.

Conclusions. These results support a scenario where high redshift, relatively bright (L ≥ 0.5L^∗) star-forming galaxies alone are unable to sustain the level of ionization observed in the cosmic intergalactic medium at z ~ 3. Star-forming galaxies at higher redshift and at fainter luminosities (L ≪ L^∗) can only be major contributors to the reionization of the Universe if their physical properties are subject to rapid changes from z ~ 3 to z ~ 6–10. Alternatively, ionizing sources could be discovered looking for fainter sources among the active galactic nuclei population at high redshift.