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Published December 16, 2015 | Published + Submitted
Journal Article Open

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


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.

Additional Information

© 2015 ESO. Received 23 April 2015. Accepted 2 August 2015. Published online December 16 2015. We thank the anonymous referee for her/his useful suggestions and comments that help us to improve this paper. We acknowledge financial contribution from the agreement ASI-INAF I/009/10/0. The LBT is an international collaboration among institutions in the United States, Italy, and Germany. LBT Corporation partners are The University of Arizona on behalf of the Arizona university system; Istituto Nazionale di Astrofisica, Italy; LBT Beteiligungsgesellschaft, Germany, representing the Max-Planck Society, the Astrophysical Institute Potsdam, and Heidelberg University; The Ohio State University; and The Research Corporation, on behalf of The University of Notre Dame, University of Minnesota, and University of Virginia. This work is supported by funding from the European Research Council Advanced Grant ERC-2010-AdG-268107-EARLY and by INAF Grants PRIN 2010, PRIN 2012 and PICS 2013. A.C., O.C., M.T. and V.S. acknowledge the grant MIUR PRIN 2010–2011. D.M. gratefully acknowledges LAM hospitality during the initial phases of the project. This work is based on data products made available at the CESAM data center, Laboratoire d'Astrophysique de Marseille. This work partly uses observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii. This work is based in part on data products produced at TERAPIX and the Canadian Astronomy Data Centre as part of the Canada-France-Hawaii Telescope Legacy Survey, a collaborative project of NRC and CNRS. Based on observations made with ESO Telescopes at the La Silla or Paranal Observatories under programme ID 175.A-0839. A.G. and E.V. warmly thank A. K. Inoue for providing the updated IGM transmission used in this paper. A.F. and J.S.D. acknowledge the contribution of the EC FP7 SPACE project ASTRODEEP (Ref. No: 312725).

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