Lyman continuum escape fraction of faint galaxies at z ~ 3.3 in the CANDELS/GOODS-North, EGS, and COSMOS fields with LBC^⋆

¹ INAF–Osservatorio Astronomico di Roma, via Frascati 33, 00078, Monte Porzio Catone, Italy
e-mail: andrea.grazian@oa-roma.inaf.it
² Carnegie Observatories, Colina El Pino, Casilla 601 La Serena, Chile
³ National Optical Astronomy Observatory, 950 North Cherry Ave, Tucson, AZ 85719, USA
⁴ School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA
⁵ Minnesota Institute for Astrophysics, University of Minnesota, 116 Church Street SE, Minneapolis, MN 55455, USA
⁶ Stockholm University, Alba Nova SCFAB, 106 91 Stockholm, Sweden
⁷ INAF–Osservatorio Astronomico di Bologna, via Ranzani, 1, 40127 Bologna, Italy
⁸ INAF–Osservatorio Astronomico di Trieste, via G.B. Tiepolo, 11 34143 Trieste, Italy
⁹ Astronomy Department, University of Massachusetts, Amherst, MA 01003, USA
¹⁰ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹¹ Cavendish Laboratory, University of Cambridge, 19 J.J. Thomson Ave., Cambridge CB30 HE, UK
¹² Kavli Institute of Cosmology c/o Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
¹³ Department of Astronomy and Astrophysics, University of California Berkeley 501 Campbell Hall Berkeley, CA 94720, USA
¹⁴ Agenzia Spaziale Italiana Science Data Center, via del Politecnico snc, 00133 Roma, Italy
¹⁵ UCO/Lick Observatory, 1156 High Street Santa Cruz, CA 95064, USA
¹⁶ INAF–IASF, via Bassini 15, 20133 Milano, Italy
¹⁷ Max Planck Institute for extraterrestrial Physics, Giessenbachstrasse 185748 Garching, Bayern, Germany
¹⁸ ESO Vitacura, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago de Chile, Chile
¹⁹ University of Kentucky, 600 Rose Street, Lexington, KY 40508, USA
²⁰ Department of Astronomy, University of Virginia, Charlottesville, VA 22904-4325, USA
²¹ Department of Astronomy and Steward Observatory, University of Arizona, Tucson, AZ 85719, USA
²² Department of Physics and Astronomy University of Missouri Columbia, MO 65211, USA

Received: 16 January 2017
Accepted: 28 February 2017

Abstract

Context. The reionization of the Universe is one of the most important topics of present-day astrophysical research. The most plausible candidates for the reionization process are star-forming galaxies, which according to the predictions of the majority of the theoretical and semi-analytical models should dominate the H i ionizing background at z ≳ 3.

Aims. We measure the Lyman continuum escape fraction, which is one of the key parameters used to compute the contribution of star-forming galaxies to the UV background. It provides the ratio between the photons produced at λ ≤ 912 Å rest-frame and those that are able to reach the inter-galactic medium, i.e. that are not absorbed by the neutral hydrogen or by the dust of the galaxy’s inter-stellar medium.

Methods. We used ultra-deep U-band imaging (U = 30.2 mag at 1σ) from Large Binocular Camera at the Large Binocular Telescope (LBC/LBT) in the CANDELS/GOODS-North field and deep imaging in the COSMOS and EGS fields in order to estimate the Lyman continuum escape fraction of 69 star-forming galaxies with secure spectroscopic redshifts at 3.27 ≤ z ≤ 3.40 to faint magnitude limits (L = 0.2L^∗, or equivalently M₁₅₀₀ ~ − 19). The narrow redshift range implies that the LBC U-band filter exclusively samples the λ ≤ 912 Å rest-frame wavelengths.

Results. We measured through stacks a stringent upper limit (<1.7% at 1σ) for the relative escape fraction of H i ionizing photons from bright galaxies (L>L^∗), while for the faint population (L = 0.2L^∗) the limit to the escape fraction is ≲ 10%. We computed the contribution of star-forming galaxies to the observed UV background at z ~ 3 and find that it is not sufficient to keep the Universe ionized at these redshifts unless their escape fraction increases significantly (≥ 10%) at low luminosities (M₁₅₀₀ ≥ − 19).

Conclusions. We compare our results on the Lyman continuum escape fraction of high-z galaxies with recent estimates in the literature, and discuss future prospects to shed light on the end of the Dark Ages. In the future, strong gravitational lensing will be fundamental in order to measure the Lyman continuum escape fraction down to faint magnitudes (M₁₅₀₀ ~ − 16) that are inaccessible with the present instrumentation on blank fields. These results will be important in order to quantify the role of faint galaxies to the reionization budget.