New constraints on the average escape fraction of Lyman continuum radiation in z ~ 4 galaxies from the VIMOS Ultra Deep Survey (VUDS) ⋆
1 INAF–Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone, Italy
2 Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
3 Geneva Observatory, University of Geneva, ch. des Maillettes 51, 1290 Versoix, Switzerland
4 INAF–Osservatorio Astronomico di Bologna, via Ranzani, 1, 40127 Bologna, Italy
5 Instituto de Fisica y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Gran Bretaña 1111, Playa Ancha, Valparaíso, Chile
6 INAF–IASF Milano, via Bassini 15, 20133 Milano, Italy
7 Institut de Recherche en Astrophysique et Planétologie – IRAP, CNRS, Université de Toulouse, UPS-OMP, 14 avenue É. Belin, 31400 Toulouse, France
8 Cavendish Laboratory, University of Cambridge, 19 JJ Thomson Avenue, Cambridge, CB3 0HE, UK
9 Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
10 Department of Physics, University of California, Davis, One Shields Ave., Davis, CA 95616, USA
11 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
Received: 14 November 2016
Accepted: 23 December 2016
Context. Determining the average fraction of Lyman continuum (LyC) photons escaping high redshift galaxies is essential for understanding how reionization proceeded in the z> 6 Universe.
Aims. We want to measure the LyC signal from a sample of sources in the Chandra Deep Field South (CDFS) and COSMOS fields for which ultra-deep VIMOS spectroscopy as well as multi-wavelength Hubble Space Telescope (HST) imaging are available.
Methods. We select a sample of 46 galaxies at z ~ 4 from the VIMOS Ultra Deep Survey (VUDS) database, such that the VUDS spectra contain the LyC part, that is, the rest-frame range 880−910 Å. Taking advantage of the HST imaging, we apply a careful cleaning procedure and reject all the sources showing nearby clumps with different colours, that could potentially be lower-redshift interlopers. After this procedure, the sample is reduced to 33 galaxies. We measure the ratio between ionizing flux (LyC at 895 Å) and non-ionizing emission (at ~ 1500 Å) for all individual sources. We also produce a normalized stacked spectrum of all sources.
Results. Assuming an intrinsic average Lν(1470) /Lν(895) of 3, we estimate the individual and average relative escape fraction. We do not detect ionizing radiation from any individual source, although we identify a possible LyC emitter with very high Lyα equivalent width (EW). From the stacked spectrum and assuming a mean transmissivity for the sample, we measure a relative escape fraction f escrel = 0.09 ± 0.04. We also look for correlations between the limits in the LyC flux and source properties and find a tentative correlation between LyC flux and the EW of the Lyα emission line.
Conclusions. Our results imply that the LyC flux emitted by V = 25−26 star-forming galaxies at z ~ 4 is at most very modest, in agreement with previous upper limits from studies based on broad and narrow band imaging.
Key words: galaxies: high-redshift / galaxies: evolution / galaxies: star formation
© ESO, 2017