Limits on the LyC signal from z ~ 3 sources with secure redshift and HST coverage in the E-CDFS field^⋆

¹ INAF–Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio (RM), Italy
e-mail: lucia.guaita@oa-roma.inaf.it
² INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ INAF–Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34143 Trieste, Italy
⁴ Department of Physics and Astronomy, Rutgers, The State University of New Jersey, Piscataway, NJ 08854, USA
⁵ Department of Astronomy, University of Massachusetts, 710 North Pleasant Street, Amherst, MA 01003, USA
⁶ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
⁷ Instituto de Fisica y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Gran Bretaña 1111, Playa Ancha, Valparaíso, Chile
⁸ INAF–IASF, via Bassini 15, 20133 Milano, Italy

Received: 16 October 2015
Accepted: 5 January 2016

Abstract

Context. Determining the strength of the Lyman continuum (LyC) and the fraction of LyC escape have implications for the properties of the emitting sources at any redshift, but also for the re-ionization of the Universe at z > 6.

Aims. We aim to measure the LyC signal from a sample of sources in the Chandra deep field south. We collect star-forming galaxies (SFGs) and active galactic nuclei (AGN) with accurate spectroscopic redshifts, for which Hubble Space Telescope (HST) coverage and multi-wavelength photometry are available.

Methods. We selected a sample of about 200 sources at z ~ 3. Taking advantage of HST resolution, we applied a careful cleaning procedure and rejected sources showing nearby clumps with different colours, which could be lower-z interlopers. Our clean sample consisted of 86 SFGs (including 19 narrow-band selected Lyα emitters) and 8 AGN (including 6 detected in X-rays). We measured the LyC flux from aperture photometry in four narrow-band filters covering wavelengths below a 912 Å rest frame (3.11 < z < 3.53). We estimated the ratio between ionizing (LyC flux) and 1400 Å non-ionizing emissions for AGN and galaxies.

Results. By running population synthesis models, we assume an average intrinsic L_ν(1400 Å)/L_ν(900 Å) ratio of 5 as the representative value for our sample. With this value and an average treatment of the lines of sight of the inter-galactic medium, we estimate the LyC escape fraction relative to the intrinsic value (fesc^rel(LyC)). We do not directly detect ionizing radiation from any individual SFG, but we are able to set a 1(2)σ upper limit of fesc^rel(LyC) < 12(24)%. This result is consistent with other non-detections published in the literature. No meaningful limits can be calculated for the sub-sample of Lyα emitters. We obtain one significant direct detection for an AGN at z = 3.46, with fesc^rel(LyC) = (72 ± 18)%.

Conclusions. Our upper limit on fesc^rel(LyC) implies that the SFGs studied here do not present either the physical properties or the geometric conditions suitable for efficient LyC-photon escape.