  Aim: 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. Method:
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 Lya 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 A rest frame (3.11<z<3.53). We estimated the ratio
between ionizing (LyC flux) and 1400 A non-ionizing emissions for AGN and
galaxies. Results: By running population synthesis models, we assume an average
intrinsic L(1400 A)/L(900 A) 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)sigma 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 Lya 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
fescrel(LyC) implies that the SFGs studied here do not present either the
physical properties or the geometric conditions suitable for efficient
LyC-photon escape.
