Probing ionizing radiation of L<~0.1L* star-forming galaxies at z>~3
  with strong lensing
  We show the effectiveness of strong lensing in the characterisation of Lyman
continuum emission from faint L<~0.1L* star-forming galaxies at redshift >~ 3.
Past observations of L>~L* galaxies at redshift >~3 have provided upper limits
of the average escape fraction of ionising radiation of fesc~5%. Galaxies with
relatively high fesc (>10%) seem to be particularly rare at these luminosities,
there is therefore the need to explore fainter limits. Before the advent of
giant ground based telescopes, one viable way to probe fesc down to 0.05-0.15L*
is to exploit strong lensing magnification. This is investigated with Monte
Carlo simulations that take into account the current observational
capabilities. Adopting a lensing cross-section of 10 arcmin^2 within which the
magnification is higher than 1 (achievable with about 4-5 galaxy clusters),
with a U-band survey depth of 30(30.5) (AB, 1-sigma), it is possible to
constrain fesc for z~3 star-forming galaxies down to 15(10)% at 3-sigma for
L<0.15L* luminosities. This is particularly interesting if fesc increases at
fainter luminosities, as predicted from various HI reionization scenarios and
radiation transfer modelling. Ongoing observational programs on galaxy clusters
are discussed and offer positive prospects for the future, even though from
space the HST/WFC3 instrument represents the only option we have to investigate
details of the spatial distribution of the Lyman continuum emission arising
from z~2-4 galaxies.
