The size-luminosity relation at z=7 in CANDELS and its implication on
  reionization
  The exploration of the relation between galaxy sizes and other physical
parameters has provided important clues for understanding galaxy formation. We
use the CANDELS Deep+Wide surveys in the GOODS-South, UDS and EGS fields,
complemented by data from the HUDF09 program, to address the relation between
size and luminosity at z\sim7. We select 153 z-band drop-out galaxies in six
different fields characterized by a wide combination of depth and areal
coverage, ideally suited to sample without biases the observed size-magnitude
plane. Detailed simulations allow us to derive the completeness as a function
of size and magnitude and to quantify measurements errors/biases. We find that
the half light radius distribution function of z\sim7 galaxies fainter than
J=26.6 is peaked at <0.1 arcsec (or equivalently 0.5 kpc proper), while at
brighter magnitudes high-z galaxies are typically larger than ~0.15 arcsec. We
also find a well defined size-luminosity relation, Rh\simL^1/2. We compute the
Luminosity Function in the HUDF and P12HUDF fields, finding large spatial
variation on the number density of faint galaxies. Adopting the size
distribution and the size-luminosity relation found for faint galaxies at z=7,
we derive a mean slope of -1.7\pm0.1 for the luminosity function of LBGs at
this redshift. Using this LF, we find that the amount of ionizing photons
cannot keep the Universe re-ionized if the IGM is clumpy (C_HII>3) and the
Lyman continuum escape fraction of high-z LBGs is relatively low (f_esc<0.3).
With future CANDELS data, we can put severe limits to the role of galaxies in
the reionization of the Universe.
