  We present a first morphological study of z~7-8 Lyman Break galaxies (LBGs)
from Oesch et al. 2009 and Bouwens et al. 2009 detected in ultra-deep
near-infrared imaging of the Hubble Ultra Deep field (HUDF) by the HUDF09
program. With an average intrinsic size of 0.7+-0.3 kpc these galaxies are
found to be extremely compact having an average observed surface brightness of
mu_J ~= 26 mag arcsec^(-2), and only two out of the full sample of 16 z~7
galaxies show extended features with resolved double cores. By comparison to
lower redshift LBGs it is found that only little size evolution takes place
from z~7 to z~6, while galaxies between z~4-5 show more extended wings in their
apparent profiles. The average size scales as (1+z)^(-m) with m=1.12+-0.17 for
galaxies with luminosities in the range (0.3-1)L*_{z=3} and with m=1.32+-0.52
for (0.12-0.3)L*_{z=3}, consistent with galaxies having constant comoving
sizes. The peak of the size distribution changes only slowly from z~7 to z~4.
However, a tail of larger galaxies (>~ 1.2 kpc) is gradually built up towards
later cosmic times, possibly via hierarchical build-up or via enhanced
accretion of cold gas. Additionally, the average star-formation surface density
of LBGs with luminosities (0.3-1)L*_{z=3} is nearly constant at Sigma_{SFR}=1.9
Msun/yr/kpc^2 over the entire redshift range z~4-7 suggesting similar
star-formation efficiencies at these early epochs. The above evolutionary
trends seem to hold out to z~8 though the sample is still small and possibly
incomplete.
