We present a first morphological study of z \sim 7 - 8 Lyman Break galaxies ( LBGs ) from Oesch et al .
( 31 ) and Bouwens et al .
( 4 ) 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 \pm 0.3 kpc these galaxies are found to be extremely compact having an average observed surface brightness of \mu _ { J } \simeq 26 mag arcsec ^ { -2 } , and only two out of the full sample of 16 z \sim 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 \sim 7 to z \sim 6 , while galaxies between z \sim 4 - 5 show more extended wings in their apparent profiles .
The average size scales as ( 1 + z ) ^ { - m } with m = 1.12 \pm 0.17 for galaxies with luminosities in the range ( 0.3-1 ) L ^ { * } _ { z = 3 } and with m = 1.32 \pm 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 \sim 7 to z \sim 4 .
However , a tail of larger galaxies ( \gtrsim 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 M _ { \odot } yr ^ { -1 } kpc ^ { -2 } over the entire redshift range z \sim 4 - 7 suggesting similar star-formation efficiencies at these early epochs .
The above evolutionary trends seem to hold out to z \sim 8 though the sample is still small and possibly incomplete .