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We present a first morphological study of z∼7-8 Lyman break galaxies (LBGs) from Oesch etal. and Bouwens etal. 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 μJ ≃ 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 toward 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 ΣSFR = 1.9 M 1 yr-1 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.
Oesch et al. (Tue,) studied this question.