2010ApJ...709L..21O

We present a first morphological study of z ∼ 7-8 Lyman break galaxies (LBGs) from Oesch et al. and Bouwens et al. 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 µ_ J_=~ 26 mag/arcsec2, 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 Σ_SFR= 1.9 M_☉/yr/kpc2 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.