STRUCTURE AND MORPHOLOGIES OF z ∼ 7-8 GALAXIES FROM ULTRA-DEEP WFC3/IR IMAGING OF THE HUBBLE ULTRA-DEEP FIELD

We present a first morphological study of z similar to 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 mu(J) less than or similar to 26 mag arcsec(-2), and only two out of the full sample of 16 z similar to 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 similar to 7 to z similar to 6, while galaxies between z similar to 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 similar to 7 to z similar to 4. However, a tail of larger galaxies (greater than or similar to 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 Sigma(SFR) = 1.9 M(circle dot) yr(-1) kpc(-2) over the entire redshift range z similar to 4-7 suggesting similar star formation efficiencies at these early epochs. The above evolutionary trends seem to hold out to z similar to 8 though the sample is still small and possibly incomplete.