Structure and Morphologies of z ~ 7-8 Galaxies from Ultra-deep WFC3/IR Imaging of the Hubble Ultra-deep Field

Abstract

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 sime 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 (gsim1.2 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 _sun 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.

Based on data obtained with the Hubble Space Telescope operated by AURA, Inc. for NASA under contract NAS5-26555.