Structure and Morphologies of Z ∼ 7 − 8 Galaxies from Ultra - Deep Wfc 3 / Ir Imaging of the Hudf

We present a first morphological study of z ∼ 7 − 8 Lyman Break galaxies (LBGs) from Oesch et al. (2009b) and Bouwens et al. (2009a) 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 arcsec −2. Only two out of the full sample of 16 z ∼ 7 galaxies show extended features with resolved double cores; the remaining galaxies are very compact and relatively symmetric in their appearance. By comparison to lower redshift LBGs it is found that only very little size evolution takes place from z ∼ 7 to z ∼ 6, while galaxies between z ∼ 4 − 5 clearly 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) with more complex morphologies 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 Σ SF R = 1.9 M ⊙ 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. 1. INTRODUCTION The newly installed WFC3/IR camera on the Hub-ble Space Telescope has opened up a new territory in the study of galaxies at z 6.5. Its increased capability has lead to numerous detections of galaxies at z 6.5 already in the first epoch data of the HUDF09 Understanding the evolution of galaxy sizes and morphologies out to z 6.5 can provide essential clues to galaxy formation models. While hydrodynamical galaxy formation simulations of LBGs (e.g. Finlator et al. 2006; Night et al. 2006; Nagamine et al. 2008) have focussed on the prediction …