2007ApJ...658..763E

The photometric redshift distributions, spectral types, Sérsic indices, and sizes of all resolved galaxies in the Hubble Space Telescope Ultra Deep Field (UDF) are studied in order to understand the environment and nature of star formation in the early universe. Clumpy disk galaxies that are bright at short wavelengths (rest-frame λ<5000 Å) dominate the UDF out to z∼5.5. Their uniformity in V/V_maxand comoving volume density suggest that they go even further, spanning a total time more than an order of magnitude larger than their instantaneous star formation times. They precede as well as accompany the formation epoch of distant red galaxies and extreme red objects. Those preceding could be the premerger objects that combined to make red spheroidal types at z∼2-3. Clumpy disks that do not undergo mergers are likely to evolve into spirals. The morphology of clumpy disks, the size and separation of the clumps, and the prevalence of this type of structure in the early universe suggests that most star formation occurs by self-gravitational collapse of disk gas.