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The photometric redshift distributions, spectral types, Sersic indices, andsizes of all resolved galaxies in the Hubble Space Telescope Ultra Deep Field(UDF) are studied in order to understand the environment and nature of starformation in the early Universe. Clumpy disk galaxies that are bright at shortwavelengths (restframe <5000 Angstroms) dominate the UDF out to z~5.5. Theiruniformity in V/V_max and co-moving volume density suggest they go evenfurther, spanning a total time more than an order of magnitude larger thantheir instantaneous star formation times. They precede as well as accompany theformation epoch of distant red galaxies and extreme red objects. Thosepreceding could be the pre-merger objects that combined to make red spheroidaltypes at z~2 to 3. Clumpy disks that do not undergo mergers are likely toevolve into spirals. The morphology of clumpy disks, the size and separation ofthe clumps, and the prevalence of this type of structure in the early Universesuggests that most star formation occurs by self-gravitational collapse of diskgas.Comment: 40 pages, 13 figures, scheduled for ApJ Vol 658, April 1, 200

Resolved Galaxies in the Hubble Ultra Deep Field: Star Formation in Disks at High Redshift