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P. Moth, R. J. Elston (University of Florida)
We fit elliptical isophotes to the Hubble Deep Field WFPC-2 and NICMOS data to study the rest-frame (Uv-U)o color profiles of 33 intermediate redshift galaxies ( 0.5 \leq z \leq 1.2) and 50 high redshift galaxies (2.0 \leq z \leq 3.5). From the weighted least-squares fit to the color profiles, we find that at intermediate redshifts, the galaxies possess negative color gradients (\langle \Delta(Uv-U)o/\Deltalog(r) \rangle =-0.098 ± 0.0083 mag/dex) indicating a reddening towards the center of the profile similar to local samples, whereas at high redshifts, the galaxies possess positive color gradients (\langle \Delta(Uv-U)o/\Deltalog(r) \rangle =+0.260 ± 0.0327 mag/dex) indicating that star formation is more centrally concentrated in disks. Whereas the presence of dust can cause some reddening to occur towards the centers of the profiles seen at intermediate redshifts, it can not explain the central blueing of light seen at high redshifts. Our results may be strongly affected by observational selection affects which lead us to detect only the bluest and brightest galaxies at high redshifts. However, if selection affects do not represent the full story, then we may be witnessing evolution in these galaxies. Mergers and interactions may be responsible for the evolution seen in the color profiles of the galaxies in the Hubble Deep Field and for igniting centrally concentrated starbursts in the high redshift systems. We compare our results with recent semi-analytical models which treat galaxy formation and evolution following the Cold Dark Matter (CDM) hierarchical framework.