2002AJ....124.1886M

We fit elliptical isophotes to the Hubble Deep Field North WFPC-2 and NICMOS data to study the rest-frame (UV₂₁₈-U₃₀₀)₀color profiles and rest-frame B surface brightness profiles of 33 intermediate-redshift galaxies (0.5≤z≤1.2) with I₈₁₄<25 and 50 high-redshift galaxies (2.0≤z≤3.5) with H₁₆₀<27. From the weighted least-squares fit to the color profiles we find that at intermediate redshifts the galaxies possess negative color gradients [<Δ(UV₂₁₈-U₃₀₀)₀/Δlogr>=-0.091±0.007 mag.dex^–1], indicating a reddening toward the center of the profile similar to local samples, whereas at high redshifts the galaxies possess positive color gradients [<Δ(UV₂₁₈-U₃₀₀)₀/Δlogr>=0.272±0.007 mag.dex^–1], indicating that star formation is more centrally concentrated. Although the presence of dust can cause some reddening to occur toward the centers of the profiles seen at intermediate redshifts, it cannot explain the strong central blueing of light seen at high redshifts. Thus, we are witnessing a population of galaxies with strong positive color gradients at high redshifts that do not seem to exist in large numbers at lower redshifts. This indicates that star formation is more centrally concentrated in the distant galaxy sample, which differs from the prevalent mode of extended disk star formation that we observe in the local universe. Additionally, we find that it is critical to correct for point-spread function (PSF) effects when evaluating the surface brightness profiles since at small scale lengths and faint magnitudes, an r^1/4 profile can be smoothed out substantially to become consistent with an exponential profile. After correcting for PSF effects, we find that at higher look-back time, the fraction of galaxies possessing exponential profiles have slightly decreased while the fraction of galaxies possessing r^1/4 profiles have slightly increased. Our results also suggest a statistically insignificant increase in the fraction of peculiar or irregular galaxies. We compare our results with recent semianalytical models that treat galaxy formation and evolution following the cold dark matter hierarchical framework.