Surface brightness profiles and color gradients of galaxies in the Hubble Deep Fields north and south

With the availability of the Hubble Deep Fields, we can now probe the properties of galaxies spanning a large range of redshifts at higher resolution and depths than before. Here, I fit elliptical isophotes to the Hubble Deep Fields North and South Wide Field Planetary Camera 2 (WFPC-2) and Near Infrared Camera and Multi-Object Spectrometer (NICMOS) imaging data to study the rest-frame B surface brightness profiles and rest-frame ((UV₂₁₈ –U₃₀₀)₀, (U ₃₀₀–B₄₅₀)₀, and ( B₄₅₀–V₆₀₆)₀ color gradients of 20 low-redshift galaxies (0 < z < 0.5), 75 intermediate-redshift galaxies (0.5 ≤ z ≤ 1.2) with I₈₁₄ < 25, and 70 high-redshift galaxies (2.0 ≤ z ≤ 3.6) with H₁₆₀ < 27. I demonstrate the importance of correcting for Point Spread Function (PSF) effects when evaluating the surface brightness profiles since at small scale lengths and faint magnitudes, an r1/4 profile can be smoothed out substantially to become consistent with an exponential profile. After taking account for profile smoothing due to the PSF, I found that at higher look-back time, there may be a slight increase in the fraction of galaxies possessing exponential profiles and a slight decrease in the fraction of galaxies possessing r1/4 profiles. Results also suggest a statistically insignificant increase in the fraction of peculiar/irregular type galaxies. From the weighted least-squares fit to the color profiles, I found that at low and intermediate-redshifts, the galaxies possess negative color gradients, indicating a reddening towards the center of the profile; whereas at high-redshifts, the galaxies possess strong positive color gradients, indicating that star formation is more centrally concentrated. This differs from the prevalent mode of extended-disk star formation that is observed in the local universe. After comparing my results with previous observations and recent semi-analytic models that treat galaxy formation and evolution using the cold dark matter (CDM) hierarchical framework, I concluded that the high-redshift galaxies are undergoing mergers. At this stage, these galaxies may predominantly possess r1/4 light profiles and are forming stars at a high rate in their central regions. Over time, mergers and interactions may cause the morphologies of galaxies to change and the locations of the star forming regions to shift from the central to the outer parts of the galaxies.