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Abstract

In this thesis, we measure for the first time the projected shapes of large samples of distant early-type galaxies in deep near-infrared surveys from the ground and with the Hubble Space Telescope, and reconstruct their intrinsic, three-dimensional shape distribution assuming random viewing angles. We find that the most massive early-type galaxies are roundest at all redshifts 0 < z < 2.5, indicating that at any cosmic time mergers dominate the growth of the most massive galaxies. However, early-type galaxies were on average more disk-like at earlier times, indicating that star formation in their progenitors occurred in disks, and that merging is the dominant evolutionary channel after star formation ceases. Together with the recent finding that typical early-type galaxy sizes increase dramatically between z ~ 2 and the present, the increased `roundness' suggests that classical elliptical galaxies emerge through gradual merging and accretion of satellites. Moreover, the observed increase in number density over the same time span suggests that new early-type galaxies form continuously, also at late cosmic times. Our findings suggest that these newly formed early types are generally disk-like at all redshifts, and subsequently become larger and rounder through merging. In addition, we provide a low-redshift benchmark for high-redshift galaxy studies, by deriving new stellar mass and star formation rate estimates for nearly a million galaxies drawn from Sloan Digital Sky Survey spectroscopic sample. Our novel approach combines photometry from multiple surveys that span a large range in wavelength (from 0.4 to 22 micron), and uses the latest modeling techniques to consistently incorporate the effects of extinction and emission by dust. We use these new properties to investigate the nature of the bimodality of star formation activity in the galaxy population.