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Most present-day galaxies with stellar masses of about 10^11 M⊙ show no ongoing star formation and are dense spheroids. Ten billion years ago, at a redshift of z~2, similarly massive galaxies were typically forming stars at rates of hundreds of solar masses per year. It is debated how star formation ceased, on which timescales, and how this “quenching” relates to the emergence of dense spheroids. This Ph.D. thesis presents results on the growth of galactic components in typical star-forming galaxies and delivers a major step forward in understanding how these galaxies stopped their star formation. The core of these results is based on observations, in particular with the Hubble Space Telescope and the Very Large Telescope in Chile. We have also developed and investigated numerical and analytical models to interpret our observations.

Contrary to traditional models of galaxy formation, recent observations suggest that some bulges form within preexisting disk galaxies. Such late-epoch bulge formation within disks seems to be linked to disk gas inflow and central star formation, caused by either internal secular processes or galaxy mergers and interactions. We identify a population of galaxies likely to be experiencing active bulge growth within disks, using the criterion that the color within the half-light radius is bluer than the outer disk color. Such blue-centered galaxies make up >10% of star-forming disk galaxies within the Nearby Field Galaxy Survey, a broad survey designed to represent the natural diversity of the low-z galaxy population over a wide range of luminosities and environments. Blue-centered galaxies correlate at 99% confidence with morphological peculiarities suggestive of minor mergers and interactions. From this and other evidence, we argue that external drivers rather than internal secular processes probably account for the majority of blue-centered galaxies. We go on to discuss quantitative plausibility arguments indicating that blue-centered evolutionary phases may represent an important mode of bulge growth for most disk galaxies, leading to significant changes in bulge-to-disk ratio without destroying disks. If this view is correct, bulge growth within disks may be a natural consequence of the repeated galaxy mergers and interactions inherent in hierarchical galaxy formation., 18 pages including 12 figures, AJ, accepted