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Galaxy interactions are expected to play a crucial role in the build--up of stellar mass in any cold dark matter cosmology. Of particular interest are the mergers between systems of a comparable mass, as they are predicted to be one of the main modes of galaxy growth and have a crucial impact in the shaping of galaxy morphologies and dynamics. In this thesis I study two key aspects of the role that mergers play in galaxy evolution: a) What is the contribution of major galaxy interactions to the star formation history of the Universe at z < 1?, and b) How important are galaxy interactions for the build--up of the massive end of the red sequence? To answer the first question I use photometric redshifts, stellar masses and UV star formation rates from COMBO-17, 24 microns star formation rates from Spitzer and galaxy morphologies from two deep Hubble Space Telescope cosmological survey fields to study the enhancement in star formation activity as a function of galaxy separation. I apply robust statistical tools to find galaxies in close pairs, augmented with morphologically-selected very close pairs (unresolved in the ground-based photometry) and merger remnants from the Hubble Space Telescope imaging, finding that, on average, major galaxy interactions between galaxies more massive than 10^10 M_sun at 0.4<z<0.8 enhance the star formation activity by a factor of less than 2. I carry out detailed modeling of the methodology using a mock galaxy catalog from the Millenium Simulation, finding that in the regime applicable to this work the recovered enhancement in SF rate is accurate to better than 10%, smaller than the other sources of uncertainty. Accounting for the fraction of merging and interacting systems, I integrate the enhanced star formation to demonstrate that less than 10% of star formation activity is directly triggered by those interactions. To answer the second question I look for close pairs of galaxies on a sample drawn from the COSMOS and COMBO-17 galaxy surveys to find that the fraction of M_* > 5x10^10M_sun galaxies in close pairs (a proxy for the fraction of objects involved in an interaction) were more common 7 Gyrs ago by a factor ~2. By converting this merger fraction to a merger rate I estimate that 70% of the very massive galaxies (M_*>10^11 M_sun) have undergone a merger since z=1.2. This merger rate is sufficient to explain the observed number density evolution of such massive galaxies in the last 7 Gyrs. Merging plays, therefore, a dominant role in the formation of massive galaxies in the Universe.
Eight Gigayears of Galaxy Mergers (Other)
HTML Summary of #10688 Eight Gigayears of Galaxy Mergers
520 Astronomie
520 Astronomy and allied sciences