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Paper: Witnessing the Assembly of Present-Day Galaxies
Volume: 379, Cosmic Frontiers
Page: 130
Authors: Meisenheimer, K.; Bell E.F.; the COMBO-17/GEMS team
Abstract: We summarize recent results of the look-back survey COMBO-17 and its HST imaging complement GEMS: We demonstrate that over the past six billion years most of the stars have always been formed in spiral and irregular galaxies while mergers play a relatively minor role. The assembly of stellar mass in disk galaxies occurs at constant central surface mass density, such that disks grow inside-out. Separating the evolution of the mass function in galaxies into red-sequence and blue galaxies shows that the overall increase in stellar mass ends up in red sequence galaxies only, while the mass function of blue galaxies remains essentially constant. Since stars form in the blue (spiral) galaxies, an ongoing transformation from blue into red-sequence galaxies is required. However, the most massive elliptical galaxies are at least three times more massive than their blue progenitors. We argue, that “dry merger” (between two red galaxies) could play an essential role in assembling these most massive ellipticals. In the outskirts of the A901/A902 cluster complex, we find a large fraction of dusty, star forming galaxies within the red sequence, which are neither present in the cluster centers nor in the field. Here environmental effects seem to affect the star formation process strongly. Although we find that quasars live in host galaxies which show indications of merging and enhanced star formation, the present sample from COMBO-17/GEMS seems too small to reach firm conclusions.
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