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Published August 20, 2010 | Published
Journal Article Open

The Rise and Fall of Passive Disk Galaxies: Morphological Evolution Along the Red Sequence Revealed by COSMOS


The increasing abundance of passive "red-sequence" galaxies since z ~ 1-2 is mirrored by a coincident rise in the number of galaxies with spheroidal morphologies. In this paper, however, we show in detail, that, the correspondence between galaxy morphology and color is not perfect, providing insight into the physical origin of this evolution. Using the COSMOS survey, we study a significant population of red-sequence galaxies with disk-like morphologies. These passive disks typically have Sa-Sb morphological types with large bulges, but they are not confined to dense environments. They represent nearly one-half of all red-sequence galaxies and dominate at lower masses (≾10^(10) M_⊙) where they are increasingly disk-dominated. As a function of time, the abundance of passive disks with M_* ≾10^(11) M_⊙ increases, but not as fast as red-sequence spheroidals in the same mass range. At higher mass, the passive disk population has declined since z ~ 1, likely because they transform into spheroidals. Based on these trends, we estimate that as much as 60% of galaxies transitioning onto the red sequence evolve through a passive disk phase. The origin of passive disks therefore has broad implications for our understanding of how star formation shuts down. Because passive disks tend to be more bulge-dominated than their star-forming counterparts, a simple fading of blue disks does not fully explain their origin. We explore the strengths and weaknesses of several more sophisticated explanations, including environmental effects, internal stabilization, and disk regrowth during gas-rich mergers. While previous work has sought to explain color and morphological transformations with a single process, these observations open the way to new insight by highlighting the fact that galaxy evolution may actually proceed through several separate stages.

Additional Information

© 2010 The American Astronomical Society. Received 2009 December 5; accepted 2010 June 26; published 2010 August 4. We thank Eliot Quataert, Arjen van der Wel, Avishai Dekel, Frédéric Bournaud, Bob Nichol, Karen Masters, and Tommaso Treu for very useful discussions and feedback. K.B. acknowledges support for this work provided by NASA through Hubble Fellowship grant HF-01215, awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. We acknowledge the entire COSMOS collaboration which has made this work possible. More information on the COSMOS survey is available at http://www.astro.caltech.edu/cosmos.

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Published - Bundy2010p11222Astrophys_J.pdf


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