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Published May 1, 2010 | Published
Journal Article Open

The Buildup of the Hubble Sequence in the Cosmos Field


We use ~8600 COSMOS galaxies at mass scales >5 × 10^(10) M_☉ to study how the morphological mix of massive ellipticals, bulge-dominated disks, intermediate-bulge disks, disk-dominated galaxies, and irregular systems evolves from z = 0.2 to z = 1. The morphological evolution depends strongly on mass. At M > 3 × 10^(11) M_☉, no evolution is detected in the morphological mix: ellipticals dominate since z = 1, and the Hubble sequence has quantitatively settled down by this epoch. At the 10^(11) M_☉ mass scale, little evolution is detected, which can be entirely explained by major mergers. Most of the morphological evolution from z = 1 to z = 0.2 takes place at masses 5 × 10^(10)-10^(11) M_☉, where (1) the fraction of spirals substantially drops and the contribution of early types increases. This increase is mostly produced by the growth of bulge-dominated disks, which vary their contribution from ~10% at z = 1 to >30% at z = 0.2 (for comparison, the elliptical fraction grows from ~15% to ~20%). Thus, at these masses, transformations from late to early types result in diskless elliptical morphologies with a statistical frequency of only 30%-40%. Otherwise, the processes which are responsible for the transformations either retain or produce a non-negligible disk component. (2) The disk-dominated galaxies, which contribute ~15% to the intermediate-mass galaxy population at z = 1, virtually disappear by z = 0.2. The merger rate since z = 1 is too low to account for the disappearance of these massive disk-dominated systems, which most likely grow a bulge via secular evolution.

Additional Information

© 2010 American Astronomical Society. Received 2009 November 7; accepted 2010 March 11; published 2010 April 1. We thank the COSMOS and zCOSMOS collaborations for many stimulating discussions. P.O. acknowledges support from the Swiss National Foundation (SNF). This work is based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA Inc, under NASA contract NAS 5-26555; also based on data collected at: the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan; the European Southern Observatory under Large Program 175.A-0839, Chile; Kitt Peak National Observatory, Cerro Tololo Inter-American Observatory, and the National Optical Astronomy Observatory, which are operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation; and the Canada–France–Hawaii Telescope with MegaPrime/ MegaCam operated as a joint project by the CFHT Corporation, CEA/DAPNIA, the National Research Council of Canada, the Canadian Astronomy Data Centre, the Centre National de la Recherche Scientifique de France, TERAPIX, and the University of Hawaii.

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