The Buildup of the Hubble Sequence in the Cosmos Field

Abstract

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.

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