THE BUILDUP OF THE HUBBLE SEQUENCE IN THE COSMOS FIELD

We use similar to 8600 COSMOS galaxies at mass scales > 5 x 10(10)M(circle dot) 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 x 10(11) M-circle dot, 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(circle dot) 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 x 10(10)-10(11) M-circle dot, 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 similar to 10% at z = 1 to >30% at z = 0.2 (for comparison, the elliptical fraction grows from similar to 15% to similar 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 similar to 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.