2006ApJ...639L...1P

We study the evolution of the stellar mass density for the separate families of bulge-dominated and disk-dominated galaxies over the redshift range 0.25≤z≤1.15. We derive quantitative morphology for a statistically significant galaxy sample of 1645 objects selected from the FORS Deep Field and the GOODS-S field. We find that the morphological mix evolves monotonically with time: the higher the redshift, the more disk systems dominate the total mass content. At z∼1, massive objects (M_*≥7x10¹⁰ M_☉) host about half of the mass contained in objects of similar mass in the local universe. The contribution from early- and late-type galaxies to the mass budget at z∼1 is nearly equal. We show that in situ star formation is not sufficient to explain the changing mass budget. Moreover, we find that the star formation rate per unit stellar mass of massive galaxies increases with redshift only for the intermediate and early morphological types while it stays nearly constant for late-type objects. This suggests that merging and/or frequent accretion of small-mass objects has a key role in the shaping of the Hubble sequence as we observe it now and also in decreasing the star formation activity of the bulge-dominated descendants of massive disk galaxies.