Using the UltraVISTA catalogs, we investigate the evolution in the 11.4 Gyr since z=3 of the progenitors of local ultra-massive galaxies (log(Mstar/M⊙)≈11.8; UMGs), providing a complete and consistent picture of how the most massive galaxies at z=0 have assembled. By selecting the progenitors with a semi-empirical approach using abundance matching, we infer a growth in stellar mass of 0.56+0.35−0.25 dex, 0.45+0.16−0.20 dex, and 0.27+0.08−0.12 dex from z=3, z=2, and z=1, respectively, to z=0.
At z<1, the progenitors of UMGs constitute a homogeneous population of only quiescent galaxies with old stellar populations. At z>1, the contribution from star-forming galaxies progressively increases, with the progenitors at 2<z<3 being dominated by massive (Mstar≈2×1011M⊙), dusty (AV∼1-2.2 mag), star-forming (SFR∼100-400~M⊙ yr−1) galaxies with a large range in stellar ages. At z=2.75, ∼15% of the progenitors are quiescent, with properties typical of post-starburst galaxies with little dust extinction and strong Balmer break, and showing a large scatter in color.
Our findings indicate that at least half of the stellar content of local UMGs was assembled at z>1, whereas the remaining was assembled via merging from z∼1 to the present. Most of the quenching of the star-forming progenitors happened between z=2.75 and z=1.25, in good agreement with the typical formation redshift and scatter in age of z=0 UMGs as derived from their fossil records. The progenitors of local UMGs, including the star-forming ones, never lived on the blue cloud since z=3. We propose an alternative path for the formation of local UMGs that refines previously proposed pictures and that is fully consistent with our findings.
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