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The growth of galaxies is a key problem in understanding the structure andevolution of the universe. Galaxies grow their stellar mass by a combination ofstar formation and mergers, with a relative importance that is redshiftdependent. Theoretical models predict quantitatively different contributionsfrom the two channels; measuring these from the data is a crucial constraint.Exploiting the UltraVISTA catalog and a unique sample of progenitors of localultra massive galaxies selected with an abundance matching approach, wequantify the role of the two mechanisms from z=2 to 0. We also compare ourresults to two independent incarnations of semi-analytic models. At allredshifts, progenitors are found in a variety of environments, ranging frombeing isolated to having 5-10 companions with mass ratio at least 1:10 within aprojected radius of 500 kpc. In models, progenitors have a systematicallylarger number of companions, entailing a larger mass growth for mergers than inobservations, at all redshifts. Generally, in both observations and models, theinferred and the expected mass growth roughly agree, within the uncertainties.Overall, our analysis confirms the model predictions, showing how the growthhistory of massive galaxies is dominated by in situ star formation at z~2, bothstar-formation and mergers at 1<z<2, and by mergers alone at z<1. Nonetheless,detailed comparisons still point out to tensions between the expected massgrowth and our results, which might be due to either an incorrectprogenitors-descendants selection, uncertainties on star formation rate andmass estimates, or the adopted assumptions on merger rates.

Mergers and Star Formation: The environment and Stellar Mass Growth of the Progenitors of Ultra-Massive Galaxies since z = 2