Early and Rapid Merging as a Formation Mechanism of Massive Galaxies: Empirical Constraints

We present the results of a series of empirical computations regarding the role of major mergers in forming the stellar masses of modern galaxies. We base these results on measurements of galaxy merger and star formation histories from z ∼ 0.5 − 3. We re-construct the merger history of normal field galaxies from z ∼ 3 to z ∼ 0 as a function of initial stellar mass using published pair fractions and merger fractions from structural analyses. We calibrate the observed merger time-scale and mass ratios for galaxy mergers using self-consistent N-body models of mergers, composed of dark matter and stars, with mass ratios from 1:1 to 1:5 with various orbital properties and viewing angles. We use these simulations to determine the time-scales and mass ratios that produce structures that would be identified as major mergers. Based on these calculations we argue that a typical massive galaxy at z ∼ 3 with M ∗ 10 10 M ⊙ undergoes 4.4 +1.6 −0.9 major mergers at z 1, with no further mergers at z 1.. We find that by z ∼ 1.5 the stellar mass of an average massive galaxy is relatively established, a scenario qualitatively favored in a Λ dominated universe, and independently suggested through stellar population analyses. Through empirical measurements of the star formation induced in these mergers, and using