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

We present the results of a series of empirical computations regarding therole of major mergers in forming the stellar masses of modern galaxies based onmeasurements of galaxy merger and star formation histories from z~0.5-3. Were-construct the merger history of normal field galaxies from z~3 to z~0 as afunction of initial mass using published pair fractions and merger fractionsfrom structural analyses. We calibrate the observed merger time-scale and massratios 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 withvarious orbital properties and viewing angles. We use these simulations todetermine the time-scales and mass ratios that produce structures that would beidentified as major mergers. Based on these calculations we argue that atypical massive galaxy at z~3 with M_{*} > 10^{10} M_0 undergoes4.4^{+1.6}_{-0.9} major mergers at z > 1. We find that by z~1.5 the stellarmass of an average massive galaxy is relatively established, a scenarioqualitatively favored in a lambda dominated universe. We argue that the finalmasses of these systems increases by as much as a factor of 100 allowingLyman-break galaxies, which tend to have low stellar masses, to become the mostmassive galaxies in today's universe with M > M^{*}. Induced star formationhowever only accounts for 10-30% of the stellar mass formed in these galaxiesat z < 3. A comparison to semi-analytic models of galaxy formation shows thatCold Dark Matter (CDM) models consistently under-predict the merger fraction,and rate of merging, of massive galaxies at high redshift. This suggests thatmassive galaxy formation occurs through more merging than predicted in CDMmodels, rather than a rapid early collapse.Comment: ApJ, in pres