THE DIMINISHING IMPORTANCE OF MAJOR GALAXY MERGERS AT HIGHER REDSHIFTS

Abstract

Using mass-selected galaxy samples from deep multiwavelength data we\ninvestigate the incidence of close galaxy pairs between z=0.4-2. Many such\nclose pairs will eventually merge, and the pair fraction is therefore related\nto the merger rate. Over this redshift range the mean pair fraction is\nessentially constant (evolving as f_pair (1+z)^{-0.4 +/- 0.6}) with about\n6+/-1% of massive galaxies having a 1:4 or greater companion within 30h^-1 kpc.\nAssuming the timescale over which pairs merge is not a strong function of\nredshift, this implies a similarly constant merger rate (per unit time) out to\nz=2. Since about three times as much cosmic time passes at z<1 as between\nz=1-2, this implies that correspondingly more mergers occur in the low-redshift\nuniverse. When minor companions (1:10 mass ratio or greater) are included, the\npair fraction increases to ~20% and still does not evolve strongly with\nredshift. We also use a rest-frame color criterion to select pairs containing\nonly quiescent galaxies (major "dry merger" progenitors), and find them to be\nsimilarly rare and constant with 4-7% of massive quiescent galaxies exhibiting\na nearby companion. Thus, even though other studies find major mergers to be\nrelatively uncommon since z=1, our results suggest that few additional mergers\noccur in the 1<z<2 range and other mechanisms may be required to explain the\nmass and size growth of galaxies over this epoch.\n