THE STRUCTURES AND TOTAL (MINOR + MAJOR) MERGER HISTORIES OF MASSIVE GALAXIES UP TO<i>z</i>∼ 3 IN THE<i>HST</i>GOODS NICMOS SURVEY: A POSSIBLE SOLUTION TO THE SIZE EVOLUTION PROBLEM

Abstract

We investigate the total major (&gt; 1:4 by stellar mass) and minor (&gt; 1:100 by\nstellar mass) merger history of a population of 80 massive (M_* &gt; 10^11 M_sol)\ngalaxies at high redshifts (z = 1.7 - 3). We utilize extremely deep and high\nresolution HST H-band imaging from the GOODS NICMOS Survey (GNS), which\ncorresponds to rest-frame optical wavelengths at the redshifts probed. We find\nthat massive galaxies at high redshifts are often morphologically disturbed,\nwith a CAS deduced merger fraction f_m = 0.23 +/- 0.05 at z = 1.7 - 3. We find\nclose accord between close pair methods (within 30 kpc apertures) and CAS\nmethods for deducing major merger fractions at all redshifts. We deduce the\ntotal (minor + major) merger history of massive galaxies with M_* &gt; 10^9 M_sol\ngalaxies, and find that this scales roughly linearly with log-stellar-mass and\nmagnitude range. We test our close pair methods by utilizing mock galaxy\ncatalogs from the Millennium Simulation. We compute the total number of mergers\nto be (4.5 +/- 2.9) / &lt;\\tau_m&gt; from z = 3 to the present, to a stellar mass\nsensitivity threshold of ~ 1:100 (where \\tau_m is the merger timescale in Gyr\nwhich varies as a function of mass). This corresponds to an average mass\nincrease of (3.4 +/- 2.2) x 10^11 M_sol over the past 11.5 Gyrs due to merging.\nWe show that the size evolution observed for these galaxies may be mostly\nexplained by this merging.\n