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Institute for the Physics and Mathematics of the UniverseWPI

IPMU seminar

Date: December 16, 2008, 16:00 - 17:30
Place: Seminar Room at IPMU Prefab. B, Kashiwa Campus of the University of Tokyo
Speaker: Kevin Bundy (UC Berkeley)
Title: The Mass-Dependent Role of Mergers in Galaxy Evolution [PDF]
Abstract: Galaxy mergers are not only important in the mass assembly history of galaxies, they can also radically alter galaxy properties. With simulations showing that major mergers can transform disk galaxies into spheroidals, mergers have been invoked to explain the rising abundance of E/S0 galaxies since z~1. And because they may trigger other processes such as starbursts and quasars, mergers have also been suggested as the mechanism ultimately responsible for the quenching of star formation that shifts galaxies onto the red sequence. But with merger-induced growth proceeding hierarchically (as small systems merge to form larger ones) while the formation of red sequence spheroidals occurs in a top-down fashion (downsizing), are we asking mergers to do too much? To answer this question, we have measured the mass-dependent merger rate at z~1 using deep near-IR pair statistics in the GOODS fields. Our results confirm previous hints that the frequency of major mergers is nearly an order of magnitude lower than what's required to explain the formation of new spheroidal and red-sequence galaxies -- other mechanisms are clearly needed. At the same time, we find that the merger fraction increases among more massive galaxies, a welcome indication of hierarchical growth. In terms of mass assembly, our observations suggest that major mergers contribute little to galaxy growth below Log M = 10.5, but play a more significant role among galaxies with Log M > 11, ~30% of which have experienced a major (mostly dry) merger over 0.4 < z < 1.4.