The Structures of Distant Galaxies - III: The Merger History of over 20,000 Massive Galaxies at z < 1.2

TL;DR

This study uses deep Hubble imaging to analyze the structural properties and merger history of over 20,000 massive galaxies at z<1.2, finding that merger fractions increase with redshift and are consistent with cosmological simulations.

Contribution

It provides a detailed measurement of galaxy merger fractions using structural parameters and compares these with pair methods, refining the understanding of galaxy merger evolution.

Findings

Merger fraction increases from ~4% to 13% between z=0.2 and z=1.2.

Merger fraction follows a power-law evolution with slope ~2.3 to 3.8 depending on local universe priors.

Galaxies with M*>10^{10} M_sun undergo 1-2 major mergers on average at z<1.2.

Abstract

Utilizing deep Hubble Space Telescope imaging from the two largest field galaxy surveys, the Extended Groth Strip (EGS) and the COSMOS survey, we examine the structural properties, and derive the merger history for 21,902 galaxies with M_*>10^{10} M_0 at z<1.2. We examine the structural CAS parameters of these galaxies, deriving merger fractions, at 0.2<z<1.2, based on the asymmetry and clumpiness values of these systems. We find that the merger fraction between z=0.2 and z=1.2 increases from roughly f_m=0.04+/-0.01 to f_m=0.13+/-0.01. We explore several fitting formalisms for parameterising the merger fraction, and compare our results to other structural studies and pair methods within the DEEP2, VVDS, and COSMOS fields. We also re-examine our method for selecting mergers, and the inherent error budget and systematics associated with identifying mergers using structure. For galaxies selected by M_*>10^{10} M_0, the merger fraction can be parameterised by f_m = f_0*(1+z)^m with the power-law slope m=2.3+/-0.4. By using the best available z = 0 prior the slope increases to m=3.8+/-0.2, showing how critical the measurement of local merger properties are for deriving the evolution of the merger fraction. We furthermore show that the merger fraction derived through structure is roughly a factor of 3-6 higher than pair fractions. Based on the latest cosmological simulations of mergers we show that this ratio is predicted, and that both methods are likely tracing the merger fraction and rate properly. We calculate, utilising merger time scales from simulations, and previously published merger fractions that the merger rate of galaxies with M_*>10^{10} M_0 increases linearly between z = 0.7 and z = 3, and that a typical M_*>10^{10} M_0 galaxy undergoes between 1-2 major mergers at z<1.2.