The structures of distant galaxies - III. The merger history of over 20 000 massive galaxies at z < 1.2

Utilizing deep Hubble Space Telescope imaging from the two largest field galaxy surveys, the Extended Groth Strip and the Cosmic Evolution Survey (COSMOS), we examine the structural properties, and derive the merger history for 21 902 (galaxies with M-* > 10(10) M-circle dot at z < 1.2. We examine the structural concentration, asymmetry and clumpiness (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 1.2 increases from roughly f(m) = 0.04 +/- 0.01 to 0.13 +/- 0.01. We furthermore detect, at a high significance, an abrupt drop in the merger fraction at z < 0.7, which appears relatively constant from z = 0.7 to 1.2. We explore several fitting formalism for parametrizing the merger fraction, and compare our results to other structural studies and pair methods within the DEEP2, VVDS and Cosmic Evolution Survey (COSMOS) fields. We also examine the basic features of these galaxies, including our selection for mergers, and the inherent error budget and systematics associated with finding mergers through structure. We find that for galaxies selected by M-* > 10(10) M-circle dot, the merger fraction can be parametrized by f(m) = f(0) x (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 is 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, utilizing merger time-scales from simulations and previously published merger fractions within the Hubble Deep and Ultra Deep Fields, that the merger rate of galaxies with M-* > 10(10) M-circle dot increases linearly between z = 0.7 and 3. Finally, we show that a typical galaxy with a stellar mass of M-* > 10(10) M-circle dot undergoes between 1 and 2 major mergers at z < 1.2.