Allison Man (Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Denmark) -- The role of galaxy merging in the life of massive galaxies

In the local Universe, the most massive galaxies of above 10^11 solar masses are typically situated at the centres of galaxy clusters or groups, and have elliptical light profiles. They have uniformly old stellar populations with the majority of stars formed when the Universe was only 2-3 Gyrs old. Merging has been invoked as an important driver for their evolution, possibly responsible for morphological transformations, size growth, ignition of active galactic nuclei as well as both triggering and quenching of star formation. Accurate measurements of the merging history of massive galaxies is thus instrumental to understand their evolution. While several measurements of the merging fraction of massive galaxies up to z~3 exist to date, they lead to discrepant conclusions of whether the fraction is increasing or diminishing. My recent work resolves these discrepancies through the accurate measurement of the galaxy merger fraction up to z=3 in the COSMOS field. Combining the large area, near-infrared survey of UltraVISTA with the smaller area, but deeper and higher resolution HST/CANDELS dataset, yields the largest, most complete photometrically identified sample of mergers at z>1. The discrepancy of previous studies is found to be due to a selection effect. Selecting galaxy pairs by stellar mass ratio leads to a diminishing merger fraction at z~2, while selecting by flux ratio leads to an increasing trend. Flux-ratio selection is biased towards low M/L satellites, while stellar mass ratio selected mergers are likely biased against gas-rich satellites at z>2. I argue that the total baryon mass ratio is the least biased probe of the "true" merger rate of galaxies, and discuss future plans for examining the role of galaxy merging in the global star formation history, as well as its relation to star formation quenching.