Time-average properties of z ∼ 0.6 major mergers: mergers significantly scatter high-z scaling relations

Interpreting the scaling relations measured by recent large kinematic surveys of z ≲ 1 galaxies has remained hampered by large observational scatter. We show that the observed ISM and morpho-dynamical properties along the average z ∼ 0.6 major merger describe a very self-consistent picture in which star formation is enhanced during first passage and fusion as a result of gravitational perturbations due to the interaction, while the gas velocity dispersion is simultaneously enhanced through shocks that convert kinematic energy associated with bulk orbital motions into turbulence at small scales. Angular momentum and rotation support in the disc are partly lost during the most perturbing phases, resulting in a morphologically compact phase. The fractions of present-day E/S0 versus later type galaxies can be predicted within only a few per cent, confirming that roughly half of local discs were reformed in the past 8–9 Gyr after gas-rich major mergers. Major mergers are shown to strongly scatter scaling relations involving kinematic quantities (e.g. the Tully–Fisher or Fall relations). Selecting high-z discs relying only on V/σ turns out to be less efficient than selecting discs from multiple criteria based on their morpho-kinematic properties, which can reduce the scatter of high-z scaling relations down to the values measured in local galaxy samples.