Morpho-kinematics of z ∼ 1 galaxies probe the hierarchical scenario

Abstract

We have studied a representative sample of intermediate-mass galaxies at z ∼ 1, observed by the kinematic survey KMOS^3D. We have re-estimated the kinematical parameters from the published kinematic maps and analysed photometric data from Hubble Space Telescope (HST) to measure optical disc inclinations and Position Angle. We find that only half of the z ∼ 1 galaxies show kinematic properties consistent with rotating discs using the same classification scheme than that adopted by the KMOS^3D team. Because merger orbital motions can also brought rotation, we have also analysed galaxy morphologies from the available HST imagery. Combining these results to those from kinematics, it leads to a full morpho-kinematic classification. To test the robustness of the latter for disentangling isolated discs from mergers, we confronted the results with an analysis of pairs from the open grism redshift survey 3D-HST. All galaxies found in pairs are affected by either kinematic and/or morphological perturbations. Conversely, all galaxies classified as virialized spirals are found to be isolated. A significant fraction (one-fourth) of rotating discs classified from kinematics by the KMOS^3D team are found in pairs, which further supports the need for a morpho-kinematic classification. It results that only one-third of z ∼ 1 galaxies are isolated and virialized spirals, while 58 per cent of them are likely involved in a merger sequence, from first approach to disc rebuilding. The latter fraction is in good agreement with the results of semi-empirical Λ cold dark matter models, supporting a merger-dominated hierarchical scenario as being the main driver of galaxy formation at least during the last 8 billion years.