Isophote Shapes of Early-Type Galaxies in Massive Clusters at z ~ 1 and 0

Dynamics of early-type galaxies (ETGs), whether they are supported by rotation or dispersion, is a clue to understand their assembly history. We compare the isophote shape parameter a₄ between z ∼ 1 and 0 as a proxy for dynamics to investigate the epoch at which the dynamical properties are established. We create cluster ETG samples with stellar masses of log(M_✽/M_⦿) ≥ 10.5 with spectroscopic redshifts. We have 130 ETGs from the Hubble Space Telescope Cluster Supernova Survey for z ∼ 1 and 355 ETGs from the Sloan Digital Sky Survey for z ∼ 0. We find similar dependence of the a₄ parameter on the mass at z ∼ 1 and 0; the main population changes from disky (a₄ > 0) to boxy (a₄  ≤ 0) at a critical mass of log(M_✽/M_⦿) ~ 11.5 with the massive end dominated by boxy ETGs. The disky ETG fraction is consistent between these redshifts. Although uncertainties are large, the results suggest that the isophote shapes and probably dynamical properties of cluster ETGs are already in place at z > 1 and do not significantly evolve in z < 1, despite significant size evolution. The constant disky fraction imply that the processes responsible for the size evolution is not enough violent to convert the dynamical properties of ETGs.