The flattening and the orbital structure of early-type galaxies and collisionless N-body binary disk mergers

TL;DR

This study investigates the orbital structures of early-type galaxies using dynamical models, revealing diverse orbital compositions and anisotropies, and discusses implications for galaxy formation through mergers.

Contribution

It introduces detailed dynamical models including dark halos to analyze orbital structures, highlighting the role of anisotropy and challenging merger-based formation scenarios.

Findings

Galaxies show diverse orbital compositions.

Flattening is often due to anisotropy in stellar velocities.

Collisionless mergers produce radially anisotropic remnants.

Abstract

We use oblate axisymmetric dynamical models including dark halos to determine the orbital structure of intermediate mass to massive Coma early-type galaxies. We find a large variety of orbital compositions. Averaged over all sample galaxies the unordered stellar kinetic energy in the azimuthal and the radial direction are of the same order, but they can differ by up to 40 percent in individual systems. In contrast, both for rotating and non-rotating galaxies the vertical kinetic energy is on average smaller than in the other two directions. This implies that even most of the rotating ellipticals are flattened by an anisotropy in the stellar velocity dispersions. Using three-integral axisymmetric toy models we show that flattening by stellar anisotropy maximises the entropy for a given density distribution. Collisionless disk merger remnants are radially anisotropic. The apparent lack of strong radial anisotropy in observed early-type galaxies implies that they may not have formed from mergers of disks unless the influence of dissipational processes was significant.