The distribution of stellar orbits in Eagle galaxies -- the effect of mergers, gas accretion, and secular evolution

TL;DR

This study uses the Eagle simulation to analyze how galaxy merger history, gas accretion, and secular processes influence the distribution of stellar orbits, revealing correlations with stellar age and merger events.

Contribution

It provides a detailed analysis of stellar orbital distributions in simulated galaxies, linking orbital types to galaxy formation history and gas accretion processes.

Findings

Young stars are in thin disc orbits

Older stars have warmer, more varied orbits

Counter-rotating orbits trace dry mergers

Abstract

The merger history of a galaxy is thought to be one of the major factors determining its internal dynamics, with galaxies having undergone different types or mergers (e.g. dry, minor or major mergers) predicted to show different dynamical properties. We study the instantaneous orbital distribution of galaxies in the Eagle simulation, colouring the orbits of the stellar particles by their stellar age, in order to understand whether stars form in particular orbits (e.g. in a thin or thick disc). We first show that Eagle reproduces well the observed stellar mass fractions in different stellar orbital families as a function of stellar mass and spin parameter at z = 0. We find that the youngest stars reside in a thin disc component that can extend to the very inner regions of galaxies, and that older stars have warmer orbits, with the oldest ones showing orbits consistent with both hot and counter-rotating classifications, which is consistent with the trend found in the Milky-Way and other disc galaxies. We also show that counter-rotating orbits trace galaxy mergers - in particular dry mergers, and that in the absence of mergers, counter-rotating orbits can also be born from highly misaligned gas accretion that leads to star formation.