Orbital Eccentricity as a probe of Thick Disk Formation Scenarios

TL;DR

This paper compares orbital eccentricity distributions in four simulated thick disk formation scenarios to identify observational signatures that distinguish their origins.

Contribution

It demonstrates that eccentricity distributions differ significantly between formation models, providing a potential observational test for thick disk origins.

Findings

Heating, migration, and gas-rich merger models predict low-eccentricity peaks.

Accretion models produce broader, higher-eccentricity distributions.

Eccentricity distributions are robust indicators of formation mechanisms.

Abstract

We study the orbital properties of stars in four (published) simulations of thick disks formed by: i) accretion from disrupted satellites, ii) heating of a pre-existing thin disk by a minor merger, iii) radial migration and iv) gas rich mergers. We find that the distribution of orbital eccentricities are predicted to be different for each model: a prominent peak at low eccentricity is expected for the heating, migration and gas-rich merging scenarios, while the eccentricity distribution is broader and shifted towards higher values for the accretion model. These differences can be traced back to whether the bulk of the stars in each case is formed 'in-situ' or is 'accreted', and are robust to the peculiarities of each model. A simple test based on the eccentricity distribution of nearby thick disk stars may thus help elucidate the dominant formation mechanism of the Galactic thick disk.