Testing formation mechanisms of the Milky Way's thick disc with RAVE

TL;DR

This study analyzes the eccentricity distribution of Milky Way's thick disc stars from RAVE data, comparing it to various formation models, and finds evidence supporting in situ formation, especially via gas-rich mergers.

Contribution

It provides observational constraints on thick disc formation mechanisms by comparing stellar eccentricity distributions with simulation predictions.

Findings

Distribution peaks at low eccentricities

Inconsistent with pure accretion models

Most consistent with gas-rich merger scenario

Abstract

We study the eccentricity distribution of a thick disc sample of stars observed in the Radial Velocity Experiment (RAVE) and compare it to that expected in four simulations of thick disc formation in the literature (accretion of satellites, heating of a primordial thin disc during a merger, radial migration, and gas-rich mergers), as compiled by Sales et al. (2009). We find that the distribution of our sample is peaked at low eccentricities and falls off smoothly and rather steeply to high eccentricities. This distribution is fairly robust to changes in distances, thin disc contamination, and the particular thick disc sample used. Our results are inconsistent with what is expected for the pure accretion simulation, since we find that the dynamics of local thick disc stars implies that the majority must have formed "in situ". Of the remaining models explored, the eccentricity distribution of our stars appears to be most consistent with the gas-rich merger case.