Simulations of minor mergers. II. The phase-space structure of thick discs

TL;DR

This study uses simulations to identify observable signatures in the phase-space structure of thick galactic discs that result from minor mergers, aiding in understanding their formation history.

Contribution

It demonstrates characteristic phase-space imprints of minor mergers on thick discs, providing robust observational tests for their merger origin.

Findings

Accreted stars rotate more slowly near the Sun.

Thick disc stars have broad velocity distributions.

A sinusoidal pattern in line-of-sight velocities is observed.

Abstract

We analyse the phase-space structure of simulated thick discs that are the result of a significant merger between a disc galaxy and a satellite. Our main goal is to establish what would be the characteristic imprints of a merger origin for the Galactic thick disc. We find that the spatial distribution predicted for thick disc stars is asymmetric, seemingly in agreement with recent observations of the Milky Way thick disc. Near the Sun, the accreted stars are expected to rotate more slowly, to have broad velocity distributions, and to occupy preferentially the wings of the line-of-sight velocity distributions. The majority of the stars in our model thick discs have low eccentricity orbits (in clear reference to the pre-existing heated disc) which gives rise to a characteristic (sinusoidal) pattern for their line of sight velocities as function of galactic longitude. The z-component of the angular momentum of thick disc stars provides a clear discriminant between stars from the pre-existing disc and those from the satellite, particularly at large radii. These results are robust against the particular choices of initial conditions made in our simulations, and thus provide clean tests of the disc heating via a minor merger scenario for the formation of thick discs.