Thin, thick and dark discs in LCDM

TL;DR

This paper uses cosmological and collisionless simulations to study how satellite mergers in a LCDM universe shape the Milky Way's thin, thick, and dark discs, revealing their formation, structure, and implications for dark matter detection.

Contribution

It provides a detailed analysis of satellite accretion effects on the Milky Way's disc structure and dark matter distribution, highlighting the formation of thick and dark discs in a LCDM cosmology.

Findings

A third of satellites merge at low impact angles, forming a thick disc.

Dark matter disc contributes significantly to local dark matter density.

Higher inclination mergers create stellar halos and induce disc warping.

Abstract

In a LCDM cosmology, the Milky Way accretes satellites into the stellar disc. We use cosmological simulations to assess the frequency of near disc plane and higher inclination accretion events, and collisionless simulations of satellite mergers to quantify the final state of the accreted material and the effect on the thin disc. On average, a Milky Way-sized galaxy has 3 subhalos with vmax>80km/s; 7 with vmax>60km/s; and 15 with vmax>40km/s merge at redshift z>1. Assuming isotropic accretion, a third of these merge at an impact angle <20 degrees and are dragged into the disc plane by dynamical friction. Their accreted stars and dark matter settle into a thick disc. The stellar thick disc qualitatively reproduces the observed thick disc at the solar neighbourhood, but is less massive by a factor ~2-10. The dark matter disc contributes 0.25-1 times the halo density at the solar position. Although not likely to be dynamically interesting, the dark disc has important implications for the direct detection of dark matter because of its low velocity with respect to the Earth. Higher inclination encounters (>20 degrees) are twice as likely as low inclination ones. These lead to structures that closely resemble the recently discovered inner/outer stellar halos. They also do more damage to the Milky Way stellar disc creating a more pronounced flare, and warp; both long-lived and consistent with current observations. The most massive mergers (vmax>80km/s) heat the thin disc enough to produce a thick disc. These heated thin disc stars are essential for obtaining a thick disc as massive as that seen in the Milky Way; they likely comprise some ~50-90% of the thick disc stars. The Milky Way thin disc must reform from fresh gas after z=1 [abridged].