Thin, thick and dark discs in LCDM

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].

19 pages; 14 figures. Accepted version to appear in MNRAS. Improved halo finding led to a slight increase in subhalo masses at high redshift; conclusions unaltered

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Cited by in corpus (8)

• Cold Dark Matter Substructure and Galactic Disks II: Dynamical Effects of Hierarchical Satellite Accretion
• The Radical Consequences of Realistic Satellite Orbits for the Heating and Implied Merger Histories of Galactic Disks
• Stellar Kinematic Constraints on Galactic Structure Models Revisited: Bar and Spiral Arm Resonances
• The Destruction of Thin Stellar Disks Via Cosmologically Common Satellite Accretion Events
• Detecting the Milky Way's Dark Disk
• Dark matter in the solar system I: The distribution function of WIMPs at the Earth from solar capture
• Dark matter in the solar system III: The distribution function of WIMPs at the Earth from gravitational capture
• The Galactic Stellar Disc