Thin, thick and dark discs in CDM

In a cold dark matter ( CDM) 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 three subhaloes with vmax > 80 km s−1; seven with vmax > 60 km s−1 and 15 with vmax > 40 km s−1 merge at redshift z 1. Assuming isotropic accretion, a third of these merge at an impact angle θ < 20◦ 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 ρDDISC = 0.25 − 1ρHALO 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 θ > 20◦ are twice as likely as low-inclination ones. These lead to structures that closely resemble the recently discovered inner and outer stellar haloes. 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 80 km s−1) 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 per cent of the thick disc stars. The Milky Way thin disc must reform from fresh gas after z = 1. Only one in four of our sample Milky Way haloes experiences mergers massive and late enough to fully destroy the thin disc. We conclude that thick, thin and dark discs occur naturally within a CDM cosmology.