A fully cosmological model of a Monoceros-like ring

We study the vertical structure of a stellar disc obtained from a fully cosmological high-resolution hydrodynamical simulation of the formation of a Milky Way-like galaxy. At the present day, the disc's mean vertical height shows a well defined and strong pattern, with amplitudes as large as 3 kpc in its outer regions. This pattern is the result of a satellite– host halo–disc interaction and reproduces, qualitatively, many of the observable properties of the Monoceros Ring. In particular we find disc material at the distance of Monoceros (R ∼ 12–16 kpc, galactocentric) extending far above the mid plane (30°, 〈Z〉 ∼ 1–2 kpc) in both hemispheres, as well as well-defined arcs of disc material at heliocentric distances ≳5 kpc. The pattern was first excited ≈3 Gyr ago as an m = 1 mode that later winds up into a leading spiral pattern. Interestingly, the main driver behind this perturbation is a low-mass low-velocity fly-by encounter. The satellite has total mass, pericentre distance and pericentric velocity of ∼5 per cent of the host, ∼80 kpc and 215 km s−1, respectively. The satellite is not massive enough to directly perturb the galactic disc but we show that the density field of the host dark matter halo responds to this interaction resulting in a strong amplification of the perturbative effects. This subsequently causes the onset and development of the Monoceros-like feature.