A fully cosmological model of a Monoceros-like ring

TL;DR

This study uses a cosmological simulation to model the formation of a Monoceros-like stellar ring, revealing that a minor satellite fly-by can induce large-scale vertical structures in a galactic disk.

Contribution

It demonstrates that a low-mass, low-velocity fly-by can generate Monoceros-like features through halo response, a novel explanation for such structures.

Findings

A Monoceros-like ring can form from halo response to a minor satellite encounter.

The pattern originated as an $m=1$ mode and evolved into a spiral.

The satellite's mass is about 5% of the host, with an 80 kpc pericentre and 215 km/s velocity.

Abstract

We study the vertical structure of a stellar disk obtained from a fully cosmological high-resolution hydrodynamical simulation of the formation of a Milky Way-like galaxy. At the present day, the disk'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 - disk interaction and reproduces, qualitatively, many of the observable properties of the Monoceros Ring. In particular we find disk material at the distance of Monoceros ($R \sim$ 12-16 kpc, galactocentric) extending far above the mid plane ( 30$^{\circ}$, $\langle Z \rangle \sim$ 1-2 kpc) in both hemispheres, as well as well-defined arcs of disk material at heliocentric distances $\gtrsim 5$ kpc. The pattern was first excited $\approx 3$ 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 $\sim 5\%$ of the host, $\sim 80$ kpc, and 215 km/s, respectively. The satellite is not massive enough to directly perturb the galactic disk 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.