A Sagittarius-Induced Origin for the Monoceros Ring

TL;DR

This paper proposes that the Monoceros ring originated from a collision with the Sagittarius dwarf galaxy, which induced a nearly-circular orbit and led to its formation, supported by cosmological and idealized simulations.

Contribution

It introduces a novel scenario where the Monoceros ring's origin is linked to a collision with Sagittarius, explaining its circular orbit and metallicity.

Findings

Cosmological simulations show low-mass satellites in circular orbits result from collisions.

Sagittarius's orbit crosses the Galactic plane near Monoceros's location.

Simulations suggest Sagittarius collision could circularize and disrupt a progenitor, forming the ring.

Abstract

The Monoceros ring is a collection of stars in nearly-circular orbits at roughly 18 kpc from the Galactic center. It may have originated (i) as the response of the disc to perturbations excited by satellite companions or (ii) from the tidal debris of a disrupted dwarf galaxy. The metallicity of Monoceros stars differs from that of disc stars at comparable Galactocentric distances, an observation that disfavours the first scenario. On the other hand, circular orbits are difficult to accommodate in the tidal-disruption scenario, since it requires a satellite which at the time of disruption was itself in a nearly circular orbit. Such satellite could not have formed at the location of the ring and, given its low mass, dynamical friction is unlikely to have played a major role in its orbital evolution. We search cosmological simulations for low-mass satellites in nearly-circular orbits and find that they result, almost invariably, from orbital changes induced by collisions with more massive satellites: the radius of the circular orbit thus traces the galactocentric distance of the collision. Interestingly, the Sagittarius dwarf, one of the most luminous satellites of the Milky Way, is in a polar orbit that crosses the Galactic plane at roughly the same Galactocentric distance as Monoceros. We use idealized simulations to demonstrate that an encounter with Sagittarius might well have led to the circularization and subsequent tidal demise of the progenitor of the Monoceros ring.