Assembly of the outer Galactic stellar halo in the hierarchical model

TL;DR

This paper uses N-body simulations to study how accretion of satellite galaxies influences the rotation properties of the Milky Way's outer stellar halo, explaining observed retrograde motions.

Contribution

It demonstrates that retrograde signals in the outer halo can result from accretion of sufficiently massive satellites on low-inclination orbits, highlighting the role of dynamical friction.

Findings

Retrograde signals can originate from accreted satellites more than 1/40 of the main halo mass.

Dynamical friction significantly influences the velocity distribution of accreted stars.

Low-inclination, retrograde satellites deposit more stars in the outer halo, producing counter-rotation.

Abstract

We provide a set of numerical N-body simulations for studying the formation of the outer Milky Ways's stellar halo through accretion events. After simulating minor mergers of prograde and retrograde orbiting satellite halo with a Dark Matter main halo, we analyze the signal left by satellite stars in the rotation velocity distribution. The aim is to explore the orbital conditions where a retrograde signal in the outer part of the halo can be obtained, in order to give a possible explanation of the observed rotational properties of the Milky Way stellar halo. Our results show that, for satellites more massive than $\sim 1/40$ of the main halo, the dynamical friction has a fundamental role in assembling the final velocity distributions resulting from different orbits and that retrograde satellites moving on low inclination orbits deposit more stars in the outer halo regions end therefore can produce the counter-rotating behavior observed in the outer Milky Way halo.