Apocenter Pile-Up: Origin of the Stellar Halo Density Break

TL;DR

This study links the stellar halo density break at ~20 kpc to the apocenters of high-eccentricity, metal-rich stars, suggesting they originate from a massive dwarf galaxy progenitor that built up the halo at its orbital apocenters.

Contribution

It provides evidence that the stellar halo density break is caused by stars accumulating at the apocenters of a common progenitor's orbit, identifying a specific dwarf galaxy origin.

Findings

High-eccentricity, metal-rich stars have apocenters at the halo break radius.

The halo density profile's steepening correlates with the orbital apocenters of these stars.

A massive dwarf galaxy progenitor likely contributed significantly to the inner halo.

Abstract

We measure the orbital properties of halo stars using 7-dimensional information provided by Gaia and the Sloan Digital Sky Survey. A metal-rich population of stars, present in both local main sequence stars and more distant blue horizontal branch stars, have very radial orbits (eccentricity ~ 0.9) and apocenters that coincide with the stellar halo "break radius" at galactocentric distance r ~ 20 kpc. Previous work has shown that the stellar halo density falls off much more rapidly beyond this break radius. We argue that the correspondence between the apocenters of high metallicity, high eccentricity stars and the broken density profile is caused by the build-up of stars at the apocenter of a common dwarf progenitor. Although the radially biased stars are likely present down to metallicities of [Fe/H] ~ -2 the increasing dominance at higher metallicities suggests a massive dwarf progenitor, which is at least as massive as the Fornax and Sagittarius dwarf galaxies, and is likely the dominant progenitor of the inner stellar halo.