New views of the distant stellar halo

TL;DR

This paper re-examines the outer stellar halo of the Milky Way using models and synthetic surveys, showing how current and future observations can identify distant stars and distinguish accretion events to better understand the galaxy's formation history.

Contribution

It demonstrates the effectiveness of current selection techniques for distant M giants and RR Lyrae, and explores how their properties can reveal the galaxy's accretion history.

Findings

Color and proper motion cuts effectively reduce dwarf contamination.

LSST will identify comparable numbers of RR Lyrae beyond 100 kpc.

Observable properties can help distinguish accreted structures.

Abstract

Currently only a small number of Milky Way (MW) stars are known to exist beyond 100 kpc from the Galactic center. Though the distribution of these stars in the outer halo is believed to be sparse, they can provide evidence of more recent accretion events than in the inner halo and help map out the MW's dark matter halo to its virial radius. We have re-examined the outermost regions of 11 existing stellar halo models with two synthetic surveys: one mimicking present-day searches for distant M giants and another mimicking RR Lyrae (RRLe) projections for LSST. Our models suggest that color and proper motion cuts currently used to select M giant candidates for follow-up successfully remove nearly all halo dwarf self-contamination and are useful for focusing observations on distant M giants, of which there are thousands to tens of thousands beyond 100 kpc in our models. We likewise expect that LSST will identify comparable numbers of RRLe at these distances. We demonstrate that several observable properties of both tracers, such as proximity of neighboring stars, proper motions, and distances (for RRLe) could help us separate different accreted structures from one another. We also discuss prospects for using ratios of M giants to RRLe as a proxy for accretion time, which in the future could provide new constraints on the recent accretion history of our Galaxy.