Unmixing the Galactic Halo with RR Lyrae tagging

TL;DR

This paper uses RR Lyrae stars' period-amplitude tagging to analyze the formation history of the Galactic stellar halo, revealing correlations with halo structure, accretion events, and stellar streams.

Contribution

It introduces a novel method of classifying RR Lyrae stars to infer the assembly history of the Galactic halo, supported by simulations and stream analysis.

Findings

RR Lyrae class fractions vary with Galactocentric radius.

At least three accretion events shaped the halo's RRab composition.

RRab class fractions in streams match halo-wide trends.

Abstract

We show that tagging RR Lyrae stars according to their location in the period-amplitude diagram can be used to shed light on the genesis of the Galactic stellar halo. The mixture of RR Lyrae of ab type, separated into classes along the lines suggested by Oosterhoff, displays a strong and coherent evolution with Galactocentric radius. The change in the RR Lyrae composition appears to coincide with the break in the halo's radial density profile at ~25 kpc. Using simple models of the stellar halo, we establish that at least three different types of accretion events are necessary to explain the observed RRab behavior. Given that there exists a correlation between the RRab class fraction and the total stellar content of a dwarf satellite, we hypothesize that the field halo RRab composition is controlled by the mass of the progenitor contributing the bulk of the stellar debris at the given radius. This idea is tested against a suite of cosmological zoom-in simulations of Milky Way-like stellar halo formation. Finally, we study some of the most prominent stellar streams in the Milky Way halo and demonstrate that their RRab class fractions follow the trends established previously.