Stellar Population Variations in the Milky Way's Stellar Halo

TL;DR

This study maps the ratio of blue horizontal branch to main-sequence turn-off stars in the Milky Way's stellar halo, revealing significant population variations that support models of halo formation through dwarf galaxy disruption.

Contribution

It introduces an improved method for selecting BHB stars using ugr colors and provides the first large-scale map of BHB/MSTO ratios across the Milky Way's halo, highlighting population differences.

Findings

Large variations in BHB/MSTO ratio across the halo

Distinctive ratios in stream-like structures and other features

Population gradients within the Sagittarius dwarf galaxy

Abstract

If the stellar halos of disk galaxies are built up from the disruption of dwarf galaxies, models predict highly structured variations in the stellar populations within these halos. We test this prediction by studying the ratio of blue horizontal branch stars (BHB stars; more abundant in old, metal-poor populations) to main-sequence turn-off stars (MSTO stars; a feature of all populations) in the stellar halo of the Milky Way using data from the Sloan Digital Sky Survey. We develop and apply an improved technique to select BHB stars using ugr color information alone, yielding a sample of ~9000 g<18 candidates where ~70% of them are BHB stars. We map the BHB/MSTO ratio across ~1/4 of the sky at the distance resolution permitted by the absolute magnitude distribution of MSTO stars. We find large variations of BHB/MSTO star ratio in the stellar halo. Previously identified, stream-like halo structures have distinctive BHB/MSTO ratios, indicating different ages/metallicities. Some halo features, e.g., the low-latitude structure, appear to be almost completely devoid of BHB stars, whereas other structures appear to be rich in BHB stars. The Sagittarius tidal stream shows an apparent variation in BHB/MSTO ratio along its extent, which we interpret in terms of population gradients within the progenitor dwarf galaxy. Our detection of coherent stellar population variations between different stellar halo substructures provides yet more support to cosmologically motivated models for stellar halo growth.