Chronography of the Milky Way's Halo System with Field Blue Horizontal-Branch Stars

TL;DR

This study creates a detailed age map of the Milky Way's halo using blue horizontal-branch stars, revealing an age gradient and identifying substructures like streams and overdensities.

Contribution

First to produce a comprehensive chronographic map of the Galaxy's halo using BHB stars, linking color gradients to stellar ages and substructure identification.

Findings

Color gradient indicates an age difference of 2-2.5 Gyrs across the halo.

Oldest stars are concentrated within 15 kpc of the Galactic center.

Substructures like Sagittarius Stream are distinguishable by BHB color contrasts.

Abstract

In a pioneering effort, Preston et al. reported that the colors of blue horizontal-branch (BHB) stars in the halo of the Galaxy shift with distance, from regions near the Galactic center to about 12 kpc away, and interpreted this as a correlated variation in the ages of halo stars, from older to younger, spanning a range of a few Gyrs. We have applied this approach to a sample of some 4700 spectroscopically confirmed BHB stars selected from the Sloan Digital Sky Survey to produce the first "chronographic map" of the halo of the Galaxy. We demonstrate that the mean de-reddened g$-$r color, <(g$-$r)o>, increases outward in the Galaxy from $-$0.22 to $-$0.08 (over a color window spanning [$-$0.3:0.0]) from regions close to the Galactic center to ~40 kpc, independent of the metallicity of the stars. Models of the expected shift in the color of the field BHB stars based on modern stellar evolutionary codes confirm that this color gradient can be associated with an age difference of roughly 2-2.5 Gyrs, with the oldest stars concentrated in the central ~15 kpc of the Galaxy. Within this central region, the age difference spans a mean color range of about 0.05 mag (~0.8 Gyrs). Furthermore, we show that chronographic maps can be used to identify individual substructures, such as the Sagittarius Stream, and overdensities in the direction of Virgo and Monoceros, based on the observed contrast in their mean BHB colors with respect to the foreground/background field population.