Chronography of the Milky Way's Halo System with Field Blue Horizontal-Branch Stars
R. M. Santucci, T. C. Beers, V. M. Placco, D. Carollo, S. Rossi, Y. S., Lee, P. Denissenkov, J. Tumlinson, P. B. Tissera

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 gr color, <(gr)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…
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