Chronography of the Milky Way's Halo System with Field Blue Horizontal-Branch Stars
Timothy Beers
University of Notre Dame
In a pioneering effort, Preston et al. (1991) 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 (SDSS) to produce the first ìchronographic mapî of the halo of the Galaxy. We also describe an extension of this approach to construct a map of some 130,000 BHB stars selected from SDSS on the basis of photometry alone.
We find that the mean colors of BHB stars shift with increasing distance from the Galactic center, out to ~50 kpc, commensurate with 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, which we refer to as the Ancient Chronographic Sphere (ACS), the age difference spans a mean color range of about 0.05 mag (~0.8 Gyrs). Interestingly, the ACS extends far enough to include the Solar Neighborhood, suggesting that ancient metal-poor stars should be readily detectable in the vicinity of the Sun.
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. Applications of this technique to other large photometric samples will be discussed.