Constraints on the galactic inner halo assembly history from the age gradient of blue horizontal-branch stars
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2019
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We present an analysis of the relative age distribution of the Milky Way halo, based on samples of blue horizontal branch (BHB) stars obtained from the Panoramic Survey Telescope and Rapid Response System and Galaxy
Evolution Explorer photometry, as well a Sloan Digital Sky Survey spectroscopic sample. A machine-learning
approach to the selection of BHB stars is developed, using support vector classification, with which we produce
chronographic age maps of the Milky Way halo out to 40 kpc from the Galactic center. We identify a characteristic
break in the relative age profiles of our BHB samples, corresponding to a Galactocentric radius of RGC ∼ 14 kpc.
Within the break radius, we find an age gradient of −63.4 ± 8.2 Myr kpc−1
, which is significantly steeper than
obtained by previous studies that did not discern between the inner- and outer-halo regions. The gradient in the
relative age profile and the break radius signatures persist after correcting for the influence of metallicity on our
spectroscopic calibration sample. We conclude that neither are due to the previously recognized metallicity
gradient in the halo, as one passes from the inner-halo to the outer-halo region. Our results are consistent with a
dissipational formation of the inner-halo population, involving a few relatively massive progenitor satellites, such
as those proposed to account for the assembly of Gaia-Enceladus, which then merged with the inner halo of the
Milky Way.
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WHITTEN, Devin D.et al. Constraints on the galactic inner halo assembly history from the age gradient of blue horizontal-branch stars. The Astrophysical Journal, Bristol, v. 884, n. 1, e67, 2019. DOI: 10.3847/1538-4357/ab4269. Disponível em: https://iopscience.iop.org/article/10.3847/1538-4357/ab4269. Acesso em: 11 maio 2023.