Abstract
Abstract 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 R GC ∼ 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.
Highlights
The Λ cold dark matter cosmological paradigm describes the hierarchical growth of structure in the universe, where galaxies assemble via mergers of smaller, low-mass systems (White & Rees 1978)
We provide the first evidence for a characteristic break in the relative age profile of the Milky Way stellar halo, using a sample of blue horizontalbranch (BHB) stars obtained from the Panoramic Rapid Response Survey Telescope (Pan-STARRS1) and the Galaxy Evolution Explorer (GALEX)
We develop our selection methodology using the sample of spectroscopically verified BHB and blue straggler stars (BSSs) from SDSS/Sloan Extension for Galactic Understanding and Exploration (SEGUE) described in Santucci et al (2015a), hereby referred to as the SDSS spectroscopic sample
Summary
The Λ cold dark matter cosmological paradigm describes the hierarchical growth of structure in the universe, where galaxies assemble via mergers of smaller, low-mass systems (White & Rees 1978). The continued process of hierarchical structure formation has largely been confirmed with the discovery of streams, tidal tails, over-densities, and numerous satellite galaxies of the Milky Way (Majewski et al 2003; Belokurov et al 2006; Helmi 2008; Martin et al 2014; Shipp et al 2018). The structural components of the Milky Way largely retain the signatures of their formation (Freeman & Bland-Hawthorn 2002), and present an opportunity to study the process of galaxy formation. Low-mass stars in the halo can be nearly as old as the universe, while their spatial, kinematic, age, and chemical abundance distributions reflect their origins, whether that be in situ, or in satellite galaxies accreted onto the primordial Milky Way
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