Examinando por Autor "Mackereth, J. Ted"

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    The chemical characterization of halo substructure in the Milky Way based on APOGEE
    (Oxford University Press, 2023-04) Horta, Danny; Schiavon, Ricardo P.; Mackereth, J. Ted; Weinberg, David H.; Hasselquist, Sten; Feuillet, Diane; O’Connell, Robert W.; Anguiano, Borja; Allende-Prieto, Carlos; Beaton, Rachael L.; Bizyaev, Dmitry; Cunha, Katia; Geisler, Doug; García-Hernández D.A.; Holtzman, Jon; Jönsson, Henrik; Lane, Richard R.; Majewski, Steve R.; Mészáros, Szabolcs; Minniti, Dante; Nitschelm, Christian; Shetrone, Matthew; Smith, Verne V.; Zasowski, Gail
    Galactic haloes in a Λ-CDM universe are predicted to host today a swarm of debris resulting from cannibalized dwarf galaxies. The chemodynamical information recorded in their stellar populations helps elucidate their nature, constraining the assembly history of the Galaxy. Using data from APOGEE and Gaia, we examine the chemical properties of various halo substructures, considering elements that sample various nucleosynthetic pathways. The systems studied are Heracles, Gaia-Enceladus/Sausage (GES), the Helmi stream, Sequoia, Thamnos, Aleph, LMS-1, Arjuna, I’itoi, Nyx, Icarus, and Pontus. Abundance patterns of all substructures are cross-compared in a statistically robust fashion. Our main findings include: (i) the chemical properties of most substructures studied match qualitatively those of dwarf Milky Way satellites, such as the Sagittarius dSph. Exceptions are Nyx and Aleph, which are chemically similar to disc stars, implying that these substructures were likely formed in situ; (ii) Heracles differs chemically from in situ populations such as Aurora and its inner halo counterparts in a statistically significant way. The differences suggest that the star formation rate was lower in Heracles than in the early Milky Way; (iii) the chemistry of Arjuna, LMS-1, and I’itoi is indistinguishable from that of GES, suggesting a possible common origin; (iv) all three Sequoia samples studied are qualitatively similar. However, only two of those samples present chemistry that is consistent with GES in a statistically significant fashion; (v) the abundance patterns of the Helmi stream and Thamnos are different from all other halo substructures. © 2022 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.
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    The origin of accreted stellar halo populations in the milky way using apogee, gaia, and the eagle simulations
    (Monthly Notices of the Royal Astronomical Society, 2019-01-21) Mackereth, J. Ted; Schiavon, Ricardo P.; Pfeffer, Joel; Hayes, Christian R.; Bovy, Jo; Anguiano, Borja; Prieto, Carlos Allende; Hasselquist, Sten; Holtzman, Jon; Johnson, Jennifer A.; Majewski, Steven R.; O’Connell, Robert; Shetrone, Matthew; Tissera, Patricia B.; Fernandez-Trincado, J. G.
    Kinematics of halo stars. We show that ∼2/3 of nearby halo stars have high orbital eccentricities (e 0.8), and abundance patterns typical of massive Milky Way dwarf galaxy satellites today, characterized by relatively low [Fe/H], [Mg/Fe], [Al/Fe], and [Ni/Fe]. The trend followed by high-e stars in the [Mg/Fe]-[Fe/H] plane shows a change of slope at [Fe/H] ∼ -1.3, which is also typical of stellar populations from relatively massive dwarf galaxies. Low-e stars exhibit no such change of slope within the observed [Fe/H] range and show slightly higher abundances of Mg, Al, and Ni. Unlike their low-e counterparts, high-e stars show slightly retrograde motion, make higher vertical excursions, and reach larger apocentre radii. By comparing the position in [Mg/Fe]-[Fe/H] space of high-e stars with those of accreted galaxies from the EAGLE suite of cosmological simulations, we constrain the mass of the accreted satellite to be in the range 108.5≲ M ≲ 109M⊙ We show that the median orbital eccentricities of debris are largely unchanged since merger time, implying that this accretion event likely happened at z≲1.5. The exact nature of the low-e population is unclear, but we hypothesize that it is a combination of in situ star formation, high-|z| disc stars, lower mass accretion events, and contamination by the low-e tail of the high-e population. Finally, our results imply that the accretion history of the Milky Way was quite unusual.