197 research outputs found
The Milky Way's stellar halo - lumpy or triaxial?
We present minimum chi-squared fits of power law and Hernquist density
profiles to F-turnoff stars in eight 2.5 deg wide stripes of SDSS data: five in
the North Galactic Cap and three in the South Galactic cap. Portions of the
stellar Galactic halo that are known to contain large streams of tidal debris
or other lumpy structure, or that may include significant contamination from
the thick disk, are avoided. The data strongly favor a model that is not
symmetric about the Galaxy's axis of rotation. If included as a free parameter,
the best fit to the center of the spheroid is surprisingly approx 3 kpc from
the Galactic center in the direction of the Sun's motion. The model fits favor
a low value of the density of halo stars at the solar position. The alternative
to a non-axisymmetric stellar distribution is that our fits are contaminated by
previously unidentified lumpy substructure.Comment: 10 pages, 10 figs, to appear in proceedings of conference "Physics at
the end of the Galactic Cosmic Ray Spectrum", Journal of Physics: Conf.
series, eds. G. Thomson and P. Sokolsk
A Swing of the Pendulum: The Chemodynamics of the Local Stellar Halo Indicate Contributions from Several Radial Merger Events
We find that the chemical abundances and dynamics of APOGEE and GALAH stars
in the local stellar halo are inconsistent with a scenario in which the inner
halo is primarily composed of debris from a single, massive, ancient merger
event, as has been proposed to explain the Gaia-Enceladus/Gaia Sausage (GSE)
structure. The data contains trends of chemical composition with energy which
are opposite to expectations for a single massive, ancient merger event, and
multiple chemical evolution paths with distinct dynamics are present. We use a
Bayesian Gaussian mixture model regression algorithm to characterize the local
stellar halo, and find that the data is best fit by a model with four
components. We interpret these components as the VRM, Cronus, Nereus, and
Thamnos; however, Nereus and Thamnos likely represent more than one accretion
event because the chemical abundance distributions of their member stars
contain many peaks. Although the Cronus and Thamnos components have different
dynamics, their chemical abundances suggest they may be related. We show that
the distinct low and high alpha halo populations from Nissen & Schuster (2010)
are explained by VRM and Cronus stars, as well as some in-situ stars. Because
the local stellar halo contains multiple substructures, different popular
methods of selecting GSE stars will actually select different mixtures of these
substructures, which may change the apparent chemodynamic properties of the
selected stars. We also find that the Splash stars in the solar region are
shifted to higher v_phi and slightly lower [Fe/H] than previously reported
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