THE CASE FOR THE DUAL HALO OF THE MILKY WAY

By Timothy C. Beers, Daniela Carollo, Željko Ivezic´, D. An, M. Chiba, J. Norris, Kenneth C. Freeman, Y. S. Lee, J. Munn, P. Fiorentin and 4 more. 2011

145 citations recorded by the source corpus.

Carollo et al. have recently resolved the stellar population of the Milky Way halo into at least two distinct components, an inner halo and an outer halo. This result has been criticized by Schönrich et al., who claim that the retrograde signature associated with the outer halo is due to the adoption of faulty distances. We refute this claim, and demonstrate that the Schönrich et al. photometric distances are themselves flawed because they adopted an incorrect main-sequence absolute magnitude relationship from the work of Ivezić et al. When compared to the recommended relation from Ivezić et al., which is tied to a Milky Way globular cluster distance scale and accounts for age and metallicity effects, the relation adopted by Schönrich et al. yields up to 18% shorter distances for stars near the main-sequence turnoff (TO). Use of the correct relationship yields agreement between the distances assigned by Carollo et al. and Ivezić et al. for low-metallicity dwarfs to within 6%–10%. Schönrich et al. also point out that intermediate-gravity stars (3.5 ⩽log g < 4.0) with colors redder than the TO region are likely misclassified, with which we concur. We implement a new procedure to reassign luminosity classifications for the TO stars that require it. New derivations of the rotational behavior demonstrate that the retrograde signature and high velocity dispersion of the outer-halo population remain. We summarize additional lines of evidence for a dual halo, including a test of the retrograde signature based on proper motions alone, and conclude that the preponderance of evidence strongly rejects the single-halo interpretation.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/84d2d56f004b54471bc79411a213b655ee3949aa

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Citations in the corpus

KINEMATICS OF THE STELLAR HALO AND THE MASS DISTRIBUTION OF THE MILKY WAY USING BLUE HORIZONTAL BRANCH STARS (2012, background): “Observational evidences of multi-component halo have been found in dynamical studies of SDSS calibration stars (Carollo et al. 2007, 2010; Beers et al. 2012), in rotationalbehavior in metallicity bins (Deason et al. 2011a), in kinematics of Galactic anti-center and North Galactic Pole population…”
OXYGEN ABUNDANCES IN NEARBY FGK STARS AND THE GALACTIC CHEMICAL EVOLUTION OF THE LOCAL DISK AND HALO (2013, methodology): “This result has been questioned by Schönrich et al. (2011) on the basis of a possibly biased distance determination (but see Beers et al. 2012).”
Mass and shape of the Milky Way’s dark matter halo with globular clusters from Gaia and Hubble (2018, background): “…halo component has a small net retrograde rotation of the order of Vrot ∼ −14 km/s at 20 kpc (roughly consistent with other independent estimates, e.g. Beers et al. 2012; Kafle et al. 2017; Koppelman, Helmi, & Veljanoski 2018), while the disc component is rotating at about Vrot ∼ 210 km/s at the…”
The GALAH survey: scientific motivation (2015, methodology): “For discussions on the state of the Galactic halo based on Sloan Digital Sky Survey, see e.g. Carollo et al. (2007), Carollo et al. (2010), Beers et al. (2012), Fermani & Schönrich (2013), Kafle et al. (2013) and Schönrich, Asplund & Casagrande (2014).”
The bifurcated age–metallicity relation of Milky Way globular clusters and its implications for the accretion history of the galaxy (2013, background): “There is conflicting observational evidence in the literature as to whether the halo of the MW shows two distinct components (Beers et al. 2012; Schönrich, Asplund & Casagrande 2011), and if so, whether one of the components is incompatible with an accretion origin.”
Evidence for two early accretion events that built the Milky Way stellar halo (2019, background): “An accretion origin for the strongly retrograde components of the stellar halo has long been suspected (e.g., Quinn & Goodman 1986; Norris & Ryan 1989; Carollo et al. 2007; Beers et al. 2012).”
The Galaxy in Context: Structural, Kinematic, and Integrated Properties (2016, background): “Observationally, evidence for a dual halo has been put forward by Carollo et al. (2007) and Beers et al. (2012), but see Schönrich et al. (2014).”
Near-Field Cosmology with Extremely Metal-Poor Stars (2015, background): “…component, but contains substructure resulting from accretion events, superimposed on a dichotomy of inner and outer components (Hartwick 1987, Norris & Ryan 1989, Carollo et al. 2007, 2010, 2014, de Jong et al. 2010, Deason, Belokurov & Evans 2011, Beers et al. 2012, and references therein).”
Gaia FGK benchmark stars: Metallicity (2013, background): “A halo dichotomy similar to that of the disk has been the subject of discussion (Carollo et al. 2007; Schönrich et al. 2011; Beers et al. 2012), where the outer halo has a net retrograde rotation and is metal-poor, contrary tothe inner halo, which is slightly more metal-rich.”