Rotational signature of the Milky Way stellar halo

Abstract

We measure the rotation of the Milky Way stellar halo on two samples of Blue Horizontal Branch (BHB) field halo stars from the Sloan Digital Sky Survey (SDSS) with four different methods. The two samples comprise 1582 and 2563 stars respectively and reach out to ~50 kpc in galactocentric distance. Two of the methods to measure rotation rely exclusively on line-of-sight velocities, namely the popular double power-law model and a direct estimate of the de-projected l.o.s. velocity. The other two techniques use the full 3D motions: the radial velocity based rotation estimator of Schönrich, Binney & Asplund (2012) and a simple 3D azimuthal velocity mean. In this context we a) critique the popular model and b) assess the reliability of the estimators. All four methods agree on a weakly prograde or non-rotating halo. Further, we observe no duality in the rotation of sub-samples with different metallicities or at different radii. We trace the rotation gradient across metallicity measured by Deason et al. (2011) on a similar sample of BHB stars back to the inclusion of regions in the apparent magnitude-surface gravity plane known to be contaminated. In the spectroscopically selected sample of Xue et al. (2011), we flag ~500 hot metal-poor stars for their peculiar kinematics w.r.t. to both their cooler metal-poor counter-parts and to the metal-rich stars in the same sample. They show a seemingly retrograde behaviour in line-of-sight velocities, which is not confirmed by the 3D estimators. Their anomalous vertical motion hints at either a pipeline problem or a stream-like component rather than a smooth retrograde population.

accepted for publication in MNRAS 20 pages, 14 figures

References in corpus (13)

• The Ninth Data Release of the Sloan Digital Sky Survey: First Spectroscopic Data from the SDSS-III Baryon Oscillation Spectroscopic Survey
• The Radial Velocity Experiment (RAVE): first data release
• The Milky Way's Circular Velocity Curve to 60 kpc and an Estimate of the Dark Matter Halo Mass from Kinematics of ~2400 SDSS Blue Horizontal Branch Stars
• The RAVE Survey: Constraining the Local Galactic Escape Speed
• Two Stellar Components in the Halo of the Milky Way
• The SEGUE Stellar Parameter Pipeline. I. Description and Initial Validation Tests
• The accretion origin of the Milky Way's stellar halo
• The SEGUE Stellar Parameter Pipeline. II. Validation with Galactic Globular and Open Clusters
• Galactic Rotation and Solar Motion from Stellar Kinematics
• Kinematic Structure in the Galactic Halo at the North Galactic Pole: RR Lyrae and BHB Stars show different kinematics
• Metallicity Bias in the Kinematics of the Milky Way Stellar Halo
• Very Metal-Poor Outer-Halo Stars with Round Orbits
• A new calibration for the Blue Horizontal Branch

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