The tale of the tail -- disentangling the high transverse velocity stars in Gaia DR2

Abstract

Although the stellar halo accounts for just ~1% of the total stellar mass of the Milky Way, the kinematics of halo stars encode valuable information about the origins and evolution of our Galaxy. It has been shown that the high transverse velocity stars in Gaia DR2 reveal a double sequence in the Hertzsprung-Russell (HR) diagram, indicating a bifurcation in the local stellar halo within 1 kpc. We fit these stars by updating the popular Besançon/Galaxia model, incorporating the latest observational results for the stellar halo and an improved kinematic description for the thick-disk from Schönrich & Binney (2012). We are able to obtain a good match to the Gaia data and provide new constraints on the properties of the Galactic disc and stellar halo. In particular, we show that the kinematically defined thick disc contribution to this high velocity tail is ~13%. We look in greater detail using chemistry from LAMOST DR5, identifying a population of retrograde stars with thick-disc chemistry. Our thick disc kinematic model cannot account for this population and so we conclude there is likely to be a contribution from heated or accreted stars in the Solar Neighbourhood. We also investigate proposed dynamical substructures in this sample, concluding that they are probably due to resonant orbits rather than accreted populations. Finally we provide new insights on the nature of the two sequences and their relation with past accretion events and the primordial Galactic disc.

Accepted for publication in MNRAS

References in corpus (13)

• The Gaia mission
• galpy: A Python Library for Galactic Dynamics
• The Milky Way Tomography with SDSS: II. Stellar Metallicity
• Evidence for Two Early Accretion Events That Built the Milky Way Stellar Halo
• The Dual Origin of Stellar Halos
• A chronicle of galaxy mass assembly in the EAGLE simulation
• Constraining the thick disc formation scenario of the Milky Way
• Distances and parallax bias in Gaia DR2
• Disentangling the Galactic Halo with APOGEE: I. Chemical and Kinematical Investigation of Distinct Metal-Poor Populations
• The imprint of clump formation at high redshift. I. A disc alpha-abundance dichotomy
• A Sequoia in the Garden: FSR 1758 - Dwarf Galaxy or Giant Globular Cluster?
• The AMBRE project: The thick thin disk and thin thick disk of the Milky Way
• The Alpha-element knee of the Sagittarius Stream

Cited by in corpus (8)

• Evidence from the H3 Survey that the Stellar Halo is Entirely Comprised of Substructure
• Dynamically Tagged Groups of Very Metal-poor Halo Stars from the HK and Hamburg/ESO Surveys
• WISEA J041451.67-585456.7 and WISEA J181006.18-101000.5: The First Extreme T-type Subdwarfs?
• A Gaia early DR3 mock stellar catalog: Galactic prior and selection function
• The stellar velocity distribution function in the Milky Way galaxy
• The metal-poor end of the Spite plateau. II. Detailed chemical investigation
• The relative efficiencies of bars and clumps in driving disc stars to retrograde motion
• A Catalog of 531 White Dwarf Candidates in the Local Galactic Halo from Gaia Data Release 2