Differential rotation of the halo traced by the K-giant stars

Abstract

We use K-giant stars selected from the LAMOST DR5 to study the variation of the rotational velocity of the galactic halo at different space positions. Modelling the rotational velocity distribution with both the halo and disk components, we find that the rotational velocity of the halo population decreases almost linearly with increasing vertical distance to the galactic disk plane, $Z$, at fixed galactocentric radius, $R$. The samples are separated into two parts with $6<R<12$ kpc and $12<R<20$ kpc. We derive that the decreasing rates along $Z$ for the two subsamples are $-3.07\pm 0.63$ and $-1.89\pm 0.37$ km s${}^{-1}$ kpc${}^{-1}$, respectively. Compared with the TNG simulations, we suggest that this trend is probably caused by the interaction between the disk and halo. The results from the simulations show that only the oblate halo can provide a decreasing rotational velocity with an increasing $Z$. This indicates that the Galactic halo is oblate with galactocentric radius $R<20$ kpc. On the other hand, the flaring of the disk component (mainly the thick disk) is clearly traced by this study, with $R$ between 12 and 20 kpc, the disk can vertically extend to $6\sim 10$ kpc above the disk plane. What is more interesting is that, we find the Gaia-Enceladus-Sausage (GES) component has a significant contribution only in the halo with $R<12$ kpc, i.e. a fraction of 23$-$47\%. While in the outer subsample, the contribution is too low to be well constrained.

Accepted for publication in ApJ, 18 pages, 12 figures, 1 table

References in corpus (18)

• The Gaia mission
• galpy: A Python Library for Galactic Dynamics
• Simulating galaxy formation with black hole driven thermal and kinetic feedback
• 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
• Two Stellar Components in the Halo of the Milky Way
• Evidence for Two Early Accretion Events That Built the Milky Way Stellar Halo
• Dynamical modelling of the Galactic bulge and bar: the Milky Way's bar pattern speed, stellar, and dark matter mass distribution
• Detection of a 63 Degree Cold Stellar Stream in the Sloan Digital Sky Survey
• Rings and Radial Waves in the Disk of the Milky Way
• The role of mergers and halo spin in shaping galaxy morphology
• A 22 Degree Tidal Tail for Palomar 5
• A Synoptic Map of Halo Substructures from the Pan-STARRS1 3π Survey
• The K giant stars from the LAMOST survey data I: identification, metallicity, and distance
• Predicted Extension of the Sagittarius Stream to the Milky Way Virial Radius
• Action-space clustering of tidal streams to infer the Galactic potential
• Deep SDSS optical spectroscopy of distant halo stars II. Iron, calcium, and magnesium abundances
• Estimation of distances to stars with stellar parameters from LAMOST
• Mapping the Milky Way with LAMOST II: the stellar halo