Two Substructures in the nearby Stellar Halo Found in Gaia and RAVE

Abstract

We use the second Gaia data release (Gaia DR2), combined with Radial Velocity Experiment spectroscopic surveys, to identify the substructures in the nearby stellar halo. We select 3845 halo stars kinematically and chemically and determine their density distribution in energy and angular momentum space. To select the substructures from overdensities, we reshuffle the velocities and estimate their significance. Two statistically significant substructures, GR-1 and GR-2, are identified. GR-1 has a high binding energy and small z -angular momentum. GR-2 is metal-rich but retrograde. They are both new substructures, and may be accretion debris of dwarf galaxies.