Constraints on the Assembly History of the Milky Way's Smooth, Diffuse Stellar Halo from the Metallicity-dependent, Radially Dominated Velocity Anisotropy Profiles Probed with K Giants and BHB Stars Using LAMOST, SDSS/SEGUE, and Gaia

• Published in: The Astrophysical journal Vol. 919; no. 1; pp. 66 - 92
• Main Authors: Bird, Sarah A., Xue, Xiang-Xiang, Liu, Chao, Shen, Juntai, Flynn, Chris, Yang, Chengqun, Zhao, Gang, Tian, Hai-Jun
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.09.2021; IOP Publishing
• Subjects: Anisotropy; Astrophysics; Galactic mass; Horizontal branch stars; Metallicity; Milky Way; Milky Way stellar halo; Sky surveys (astronomy)
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/abfa9e

We analyze the anisotropy profile of the Milky Way’s smooth, diffuse stellar halo using the Sloan Digital Sky Survey (SDSS) SEGUE blue horizontal branch stars and the SDSS/SEGUE and LAMOST K giants. These intrinsically luminous stars allow us to probe the halo to approximately 100 kpc from the Galactic center. Line-of-sight velocities, distances, metallicities, and proper motions are available for all stars via SDSS/SEGUE, LAMOST, and Gaia, and we use these data to construct a full 7D set consisting of positions, space motions, and metallicity. We remove substructure from our samples using integrals of motion based on the method of Xue et al. We find radially dominated kinematic profiles with nearly constant anisotropy within 20 kpc, beyond which the anisotropy profile gently declines but remains radially dominated to the farthest extents of our sample. Independent of star type or substructure removal, the anisotropy depends on metallicity, such that the orbits of the stars become less radial with decreasing metallicity. For −1.7 < [Fe/H] < −1, the smooth, diffuse halo anisotropy profile begins to decline at Galactocentric distances ∼20 kpc, from β ∼ 0.9 to 0.7 for K giants and from β ∼ 0.8 to 0.1 for blue horizontal branch stars. For [Fe/H] < −1.7, the smooth, diffuse halo anisotropy remains constant along all distances with 0.2 < β < 0.7 depending on the metallicity range probed, but independent of star type. These samples are ideal for estimating the total Galactic mass as they represent the virialized stellar halo system.