20) Galactic Mass and Anisotropy Profile with Halo K-Giant and Blue Horizontal Branch Stars from LAMOST/SDSS and Gaia

Abstract: A major uncertainty in the determination of the mass profile of the Milky Way using stellar kinematics is the poorly determined anisotropy parameter, beta = 1 - ( ( (sigma_theta)**2 + (sigma_phi)**2 ) / ( 2 * (sigma_r)**2 ), where sigma_r is the Galactocentric radial velocity dispersion, and sigma_theta and sigma_phi are the tangential components of the velocity dispersion. We use a sample of over 22,000 Galactic halo K giant and blue horizontal branch stars from the ground-based LAMOST and SDSS/SEGUE stellar spectroscopic surveys, combined with proper motions from the space telescope Gaia, to measure beta(r) over a wide range of Galactocentric distances r from 4 to 100 kpc. We find beta is positive along all radii, indicating that the orbits of the halo stars are predominately radial. This is in agreement with simulations where the satellite(s) depositing a large majority of halo stars are also on radial orbits. We investigate the extent to which a much improved knowledge of beta influences the determination of the Galactic mass profile M(r) as a function of r.

Bio: Dr. Sarah A. Bird received her Ph.D. from the University of Turku in Finland in 2014, upon which she began as a LAMOST Fellow at the Shanghai Astronomical Observatory and later continued as a PIFI Postdoctoral Fellow. In 2018 she moved to NAOC in Beijing with the Aliyun Fellowship. Her research focuses on the stellar halos of the Milky Way and other galaxies which she investigates using both observations and simulations.