ON THE SHOULDERS OF GIANTS: PROPERTIES OF THE STELLAR HALO AND THE MILKY WAY MASS DISTRIBUTION

Abstract

Halo stars orbit within the potential of the Milky Way, and hence their kinematics can be used to understand the underlying mass distribution. However, the inferred mass distribution depends sensitively on assumptions made on the density and the velocity anisotropy profiles of the tracer population. Also, there is a degeneracy between the parameters of the halo and those of the disk or bulge. Most previous attempts that use halo stars have made arbitrary assumptions about these. In this paper, we decompose the Galaxy into three major components—a bulge, a Miyamoto–Nagai disk, and a Navarro–Frenk–White dark matter halo – and then model the kinematic data of the halo blue horizontal branch and K-giant stars from the Sloan Extension for Galactic Understanding and Exploration. Additionally, we use the gas terminal velocity curve and the Sgr A* proper motion. With the distance of the Sun from the center of the Galaxy R☉ = 8.5 kpc, our kinematic analysis reveals that the density of the stellar halo has a break at and an exponential cutoff in the outer parts starting at . Also, we find that the tracer velocity anisotropy is radially biased with βs = 0.4 ± 0.2 in the outer halo. We measure halo virial mass Mvir to be , concentration c to be , disk mass to be , disk scale length to be , and bulge mass to be . The halo mass is found to be small, and this has important consequences. The giant stars reveal that the outermost halo stars have low velocity dispersion, but interestingly this suggests a truncation of the stellar halo density rather than a small overall mass of the Galaxy. Our estimates of local escape velocity and dark matter density ( GeV cm−3) are in good agreement with recent estimates. Some of the above estimates, in particular Mvir, are dependent on the adopted value of R☉ and also on the choice of the outer power-law index of the tracer number density.