The cold veil of the Milky Way stellar halo
  We build a sample of distant (D > 80 kpc) stellar halo stars with measured
radial velocities. Faint ($20 < g <22$) candidate blue horizontal branch (BHB)
stars were selected using the deep, but wide, multi-epoch Sloan Digital Sky
Survey photometry. Follow-up spectroscopy for these A-type stars was performed
using the VLT-FORS2 instrument. We classify stars according to their Balmer
line profiles, and find 7 are bona fide BHB stars and 31 are blue stragglers
(BS). Owing to the magnitude range of our sample, even the intrinsically
fainter BS stars can reach out to D ~ 90 kpc. We complement this sample of
A-type stars with intrinsically brighter, intermediate-age, asymptotic giant
branch stars. A set of 4 distant cool carbon stars is compiled from the
literature and we perform spectroscopic follow-up on a further 4 N-type carbon
stars using the WHT-ISIS instrument. Altogether, this provides us with the
largest sample to date of individual star tracers out to r ~ 150 kpc. We find
that the radial velocity dispersion of these tracers falls rapidly at large
distances and is surprisingly cold (sigma_r ~ 50-60 km/s) between 100-150 kpc.
Relating the measured radial velocities to the mass of the Milky Way requires
knowledge of the (unknown) tracer density profile and anisotropy at these
distances. Nonetheless, by assuming the stellar halo stars between 50-150 kpc
have a moderate density fall-off (with power-law slope alpha < 5) and are on
radial orbits (sigma^2_t/sigma^2_r < 1), we infer that the mass within 150 kpc
is less than 10^12 M_solar and suggest it probably lies in the range (5-10) x
10^11 M_solar. We discuss the implications of such a low mass for the Milky
Way.
