Touching The Void: A Striking Drop in Stellar Halo Density Beyond 50 kpc
  We use A-type stars selected from Sloan Digital Sky Survey data release 9
photometry to measure the outer slope of the Milky Way stellar halo density
profile beyond 50 kpc. A likelihood-based analysis is employed that models the
ugr photometry distribution of blue horizontal branch (BHB) and blue straggler
(BS) stars. In the magnitude range, 18.5 < g < 20.5, these stellar populations
span a heliocentric distance range of: 10 kpc < D_BS < 75 kpc, 40 kpc < D_BHB <
100 kpc. Contributions from contaminants, such as QSOs, and the effect of
photometric uncertainties, are also included in our modeling procedure. We find
evidence for a very steep outer halo profile, with power-law index alpha ~ 6
beyond Galactocentric radii r=50 kpc, and even steeper slopes favored (alpha ~
6-10) at larger radii. This result holds true when stars belonging to known
overdensities, such as the Sagittarius stream, are included or excluded. We
show that, by comparison to numerical simulations, stellar halos with shallower
slopes at large distances tend to have more recent accretion activity. Thus, it
is likely that the Milky Way has undergone a relatively quiet accretion history
over the past several Gyr. Our measurement of the outer stellar halo profile
may have important implications for dynamical mass models of the Milky Way,
where the tracer density profile is strongly degenerate with total
mass-estimates.
