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 \lesssim D _ { BS } / \mathrm { kpc } \lesssim 75 , 40 \lesssim D _ { BHB } / \mathrm { kpc } \lesssim 100 .
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 \sim 6 beyond Galactocentric radii r = 50 kpc , and even steeper slopes favored ( \alpha \sim 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 .