  Using the Sloan Digital Sky Survey (SDSS), we probe the halo mass
distribution by studying the velocities of satellites orbiting isolated
galaxies. In a subsample that covers 2500 sq. degrees on the sky, we detect
about 3000 satellites with absolute blue magnitudes going down to M_B = -14;
most of the satellites have M_B=-16 to -18, comparable to the magnitudes of M32
and the Magellanic Clouds. After a careful, model-independent removal of
interlopers, we find that the line-of-sight velocity dispersion of satellites
declines with distance to the primary. For an L* galaxy the r.m.s.
line-of-sight velocity changes from ~120 km/s at 20 kpc to ~60 km/s at 350 kpc.
This decline agrees remarkably well with theoretical expectations, as all
modern cosmological models predict that the density of dark matter in the
peripheral parts of galaxies declines as rho_DM propto r^{-3}. Thus, for the
first time we find direct observational evidence of the density decline
predicted by cosmological models; we also note that this result contradicts
alternative theories of gravity such as MOND. We also find that the velocity
dispersion of satellites within 100 kpc scales with the absolute magnitude of
the central galaxy as sigma propto L^{0.3}; this is very close to the
Tully-Fisher relation for normal spiral galaxies.
