  Star streams in galactic halos are long, thin, unbound structures that will
be disturbed by the thousands dark matter sub-halos that are predicted to be
orbiting within the main halo. A sub-halo generally induces a localized wave in
the stream which often evolves into a "z-fold" as an initially trailing
innermost part rotates faster than an initially leading outermost part. The
folding, which becomes increasingly complex with time, leads to an apparent
velocity dispersion increase and thickening of the stream. We measure the
equivalent velocity dispersion around the local mean in the simulations,
finding that it rises to about 10 km/s after 5 Gyr and 20 km/s after 13 Gyr.
The currently available measurements of the velocity dispersion of halo star
streams range from as small as 2 km/s to slightly over 20 km/s. The streams
with velocity dispersions of 15-20 km/s are compatible with what sub-halo
heating would produce. A dynamical understanding of the low velocity dispersion
streams depends on the time since the progenitor's tidal disruption into a thin
stream. If the streams are nearly as old as their stars then sub-halos cannot
be present with the predicted numbers and masses. However, the dynamical age of
the streams can be significantly less than the stars. If the three lowest
velocity streams are assigned ages of 3 Gyr, they are in conflict with the
sub-halo heating. The main conclusion is that star stream heating is a powerful
and simple test for sub-halo structure.
