Star Stream Folding by Dark Galactic Subhalos

Star streams in galactic halos are long, thin, unbound structures that will be disturbed by the thousands of dark matter subhalos that are predicted to be orbiting within the main halo. A subhalo 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^-1 after 5 Gyr and 20 km s^-1 after 13 Gyr. The currently available measurements of the velocity dispersion of halo star streams range from as small as 2 km s^-1 to slightly over 20 km s^-1. The streams with velocity dispersions of 15-20 km s^-1 are compatible with what subhalo 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 subhalos 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 subhalo heating. The main conclusion is that star stream heating is a powerful and simple test for subhalo structure.