Sub-Halo Spreading of Thin Tidal Star Streams

Dark matter sub-halos that pass near or through a thin tidal star stream locally increase its velocity dispersion. Subsequent orbital evolution further increases the velocity dispersion and stream width, lowering the surface density of a stream. The kinematic properties of streams are measured in cosmological Milky Way-like halo simulations. The distance along a stream is a proxy for the time a star has spent in the stream, although there are a range of ages at any distance. Power law fits to the velocity dispersion with angular distance for the average of the streams in the 10-60 kpc range finds sigma_theta=6 phi^{0.25} km/s, sigma_phi=8 phi^{0.39} km/s, and sigma_r=10 phi^{0.44} km/s for |phi|< 34 degrees, for stars within theta=+/-5 degrees of the stream equator. The errors of the coefficients are about 10% for these streams, with comparable systematic errors depending on exactly which streams are selected and the stream width and length fitted. The stream velocity dispersions close to the clusters generally increase with the sub-halo numbers.