The difference in phase space between gas and dark matter also prevents the accretion of tidal gas onto any dark matter sub-halos that may be present. It does not suffice for a tidal tail to intersect the location of a sub-halo in coordinate space, they must also dock in velocity space. The gas is moving at the characteristic velocity of the entire system (typically ∼ 200 km s−1), which, by definition, exceeds the escape speed of typical sub-halos (usually < 100 km s−1). Therefore, the odds of capture are effectively zero unless the tail and sub-halo happen to be on very nearly the same orbit initially, which is itself very unlikely because of the initial difference in their phase space distribution.