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
), which, by definition, exceeds the escape speed of typical sub-halos (usually
). 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.