  We investigate the figure rotation of dark matter halos identified in Lambda
CDM simulations. We find that when strict criteria are used to select suitable
halos for study, 5 of the 222 halos identified in our z=0 simulation output
undergo coherent figure rotation over a 5h^{-1}Gyr period. We discuss the
effects of varying the selection criteria and find that pattern speeds for a
much larger fraction of the halos can be measured when the criteria are
relaxed. Pattern speeds measured over a 1h^{-1}Gyr period follow a log-normal
distribution, centred at Omega_p = 0.25h rad/Gyr with a maximum value of 0.94h
rad/Gyr. Over a 5h^{-1}Gyr period, the average pattern speed of a halo is about
0.1h rad/Gyr and the largest pattern speed found is 0.24h rad/Gyr. Less than
half of the selected halos showed alignment between their figure rotation axis
and minor axis, the exact fraction being somewhat dependent on how one defines
a halo. While the pattern speeds observed are lower than those generally
thought capable of causing spiral structure, we note that coherent figure
rotation is found over very long periods and argue that further simulations
would be required before strong conclusions about spiral structure in all
galaxies could be drawn. We find no correlation between halo properties such as
total mass and the pattern speed.

Astrophysics