Figure Rotation of Dark Halos in Cold Dark Matter Simulations

We investigate the figure rotation of dark matter halos identified in ΛCDM 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 5 h 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 1 h Gyr period follow a log-normal distribution, centred at Ωp = 0.25 h rad Gyr with a maximum value of 0.94 h rad Gyr. Over a 5 h Gyr period, the average pattern speed of a halo is about 0.1 h rad Gyr and the largest pattern speed found is 0.24 h 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.