Figure rotation of dark halos in CDM simulations

TL;DR

This study examines the figure rotation of dark matter halos in Lambda CDM simulations, finding that a small fraction exhibit coherent rotation over billions of years with low pattern speeds, and exploring how selection criteria affect measurements.

Contribution

The paper provides the first detailed analysis of figure rotation in dark halos from Lambda CDM simulations, including the distribution of pattern speeds and their dependence on selection criteria.

Findings

5 out of 222 halos show coherent figure rotation over 5h^{-1}Gyr

Pattern speeds follow a log-normal distribution centered at 0.25h rad/Gyr

Less than half of halos show alignment between rotation and minor axes

Abstract

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.