Unexpected Dancing Partners: Tracing the Coherence between the Spin and Motion of Dark Matter Halos

Abstract

A recent study conducted using CALIFA survey data has found that the orbital motions of neighbor galaxies are coherent with the spin direction of a target galaxy on scales of many megaparsecs. We study this so-called “large-scale coherence” phenomenon using N-body cosmological simulations. We confirm a strong coherence signal within 1 Mpc h −1 of a target galaxy, reaching out to 6 Mpc h −1. We divide the simulation halos into subsamples based on mass, spin, merger history, and local halo number density for both target and neighbor halos. We find a clear dependency on the mass of the target halo only. Another key parameter is the local number density of both target and neighbor halos, with high-density regions such as clusters and groups providing the strongest coherence signals, rather than filaments or lower-density regions. However we do not find a clear dependency on halo spin or time since last major merger. The most striking result we find is that the signal can be detected up to 15 Mpc h −1 from massive halos. These results provide valuable lessons on how observational studies could more clearly detect coherence, and we discuss the implications of our results for the origins of large-scale coherence.