Solving the puzzle of subhalo spins

TL;DR

This paper studies the distribution of subhalo spins in cosmological simulations, confirming that subhalo spins are lower than field haloes and decrease with mass, proximity to the host, and redshift, due to tidal stripping effects.

Contribution

It provides the first large-volume simulation analysis confirming the lower and evolving spin distribution of subhaloes compared to field haloes, highlighting tidal stripping as a key factor.

Findings

Subhalo spins are lower than those of field haloes.

Subhalo spin decreases with decreasing mass and proximity to the host center.

Subhalo spin is smaller at lower redshifts, indicating evolution over time.

Abstract

Investigating the spin parameter distribution of subhaloes in two high resolution isolated halo simulations, re- cent work by Onions et al. suggested that typical subhalo spins are consistently lower than the spin distribution found for field haloes. To further examine this puzzle, we have analyzed simulations of a cosmological volume with sufficient resolution to resolve a significant subhalo population. We confirm the result of Onions et al. and show that the typical spin of a subhalo decreases with decreasing mass and increasing proximity to the host halo center. We interpret this as the growing influence of tidal stripping in removing the outer layers, and hence the higher angular momentum particles, of the subhaloes as they move within the host potential. Investigating the redshift dependence of this effect, we find that the typical subhalo spin is smaller with decreasing redshift. This indicates a temporal evolution as expected in the tidal stripping scenario.