Spin Flips - II. Evolution of dark matter halo spin orientation, and its correlation with major mergers

TL;DR

This study investigates how dark matter halo spin orientations change over time, revealing that many significant spin flips occur independently of major mergers, which may influence galaxy evolution.

Contribution

It extends previous research by analyzing the full mass range of haloes and quantifies the frequency of spin flips unrelated to major mergers.

Findings

35% of haloes experience significant spin flips without major mergers.

Over 75% of large spin flips coincide with non-major mergers.

Inner-halo spin orientation changes are more likely to flip.

Abstract

We expand our previous study on the relationship between changes in the orientation of the angular momentum vector of dark matter haloes ("spin flips") and changes in their mass (Bett & Frenk 2012), to cover the full range of halo masses in a simulation cube of length 100 $h^{-1}$ Mpc. Since strong disturbances to a halo (such as might be indicated by a large change in the spin direction) are likely also to disturb the galaxy evolving within, spin flips could be a mechanism for galaxy morphological transformation without involving major mergers. We find that 35% of haloes have, at some point in their lifetimes, had a spin flip of at least $45\deg$ that does not coincide with a major merger. Over 75% of large spin flips coincide with non-major mergers; only a quarter coincide with major mergers. We find a similar picture for changes to the inner-halo spin orientation, although here there is an increased likelihood of a flip occurring. Changes in halo angular momentum orientation, and other such measures of halo perturbation, are therefore very important quantities to consider, in addition to halo mergers, when modelling the formation and evolution of galaxies and confronting such models with observations.