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