Spin flips I: Evolution of the angular momentum orientation of Milky
  Way-mass dark matter haloes
  During the growth of a cold dark matter halo, the direction of its spin can
undergo rapid changes. These could disrupt or even destroy a stellar disc
forming in the halo, possibly resulting in the generation of a bulge or
spheroid. We investigate the frequency of significant changes in the
orientation of the angular momentum vector of dark matter haloes ("spin
flips"), and their degree of correlation with mergers. We focus on haloes of
mass similar to that of the Milky Way (MW) halo at redshift z=0 (log M/(Msol/h)
= 12.0 to 12.5) and consider flips in the spin of the whole halo or just its
inner parts. We find that a greater fraction of major mergers are associated
with large spin flips than minor mergers. However, since major mergers are
rare, the vast majority (93%) of large whole-halo spin flips (angle change >=
45 degrees) coincide with small mass changes, not major mergers. The spin
vector of the inner halo experiences much more frequent flips than the halo as
a whole. Over their entire lifetimes (i.e. after a halo acquires half of its
final mass), over 10% of halos experience a flip of at least 45 degrees in the
spin of the entire halo and nearly 60 percent experience a flip this large in
the inner halo. These numbers are reduced to 9 percent for the whole halo and
47 percent for the inner halo when we consider only haloes with no major
mergers after formation. Our analysis suggests that spin flips (whose effects
are not currently included in galaxy formation models) could be an important
factor in the morphological transformation of disc galaxies.
