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 _ { 10 } M / h ^ { -1 } \mathrm { M _ { \sun } } = 12.0 \rightarrow 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 ( \theta \geq 45 \degr ) 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 \degr 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 .