Frequent Spin Reorientation of Galaxies due to Local Interactions
  We study the evolution of angular momenta of ($M_*=10^{10}-10^{12}\msun$)
galaxies utilizing large-scale ultra-high resolution cosmological hydrodynamic
simulations and find that spin of the stellar component changes direction
frequently, caused by major mergers, minor mergers, significant gas inflows and
torques by nearby systems. The rate and nature of change of spin direction can
not be accounted for by large-scale tidal torques, because the latter fall
short in rates by orders of magnitude and because the apparent random swings of
the spin direction are inconsistent with alignment by linear density field. The
implications for galaxy formation as well as intrinsic alignment of galaxies
are profound. Assuming the large-scale tidal field is the sole alignment agent,
a new picture emerging is that intrinsic alignment of galaxies would be a
balance between slow large-scale coherent torquing and fast spin reorientation
by local interactions. What is still open is whether other processes, such as
feeding galaxies with gas and stars along filaments or sheets, introduce
coherence for spin directions of galaxies along the respective structures.
