Frequent Spin Reorientation of Galaxies due to Local Interactions

Abstract

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.Comment: 13 pages, 4 figures, ApJ Letters in press, comments always welcom