Galactic accretion and angular momentum re-orientation

The generation of galactic angular momentum by tidal forces is re-examined. We calculate the tidal torques in linear theory on a spherical shell centred either on a random point or on a peak in the smoothed density field. We show that when the torque is decomposed into contributions from multipoles, the dipole term is more important than the quadrupole, and higher multipoles are unimportant. When the dipole contribution is taken into account, the torques of thin shells of radii r and rʹ are anticorrelated for |r/r^\,\,2|⁠. Significant contributions to the torque on any shell come from shells that are more than five times as big. The magnitude of the torque on a shell does not depend greatly on whether the shell is centred on a peak in the smoothed density field rather than on a random point. The dimensionless angular momentum parameter λ of virialized objects remains fairly constant at |λ\,\,0.05| as the object grows as a result of cosmic infall.