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In the standard model of cosmology, structure emerges out of non-rotationalflow and the angular momentum of collapsing halos is induced by tidal torques.The growth of halo angular momentum in the linear and quasi-linear phases isassociated with a shear, curl-free, flow and it is well described within thelinear framework of tidal torque theory (TTT). However, TTT is renderedirrelevant as haloes approach turn around and virialization. At that stage theflow field around halos has non-zero vorticity. Using a cosmologicalsimulation, we have examined the importance of the curl of the velocity field(vorticity) in determining halo spin, finding a strong alignment between thetwo. We have also examined the alignment of vorticity with the principle axesof the shear tensor, finding that it tends to be perpendicular to the axisalong which material is collapsing fastest (e1). This behavior is independentof halo masses and cosmic web environment. Our results agree with previousfindings on the tendency of halo spin to be perpendicular to e1, and of thespin of (simulated) halos and (observed) galaxies to be aligned with thelarge-scale structure. Our results imply that angular momentum growth proceedsin two distinct phases. In the first phase angular momentum emerges out of ashear, curl-free, potential flow, as described by TTT. In the second phase, inwhich haloes approach virialization, the angular momentum emerges out of avortical flow and halo spin becomes strongly aligned with the vorticity of theambient flow field.

Cosmic vorticity and the origin of halo spins