The origin of spin in galaxies: Clues from simulations of atomic cooling haloes

Abstract

In order to elucidate the origin of spin in both dark matter and baryons in galaxies, we have performed hydrodynamical simulations from cosmological initial conditions. We study atomic cooling haloes in the redshift range 100 > z > 9 with masses of the order of 109M⊙ at redshift z = 10. We assume that the gas has primordial composition and that H2-cooling and prior star formation in the haloes have been suppressed. We present a comprehensive analysis of the gas and dark matter properties of four haloes with very low (λ ≈ 0.01), low (λ ≈ 0.04), high (λ ≈ 0.06) and very high (λ ≈ 0.1) spin parameter. Our main conclusion is that the spin orientation and magnitude is initially well described by tidal torque linear theory, but later on is determined by the merging and accretion history of each halo. We provide evidence that the topology of the merging region, i.e. the number of colliding filaments, gives an accurate prediction for the spin of dark matter and gas: haloes at the centre of knots will have low spin while those in the centre of filaments will have high spin. The spin of a halo is given by λ ≈ 0.05 × (7.6/number of filaments)5.1.