The origin of spin in galaxies: clues from simulations of atomic cooling halos

TL;DR

This study uses hydrodynamical simulations of atomic cooling halos to explore how galaxy spins originate, showing that initial conditions and merging history, especially filament topology, determine spin magnitude and orientation.

Contribution

It demonstrates that halo spin is initially set by tidal torque theory and later shaped by merging and filamentary accretion, with a predictive relation based on filament number.

Findings

Spin aligns with tidal torque predictions initially.

Halo spin correlates with the number of filaments.

Filament topology predicts halo spin magnitude.

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 order $10^9{\rm M_{\odot}}$ at redshift $z=10$. We assume that the gas has primordial composition and that ${\rm H_2}$-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 halos with very low ($\lambda \approx 0.01$), low ($\lambda \approx 0.04$), high ($\lambda \approx 0.06$) and very high ($\lambda \approx 0.1$) 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: halos at the center of knots will have low spin while those in the center of filaments will have high spin. The spin of a halo is given by $\lambda \approx 0.05 \times \left(\frac{7.6}{\rm number\,\,\, of \,\,\, filaments}\right)^{5.1}$.