Nihao XII: Galactic uniformity in a CDM universe

We use a sample of 83 high-resolution cosmological zoom-in simulations and a semi-analytic model (SAM) to study the stochasticity of galaxy formation in haloes ranging from dwarf to Milky Way masses. Our simulated galaxies reproduce the observed inefficiency of galaxy formation as expressed through the stellar, gas and baryonic Tully–Fisher relations. For H I velocities in the range (70 V 220 km s⁻¹), the scatter is just 0.08 to 0.14 dex, consistent with the observed intrinsic scatter at these scales. At low velocities (20 V 70 km s⁻¹), the simulated scatter is 0.2–0.25 dex, which could be tested with future observations. The scatter in the stellar mass versus dark halo velocity relation is constant for 30 V 180 km s⁻¹, and smaller (0.17 dex) when using the maximum circular velocity of the dark-matter-only simulation, V_max^DMO , compared to the virial velocity (V₂₀₀ or V₂₀₀^DMO). The scatter in stellar mass is correlated with halo concentration, and is minimized when using a circular velocity at a fixed fraction of the virial radius 0.4R₂₀₀ or with V_α = V₂₀₀^DMO(V_max^DMO /V₂₀₀^DMO)^α with α 0.7, consistent with constraints from halo clustering. Using the SAM we show the correlation between halo formation time and concentration is essential in order to reproduce this result. This uniformity in galaxy formation efficiency we see in our hydrodynamical simulations and an SAM proves the simplicity and self-regulating nature of galaxy formation in a cold dark matter (CDM) universe.