Dwarf Galaxy Formation with and without Dark Matter–Baryon Streaming Velocities

Abstract

We study how supersonic streaming velocities of baryons relative to dark matter—a large-scale effect imprinted at recombination and coherent over ∼3 Mpc scales—affect the formation of dwarf galaxies at z ≳ 5. We perform cosmological hydrodynamic simulations, including and excluding streaming velocities, in regions centered on halos with M vir ( z = 0) ≈ 10 10 M ⊙ ; the simulations are part of the Feedback In Realistic Environments (FIRE) project and run with FIRE-3 physics. Our simulations comprise many thousands of systems with halo masses between M vir = 2 × 10 5 M ⊙ and 2 × 10 9 M ⊙ in the redshift range z = 20–5. A few hundred of these galaxies form stars and have stellar masses ranging from 100 to 10 7 M ⊙ . While star formation is globally delayed by approximately 50 Myr in the streaming relative to nonstreaming simulations and the number of luminous galaxies is correspondingly suppressed at high redshift in the streaming runs, these effects decay with time. By z = 5, the properties of the simulated galaxies are nearly identical in the streaming versus nonstreaming runs, indicating that any effects of streaming velocities on the properties of galaxies at the mass scale of classical dwarfs and larger do not persist to z = 0.