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 $\sim 3$ Mpc scales -- affects the formation of dwarf galaxies at $z \gtrsim 5$. We perform cosmological hydrodynamic simulations, including and excluding streaming velocities, in regions centered on halos with $M_{\rm vir}(z=0) \approx 10^{10}$ M$_{\odot}$; 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 $M_{\rm vir} = 2\times10^{5}$ M$_{\odot}$ and $2\times10^9$ M$_{\odot}$ 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$_{\odot}$. While star formation is globally delayed by approximately 50 Myr in the streaming relative to non-streaming 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 non-streaming 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 $z=0$.