Hybrid Cosmological Simulations with Stream Velocities

Abstract

In the early universe, substantial relative "stream" velocities between the gas and dark matter arise due to radiation pressure and persist after recombination. To asses the impact of these velocities on high-redshift structure formation, we carry out a suite of high-resolution Adaptive Mesh Refinement (AMR) cosmological simulations, which use Smoothed Particle Hydrodynamic datasets as initial conditions, converted using a new tool developed for this work. These simulations resolve structures with masses as small as a few 100 M$_\odot$, and we focus on the $10^6$ M$_\odot$ "mini-halos" in which the first stars formed. At $z \approx 17,$ the presence of stream velocities has only a minor effect on the number density of halos below $10^6$ M$_\odot$, but it greatly suppresses gas accretion onto all halos and the dark matter structures around them. Stream velocities lead to significantly lower halo gas fractions, especially for $\approx 10^5$ M$_\odot$ objects, an effect that is likely to depend on the orientation of a halo's accretion lanes. This reduction in gas density leads to colder, more compact radial profiles, and it substantially delays the redshift of collapse of the largest halos, leading to delayed star formation and possibly delayed reionization. These many differences suggest that future simulations of early cosmological structure formation should include stream velocities to properly predict gas evolution, star-formation, and the epoch of reionization.