Hybrid Cosmological Simulations with Stream Velocities
  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.
