The onset of star formation in primordial haloes
  Star formation remains an unsolved problem in astrophysics. Numerical studies
of large-scale structure simulations cannot resolve the whole process and their
approach usually assumes that only gas denser than a typical threshold can host
and form stars. We investigate the onset of cosmological star formation and
compare several very-high-resolution, three-dimensional, N-body/SPH simulations
that include non-equilibrium, atomic and molecular chemistry, star formation
prescriptions, and feedback effects. We study how primordial star formation
depends on gas density thresholds, cosmological parameters and initial set-ups.
For mean-density initial conditions, we find that standard low-density
star-formation threshold (0.2 h^2/cm3) models predict the onset of star
formation at z~25-31, depending on the adopted cosmology. In these models stars
are formed regardless of the time between the moment when the threshold is
reached and the effective runaway collapse. At high redshift, this time
interval represents a significant fraction of the Hubble time and thus this
assumption can induce large artificial off-sets to the onset of star formation.
Choosing higher density thresholds (135 h^2/cm3) allows the entire cooling
process to be followed, and the onset of star formation is then estimated to be
at redshift z~12-16. When isolated, rare, high-density peaks are considered,
the chemical evolution is much faster and the first star formation episodes
occur at z > 40, almost regardless of the choice for the density threshold.
These results could have implications for the formation redshift of the first
cosmological objects, as inferred from direct numerical simulations of
mean-density environments, and on the studies of the reionization history of
the universe.
