The complexity that the first stars brought to the Universe: Fragility
  of metal enriched gas in a radiation field
  The initial mass function (IMF) of the first (Population III) stars and
Population II (Pop II) stars is poorly known due to a lack of observations of
the period between recombination and reionization. In simulations of the
formation of the first stars, it has been shown that, due to the limited
ability of metal-free primordial gas to cool, the IMF of the first stars is a
few orders of magnitude more massive than the current IMF. The transition from
a high-mass IMF of the first stars to a lower-mass current IMF is thus
important to understand. To study the underlying physics of this transition, we
performed several simulations using the cosmological hydrodynamic adaptive mesh
refinement code Enzo for metallicities of 10^{-4}, 10^{-3}, 10^{-2}, and
10^{-1} Z_{\odot}. In our simulations we include a star formation prescription
that is derived from a metallicity dependent multi-phase ISM structure, an
external UV radiation field, and a mechanical feedback algorithm. We also
implement cosmic ray heating, photoelectric heating and gas-dust
heating/cooling, and follow the metal enrichment of the ISM. It is found that
the interplay between metallicity and UV radiation leads to the co-existence of
Pop III and Pop II star formation in non-zero metallicity (Z/Z_{\odot}
\geq10^{-2}) gas. A cold (T<100 K) and dense (\rho>10^{-22} g cm^{-3}) gas
phase is fragile to ambient UV radiation. In a metal-poor (Z/Z_{\odot}
\leq10^{-3}) gas, the cold and dense gas phase does not form in the presence of
a radiation field of F_{0}\sim10^{-5}-10^{-4} erg cm^{-2} s^{-1}. Therefore,
metallicity by itself is not a good indicator of the Pop III-Pop II transition.
Metal-rich (Z/Z_{\odot}\geq10^{-2}) gas dynamically evolves two to three orders
of magnitude faster than metal poor gas (Z/Z_{\odot}\leq10^{-3}). The
simulations including SNe show that pre-enrichment of the halo does not affect
the mixing of metals.
