Simulating extremely metal-poor gas and DLA metal content at redshift
  z=7
  We present the first theoretical study of metals in damped Ly-alpha (DLA)
systems at redshift z=7. The features of cold, primordial gas are studied by
means of N-body, hydro, chemistry simulations, including atomic and molecular
non-equilibrium chemistry, cooling, star formation for population III and
population II-I regimes, stellar evolution, cosmic metal spreading according to
proper yields (for He, C, O, Si, Fe, Mg, S, etc.) and lifetimes, and feedback
effects. Theoretical expectations are then compared to recently available
constraints from DLA observations. We find that DLA galaxies at z=7 account for
10 per cent of the whole galaxy population and for most of the metal-poor
galaxies at these epochs. About 7 per cent of these DLA galaxies contain purely
pristine material and 34 per cent of them consist of very weakly polluted gas,
being, therefore, suitable candidate s as population III sites. The remaining
59 per cent are enriched above ~10^{-4} Zsun. Additionally, DLA candidates
appear to have: gas masses <~2x10^8 Msun; very low star formation rate,
~10^{-3} - 10^{-2} Msun/yr (significantly weaker than late-time counterparts);
mean molecular fractions covering a fairly wide range, x_mol~10^{-3}- 10^{-6};
typical metallicities Z<~3x10^{-3}Zsun and HI column densities NHI>~3x10^{20}
cm^{-2}. They present no or weak correlations between their gas mass and Z,
NHI, or x_mol; a moderate correlation between x_mol and Z, linked to the
ongoing molecular-driven star formation and metal pollution processes; a mild
anti-correlation between NHI and x_mol, due to H depletion into molecules; and
a chemical content that is subject to environmental dependencies.
