We present the first theoretical study of metals in damped-Ly \alpha ( DLA ) systems at redshift z \simeq 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 \simeq 7 account for \sim 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 \sim 34 per cent of them consist of very weakly polluted gas , being , therefore , suitable candidates as population III sites .
The remaining \sim 59 per cent are enriched above \sim 10 ^ { -4 } { Z _ { \odot } } .
Additionally , DLA candidates appear to have : gas masses \la 2 \times 10 ^ { 8 } ~ { } { M _ { \odot } } ; very low star formation rate , \sim 10 ^ { -3 } -10 ^ { -2 } ~ { } { M _ { \odot } } / yr ( significantly weaker than late-time counterparts ) ; mean molecular fractions covering a fairly wide range , x _ { mol } \sim 10 ^ { -3 } -10 ^ { -6 } ; typical metallicities Z \la 3 \times 10 ^ { -3 } ~ { } { Z _ { \odot } } and HI column densities N _ { HI } \ga 3 \times 10 ^ { 20 } ~ { } cm ^ { -2 } .
They present no or weak correlations between their gas mass and Z , N _ { HI } , 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 N _ { HI } and x _ { mol } , due to H depletion into molecules ; and a chemical content that is subject to environmental dependencies .