Metallicity evolution in the early universe

Observations of the damped Ly alpha systems provide direct measurements on the chemical enrichment history of neutral gas in the early universe. In th is Letter, we present new measurements for four damped Ly alpha systems at high redshift. Combining these data with [Fe/H] values culled from the lite rature, we investigate the metallicity evolution of the universe from z app roximate to 1.5 to 4.5. Contrary to our expectations and the predictions of essentially every chemical evolution model, the N(H (I))-weighted mean [Fe /H] metallicity exhibits minimal evolution over this epoch. For the individ ual systems, we report tentative evidence for an evolution in the unweighte d [Fe/H] mean and the scatter in [Fe/H], with the higher redshift systems s howing lower scatter and lower typical [Fe/H] values. We also note that no damped Ly alpha system has [Fe/H] < -2.7 dex. Finally, we discuss the poten tial impact of small number statistics and dust on our conclusions and cons ider the implications of these results on chemical evolution in the early u niverse.