The Chemical Evolution of High-z Galaxies from the Relative Abundances of N, Si, S, and Fe in Damped Lyα Systems
Abundances of N, Si, S, and Fe for 45 damped Lyα systems (DLAs) have been compiled and detailed one-zone chemical evolution models have been constructed for 30 of them. Assuming continuous star formation, we found that final abundances in each object can be modeled by adjusting only two parameters, i.e., its time-averaged star formation efficiency and evolutionary age, with ranges in our sample of 0.01-1.5 Gyr-1 and 0.18-2.0 Gyr, respectively. In addition, average star formation efficiency and evolutionary age appear to be anticorrelated for the sample, suggesting that the star formation efficiency in a typical DLA decreases with age. At the same time, N/Si in DLAs is directly linked to an object's age. There is an apparent bimodality in the distribution of N/Si values which could be the result of a statistical accident or an effect produced by a truncated or flattened initial mass function. We find that the mean and small dispersion of Si/Fe values is related to the generally young ages of DLAs, wherein not all Fe has yet been expelled by Type Ia supernovae. Finally, the large scatter and generally lower values of N/Si of DLAs with respect to blue compact galaxies, despite their partially overlapping metallicities, indicate that DLAs are generally younger than the latter.