Abundances of N , Si , S , and Fe for 45 damped Lyman alpha 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 modelled 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 IMF .
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 .