Metallicity and HI column density properties of damped Lyα systems

On the basis of the disk galaxy formation theory within the framework of the standard Lambda CDM hierarchical picture (Mo, Mao,& White), we selected modeled damped Ly alpha systems (DLAs), according to their observational criterion N-H (I) greater than or similar to 10(20.3) cm(-2) by Monte Carlo simulation using random inclinations to examine their observed properties. By best-fitting the predicted metallicity distribution to the observed ones, we get an effective yield of about 0.25 Z(circle dot) for DLAs, which is comparable to those for the SMC and LMC. And the predicted distribution is the same as that of the observations at a significance level of higher than 60%. The predicted column density distribution of modeled DLAs is compared with the observed ones with the corresponding number density, with a discussion of the gas content. We found that the predicted number density n(z) at redshift 3 agrees well with the observed value, but the gas content Omega(DLA) is about 3 times larger than that observed, since our model predicts more DLAs with higher column density. It should be noted that the predicted star formation rate density contributed by DLAs is consistent with the most recent observations if the star formation timescale in DLAs is assumed to be 1-3 Gyr. Meanwhile, the connection between DLAs and Lyman break galaxies (LBGs) is discussed by comparing their UV luminosity functions, which shows that the DLAs' host galaxies are much fainter than LBGs. We also predict that only a few percent of DLAs can host LBGs, which is also consistent with current observations. However, there is a discrepancy between model prediction and observation in the correlation between metallicity and H I column density for DLAs. We suggest that this could result from either the inadequacy of a Schmidt-type star formation law at high redshift, the diversities of DLA populations, or the model limitations. Although our current simple model cannot fully reproduce the observed DLA velocity distribution, we argue that this kind of model can still provide valuable information about the nature of DLAs.