The star formation history of damped lyα absorbers

The local power-law relationship between the surface densities of neutral hydrogen gas and star formation rate (SFR) can be used to explore the SFR properties of damped Lyα (DLA) systems at higher redshift. We find that while the SFR densities for DLA systems are consistent with luminous star-forming galaxies at redshifts below z ≈ 0.6, at higher redshifts their SFR density is too low for them to provide a significant contribution to the cosmic star formation history (SFH). This suggests that the majority of DLAs may be a distinct population from the Lyman break galaxies (LBGs) or submillimeter star-forming galaxies that together dominate the SFR density at high redshift. It is also possible that the DLAs do not trace the bulk of the neutral gas at high redshift. The metallicity properties of DLAs are consistent with this interpretation. The DLAs show a metal mass density lower by 2 orders of magnitude at all redshifts than that inferred from the SFH of the universe. These results are consistent with DLAs being dominated by low-mass systems having low SFRs or a late onset of star formation, similar to the star formation histories of dwarf galaxies in the local universe.