Star formation in neutral hydrogen gas reservoirs at cosmic noon

We aim to constrain the average star formation associated with neutral hydrogen gas reservoirs at cosmic noon. We used a unprecedented sample of 1716 high-column-density, damped Ly-α absorbers (DLAs) from the Sloan Digital Sky Survey with log(N(HI)/cm-2) ≥ 21. This allowed us to generate the average Ly-α emission spectrum associated to DLAs, free from any emission coming from the background quasar. We measured the Lyα emission at > 5.8σ level with a luminosity of 8.95 ± 1.54 × 1040 erg s-1 (corresponding to about 0.02 L∗ at z∼2 - 3) in systems with average log(N(HI) /cm-2) of ≈21.2 and at a median redshift of z∼2.64. The peak of the Lyα emission is apparently redshifted by ∼300 km s-1 relative to the absorption redshift, which appears to be due to suppression of blue Ly-α photons by radiative transfer through expanding gas. We infer that DLAs form stars with an average rate of (0.08 ± 0.01)/fescM· yr-1; namely, ≈(0.54 ± 0.09) M· yr-1 for a typical escape fraction (fesc=0.15) of Lyman-α emitting galaxies. DLA galaxies follow the main sequence of star-forming galaxies at high redshift, suggesting that the DLA population is dominated by the lower mass end of Lyman-α emitting galaxies.