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 (H I )/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 Ly α emission at > 5.8 σ level with a luminosity of 8.95 ± 1.54 × 10 40 erg s −1 (corresponding to about 0.02 L ⋆ at z ∼ 2 − 3) in systems with average log( N (H I ) /cm −2 ) of ≈21.2 and at a median redshift of z ∼ 2.64. The peak of the Ly α 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)/f esc M ⊙ yr −1 ; namely, ≈ (0.54 ± 0.09) M ⊙ yr −1 for a typical escape fraction (f esc = 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.