Letter to the Editor

Star formation in neutral hydrogen gas reservoirs at cosmic noon

¹ Central University of Himachal Pradesh, Dharamshala, Kangra, India
² Indian Institute of Astrophysics (IIA), Bengaluru, Karnataka, India
³ Università degli Studi di Milano-Bicocca, Dip. di Fisica G. Occhialini, Piazza della Scienza 3, 20126 Milano, Italy
⁴ INAF–Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, I-34143 Trieste, Italy
⁵ Institut d’Astrophysique de Paris, CNRS-SU, UMR 7095, 98bis bd Arago, 75014 Paris, France
⁶ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
⁷ Department of Astronomy, School of Physics, Peking University, Beijing 100871, PR China

^⋆ Corresponding authors; dharmender98thakur@gmail.com, rvjoshirv@gmail.com

Received: 9 October 2024
Accepted: 15 November 2024

Abstract

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 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(H I) /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)/f_escM_⊙ 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.