Detection of star formation in a near-pristine pocket of the Universe

Abstract

We detect two high-confidence star-forming sources in the environment of a chemically near-pristine ([Fe/H] <−3.66 ; 2σ) Damped Lyα system (DLA) using Keck/KCWI. These are found via the hydrogen Lyα emission line at Δv = −1200 km s−1 (LAE1) and Δv = −200 km s−1 (LAE2) relative to the DLA redshift (zabs = 3.078) at an impact parameter of b = 66 physical kpc and b = 137 physical kpc. We measure Lyα fluxes of $F(\rm Ly\alpha )_{\rm LAE1} =4.4\pm 0.7\times 10^{-18}~\rm erg\, s^{-1}\, cm^{-2}$ and $F(\rm Ly\alpha )_{\rm LAE2} =3.8\pm 0.9\times 10^{-18}~\rm erg\, s^{-1}\, cm^{-2}$. This marks the first detection of star formation in the environment of a chemically near-pristine DLA. We use red channel Keck/KCRM data to place limits on the associated He ii 1640Å emission flux of these sources, $F(\rm He\textsc {ii}) <3.8\times 10^{-19}~\rm erg\, s^{-1}\, cm^{-2}~(3\sigma )$ and $F(\rm He\textsc {ii}) <3.7\times 10^{-19}~\rm erg\, s^{-1}\, cm^{-2}~(3\sigma )$. The weak and narrow properties of the Lyα emission, combined with the inferred position on the mass-metallicity relation, suggest a metal-paucity of these galaxies. Future rest-frame optical observations are required to pin down the metallicity of LAE1 and LAE2 and test if chemically near-pristine DLAs can be used as a signpost to search for primitive, metal-poor galaxies 2 Gyrs after the Big Bang.