Witnessing Galaxy Assembly at the Edge of the Reionization Epoch*

Abstract

Abstract We report the discovery of Serenity-18, a galaxy at z ≃ 5.939 for which we could measure the content of molecular gas, M(H2) ≃ 5 × 109 M ⊙, traced by the CO(6–5) emission, together with the metal-poor ([Fe/H] =−3.08 ± 0.12, [Si/H] = −2.86 ± 0.14) gas clump/filament which is possibly feeding its growth. The galaxy has an estimated star formation rate of ≈100 M ⊙ yr−1, implying that it is a typical main sequence galaxy at these redshifts. The metal-poor gas is detected through a damped Lyα absorber (DLA) observed at a spatial separation of 40 kpc and at the same redshift of Serenity-18, along the line of sight to the quasar SDSS J2310+1855 (z em ≃ 6.0025). The chemical abundances measured for the damped Lyα system are in very good agreement with those measured for other DLAs discovered at similar redshifts, indicating an enrichment due to massive PopII stars. The galaxy/damped system that we discovered is a direct observational evidence of the assembly of a galaxy at the edge of the reionization epoch.