Witnessing galaxy assembly at the reionization epoch

Damped Lyman-alpha systems are the highest column density HI absorbers observed in the spectra of high redshift QSOs. They arise in the overdense knots of the cosmic web and they trace galactic disks, or the dense clumps and filaments building primordial galaxies. They also provide the most accurate measurements of chemical enrichment of gas in the high redshift Universe. Up to now, only a small sample of z>2, metal rich DLAs has been observed in emission with ALMA, to look for their galactic counterpart. I will report the detection of a pair DLA absorber/CO emitting galaxy at z=5.939 and describe the interpretation of this system in the context of hydrodynamical simulations.