Enhanced [CII] emission in a z = 4.76 submillimetre galaxy

We present the detection of bright [CII] emission in the z = 4.76 submillimetre galaxy LESS J033229.4-275619 using the Atacama Pathfinder EXperiment. This represents the highest redshift [CII] detection in a submm selected, star-formation dominated system. The AGN contributions to the [CII] and far-infrared (FIR) luminosities are small. We find an atomic mass derived from [CII] comparable to the molecular mass derived from CO. The ratio of the [CII], CO and FIR luminosities imply a radiation field strength G ∼ 10 and a density ∼ 10 cm in a kpc-scale starburst, as seen in local and high redshift starbursts. The high L/L = 2.4 × 10 and the very high L/L 10 are reminiscent of low metallicity dwarf galaxies, suggesting that the highest redshift star-forming galaxies may also be characterised by lower metallicities. We discuss the implications of a reduced metallicity on studies of the gas reservoirs, and conclude that especially at very high redshift, [CII] may be a more powerful and reliable tracer of the interstellar matter than CO.