The essential signature of a massive starburst in a distant quasar

Observations of carbon monoxide emission in high-redshift (z > 2) galaxies indicate the presence of large amounts of molecular gas. Many of these galaxies contain an active galactic nucleus powered by accretion of gas onto a supermassive black hole, and a key question is whether their extremely high infrared luminosities result from the active galactic nucleus, from bursts of massive star formation (associated with the molecular gas), or both. In the Milky Way, high-mass stars form in the dense cores of interstellar molecular clouds, where gas densities are n(H2) > 105 cm-3 (refs 1, 2). Recent surveys show that virtually all galactic sites of high-mass star formation have similarly high densities3. The bulk of the cloud material traced by CO observations, however, is at a much lower density. For galaxies in the local Universe, the HCN molecule is an effective tracer of high-density molecular gas4. Here we report observations of HCN emission from the infrared-luminous ‘Cloverleaf’ quasar (at a redshift z = 2.5579). The HCN line luminosity indicates the presence of 10 billion solar masses of very dense gas, an essential feature of an immense starburst, which contributes, together with the active galactic nucleus it harbours, to its high infrared luminosity.