CO in GN20: The Nature of a z=4 Submillimeter Galaxy

We present a study of the formation of clustered, massive galaxies at large look--back times, via high resolution spectroscopic imaging of CO in the unique GN20 proto-cluster. The data reveal evidence for rich structure and gas dynamics in unprecedented detail, allowing us to image the molecular gas with a resolution of only 1 kpc just 1.5 Gyr after the Big Bang. These state-of-the-art data give new insight into the detailed physical processes involved in early massive galaxy formation, and they provide a first glimpse of the morphological studies that will become feasible on a regular basis with ALMA.