explain the ubiquity of “clumpy” disks seen in images of high-redshift galaxies, numerical simulations have also suggested that most massive, star-forming galaxies at $z$=1–3 continually accrete gas from the inter-galactic medium along cold and clumpy streams from the cosmic web [@Keres05; @Dekel09; @Bournaud09; @VandeVoort11]. This mode of accretion is at its most efficient at $z\sim $1–2, and offers a natural route for maintaining the high gas surface densities, star formation rates and clumpy morphologies of galaxies at these epochs. In such models, the gas disks fragment into a few bound clumps which are a factor 10–100$\times$ more massive than star-forming complexes in local galaxies. The gravitational release of energy as the most massive clumps form, torques between in-spiraling clumps and energy injection from star formation are all likely to contribute to maintaining the high turbulence velocity dispersion of the inter-stellar medium (ISM) [e.g.  @Bournaud09; @Lehnert09; @Genzel08; @Genzel11].

In order to refine or refute these models, the observational challenge is now to quantitatively measure the internal properties of high-redshift galaxies, such as their cold molecular gas mass and surface density, disk scaling relations, chemical make up, and distribution and intensity of star formation. Indeed, constraining the evolution of the star formation 