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We present observations of the molecular gas in the GN20 proto-cluster ofgalaxies at $z =4.05$ using the Expanded Very Large Array (EVLA). This group ofgalaxies is the ideal laboratory for studying the formation of massive galaxiesvia luminous, gas-rich starbursts within 1.6 Gyr of the Big Bang. We detectthree galaxies in the proto-cluster in CO 2-1 emission, with gas masses (H$_2$)between $10^{10}$ and $10^{11} \times (\alpha/0.8)$ M$_\odot$. The emissionfrom the brightest source, GN20, is resolved with a size $\sim 2"$, and has aclear north-south velocity gradient, possibly indicating ordered rotation. Thegas mass in GN20 is comparable to the stellar mass ($1.3\times 10^{11} \times(\alpha/0.8)$ M$_\odot$ and $2.3\times 10^{11}$ M$_\odot$, respectively), andthe sum of gas plus stellar mass is comparable to the dynamical mass of thesystem ($\sim 3.4\times 10^{11} [sin(i)/sin(45^o)]^{-2}$ M$_\odot$), within a5kpc radius. There is also evidence for a tidal tail extending another $2"$north of the galaxy with a narrow velocity dispersion. GN20 may be a massive,gas rich disk that is gravitationally disturbed, but not completely disrupted.There is one Lyman-break galaxy (BD29079) in the GN20 proto-cluster with anoptical spectroscopic redshift within our search volume, and we set a 3$\sigma$limit to the molecular gas mass of this galaxy of $1.1\times 10^{10} \times(\alpha/0.8)$ M$_\odot$.

EVLA observations of a proto-cluster of molecular gas rich galaxies at z = 4.05