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

We present observations of the molecular gas in the GN20 proto-cluster of galaxies at $z=4.05$ using the Expanded Very Large Array (EVLA). This group of galaxies is the ideal laboratory for studying the formation of massive galaxies via luminous, gas-rich starbursts within 1.6 Gyr of the Big Bang. We detect three galaxies in the proto-cluster in CO 2-1 emission, with gas masses (H${}_2$) between ${10}^{10}$ and ${10}^{11}\times (\alpha \mathrm{/}0.8)$ M${}_{\odot }$. The emission from the brightest source, GN20, is resolved with a size $\sim 2\mathrm{"}$, and has a clear north-south velocity gradient, possibly indicating ordered rotation. The gas 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), and the sum of gas plus stellar mass is comparable to the dynamical mass of the system ($\sim 3.4\times {10}^{11}[sin(i)\mathrm{/}sin({45}^o){]}^{-2}$ M${}_{\odot }$), within a 5kpc radius. There is also evidence for a tidal tail extending another $2\mathrm{"}$ 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 an optical 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$_\odot$.