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 $10^{11} \times (\alpha/0.8)$ M$_\odot$. The emission
from the brightest source, GN20, is resolved with a size $\sim 2"$, 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 ($\sim 3.4\times 10^{11} [sin(i)/sin(45^o)]^{-2}$ M$_\odot$), within a
5kpc 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 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$_\odot$.
