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 } .