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

Abstract

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 $10^{11}\times (\hat{I}\pm /0.8)$ 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 ($1.3\times 10^{11}\times (\hat{I}\pm /0.8)$ 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$\ddot{I}ƒ$ limit to the molecular gas mass of this galaxy of $1.1\times 10^{10}\times (\hat{I}\pm /0.8)$ M${}_{\odot }$.

AAStex format, 4 figures; prepared for the ApJ Letters EVLA special issue