Expanded Very Large Array 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 (H₂) between 10¹⁰ and 10¹¹ × (α/0.8) M _sun. The emission from the brightest source, GN20, is resolved with a size ~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 × 10¹¹ × (α/0.8) M _sun and 2.3 × 10¹¹ M _sun, respectively), and the sum of gas plus stellar mass is comparable to the dynamical mass of the system (~3.4 × 10¹¹[sin (i)/sin (45°)]^-2 M _sun), within a 5 kpc 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 3σ limit to the molecular gas mass of this galaxy of 1.1 × 10¹⁰ × (α/0.8) M _sun.

The Very Large Array of the National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.