2013ApJ...776...22H

We present a study of the formation of clustered, massive galaxies at large look-back times via spectroscopic imaging of CO in the unique GN20 proto-cluster at z = 4.05. Existing observations show that this is a dense concentration of gas-rich, very active star forming galaxies, including multiple bright submillimeter galaxies (SMGs). Using deep, high-resolution Karl G. Jansky Very Large Array CO(2-1) observations, we image the molecular gas with a resolution of ∼1 kpc just 1.6 Gyr after the big bang. The SMGs GN20.2a and GN20.2b have deconvolved sizes of ∼5 kpcx3 kpc and ∼8 kpcx5 kpc (Gaussian FWHM) in CO(2-1), respectively, and we measure gas surface densities up to ∼12,700/1700x(sin i) (α_CO/0.8) M_☉/pc2 for GN20.2a/GN20.2b in the highest-resolution maps. Dynamical mass estimates allow us to constrain the CO-to-H₂ conversion factor to α_CO= 1.7±0.8 M_☉(K km/s pc²)^–1 for GN20.2a and α_CO = 1.1 ± _1.1^1.5 M_☉(K km/s pc²)^–1 for GN20.2b. We measure significant offsets (0.''5-1'') between the CO and optical emission, indicating either dust obscuration on scales of tens of kiloparsecs or that the emission originates from distinct galaxies. CO spectral line energy distributions imply physical conditions comparable to other SMGs and reveal further evidence that GN20.2a and GN20.2b are in different merging stages. We carry out a targeted search for CO emission from the 14 known B-band Lyman break galaxies (LBGs) in the field, tentatively detecting CO in a previously undetected LBG and placing 3σ upper limits on the CO luminosities of those that may lie within our bandpass. A blind search for emission-line sources down to a 5σ limiting CO luminosity of L'_CO(2–1) = 8 x 10⁹ K km/s pc² and covering Δz = 0.0273 (∼20 comoving Mpc) produces no other strong contenders associated with the proto-cluster.