Аннотация
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 VLA
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 kpc X 3 kpc and ~8 kpc X 5 kpc (Gaussian FWHM) in CO(2-1),
respectively, and we measure gas surface densities up to ~12,700/1,700X(sin i)
(\alpha_CO/0.8) M_sun/pc^2 for GN20.2a/GN20.2b in the highest-resolution maps.
Dynamical mass estimates allow us to constrain the CO-to-H_2 conversion factor
to \alpha_CO = 1.7+/-0.8 M_sun (K km s^-1 pc^2)^-1 for GN20.2a and \alpha_CO
= 1.1+/-^1.5_1.1 M_sun (K km/s pc^2)^-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 kpc 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 3sigma
upper limits on the CO luminosities of those that may lie within our bandpass.
A blind search for emission line sources down to a 5sigma limiting CO
luminosity of L'_CO(2-1) = 8 X 10^9 K km/s pc^2 and covering \Delta z = 0.0273
(~20 comoving Mpc) produces no other strong contenders associated with the
proto-cluster.
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