Imaging the Molecular Gas in a Submillimeter Galaxy at z = 4.05: Cold Mode Accretion or a Major Merger?

Abstract

We present a high-resolution (down to 0."18), multi-transition imaging study of the molecular gas in the z = 4.05 submillimeter galaxy GN20. GN20 is one of the most luminous starburst galaxy known at z>4, and is a member of a rich proto-cluster of galaxies at z = 4.05 in GOODS-North. We have observed the CO 1-0 and 2-1 emission with the Very Large Array (VLA), the CO 6-5 emission with the Plateau de Bure Interferometer, and the 5-4 emission with Combined Array for Research in Millimeter Astronomy. The H_2 mass derived from the CO 1-0 emission is 1.3 × 10^(11)(α/0.8) M_☉. High-resolution imaging of CO 2-1 shows emission distributed over a large area, appearing as partial ring, or disk, of ~10 kpc diameter. The integrated CO excitation is higher than found in the inner disk of the Milky Way, but lower than that seen in high-redshift quasar host galaxies and low-redshift starburst nuclei. The CO 4-3 integrated line strength is more than a factor of 2 lower than expected for thermal excitation. The excitation can be modeled with two gas components: a diffuse, lower excitation component with a radius ~4.5 kpc and a filling factor ~0.5, and a more compact, higher excitation component (radius ~2.5 kpc, filling factor ~0.13). The lower excitation component contains at least half the molecular gas mass of the system, depending on the relative conversion factor. The VLA CO 2-1 image at 0"2 resolution shows resolved, clumpy structure, with a few brighter clumps with intrinsic sizes ~2 kpc. The velocity field determined from the CO 6-5 emission is consistent with a rotating disk with a rotation velocity of ~570 km s^(–1) (using an inclination angle of 45°), from which we derive a dynamical mass of 3 × 10^(11) M_☉ within about 4 kpc radius. The star formation distribution, as derived from imaging of the radio synchrotron and dust continuum, is on a similar scale as the molecular gas distribution. The molecular gas and star formation are offset by ~1" from the Hubble Space Telescope I-band emission, implying that the regions of most intense star formation are highly dust obscured on a scale of ~10 kpc. The large spatial extent and ordered rotation of this object suggests that this is not a major merger, but rather a clumpy disk accreting gas rapidly in minor mergers or smoothly from the proto-intracluster medium. Qualitatively, the kinematic and structural properties of GN20 compare well to the most rapid star formers fed primarily by cold accretion in cosmological hydrodynamic simulations. Conversely, if GN20 is a major, gas-rich merger, then some process has managed to ensure that the star formation and molecular gas distribution has not been focused into one or two compact regions.

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