Evidence for a clumpy, rotating gas disk in a submillimeter galaxy at z=4
Abstract
We present Karl G. Jansky Very Large Array (VLA) observations of the CO(2-1) emission in the z=4.05 submillimeter galaxy (SMG) GN20. These high-resolution data allow us to image the molecular gas at 1.3 kpc resolution just 1.6 Gyr after the Big Bang. The data reveal a clumpy, extended gas reservoir, 14 +/- 4 kpc in diameter, in unprecedented detail. A dynamical analysis shows that the data are consistent with a rotating disk of total dynamical mass 5.4 +/- 2.4 X 1011 Msun. We use this dynamical mass estimate to constrain the CO-to-H2 mass conversion factor (alphaCO), finding alphaCO=1.1 +/- 0.6 Msun (K km s-1 pc2)-1. We identify five distinct molecular gas clumps in the disk of GN20 with masses a few percent of the total gas mass, brightness temperatures of 16-31K, and surface densities of >3,200-4,500 X (alphaCO/0.8) Msun pc-2. Virial mass estimates indicate they could be self-gravitating, and we constrain their CO-to-H2 mass conversion factor to be <0.2-0.7 Msun (K km s-1 pc2)-1. A multiwavelength comparison demonstrates that the molecular gas is concentrated in a region of the galaxy that is heavily obscured in the rest-frame UV/optical. We investigate the spatially-resolved gas excitation and find that the CO(6-5)/CO(2-1) ratio is constant with radius, consistent with star formation occuring over a large portion of the disk. We discuss the implications of our results in the context of different fueling scenarios for SMGs.
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