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 \pm 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 \pm 2.4 \times 10 ^ { 11 } M _ { \odot } .
We use this dynamical mass estimate to constrain the CO–to–H _ { 2 } mass conversion factor ( \alpha _ { CO } ) , finding \alpha _ { CO } = 1.1 \pm 0.6 M _ { \sun } ( K km s ^ { -1 } pc ^ { 2 } ) ^ { -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 \times ( \alpha _ { CO } / 0.8 ) M _ { \odot } pc ^ { -2 } .
Virial mass estimates indicate they could be self–gravitating , and we constrain their CO–to–H _ { 2 } mass conversion factor to be < 0.2–0.7 M _ { \sun } ( K km s ^ { -1 } pc ^ { 2 } ) ^ { -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 .

Key words : galaxies : evolution - - galaxies : formation - - galaxies : high-redshift - - galaxies : ISM - - galaxies : star formation