We report the detection of CO 2-1, 5-4, and 6-5 emission in the highest-redshift submillimeter galaxy (SMG) AzTEC-3 at z=5.298, using the Expanded Very Large Array and the Plateau de Bure Interferometer. These observations ultimately confirm the redshift, making AzTEC-3 the most submillimeter-luminous galaxy in a massive z=5.3 protocluster structure in the COSMOS field. The strength of the CO line emission reveals a large molecular gas reservoir with a mass of 5.3e10 (alpha_CO/0.8) Msun, which can maintain the intense 1800 Msun/yr starburst in this system for at least 30 Myr, increasing the stellar mass by up to a factor of six in the process. This gas mass is comparable to `typical z~2 SMGs, and constitutes >~80% of the baryonic mass (gas+stars) and 30%-80% of the total (dynamical) mass in this galaxy. The molecular gas reservoir has a radius of <4 kpc and likely consists of a `diffuse, low-excitation component, containing (at least) 1/3 of the gas mass (depending on the relative conversion factor alpha_CO), and a `dense, high-excitation component, containing ~2/3 of the mass. The likely presence of a substantial diffuse component besides highly-excited gas suggests different properties between the star-forming environments in z>4 SMGs and z>4 quasar host galaxies, which perhaps trace different evolutionary stages. The discovery of a massive, metal-enriched gas reservoir in a SMG at the heart of a large z=5.3 protocluster considerably enhances our understanding of early massive galaxy formation, pushing back to a cosmic epoch where the Universe was less than 1/12 of its present age.
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