A MASSIVE MOLECULAR GAS RESERVOIR IN THE z=5.3 SUBMILLIMETER GALAXY AzTEC-3

We report the detection of CO J = 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 similar or equal to 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 5.3 x 10(10) (alpha(CO)/0.8) M(circle dot), which can maintain the intense 1800 M(circle dot) yr(-1) 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 similar to 2 SMGs and constitutes greater than or similar to 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 similar to 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 an 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.