The Redshift and Star Formation Mode of AzTEC2: A Pair of Massive Galaxies at z = 4.63

We combine observations from the Atacama Large Millimeter/submillimeter Array and the NOrthern Extended Millimeter Array to assess the redshift and to study the star formation conditions in AzTEC2, one of the brightest submillimeter galaxies (SMGs) in the COSMOS field ( ${S}_{1.1\mathrm{mm}}=10.5\pm 1.4$ mJy). Our high-resolution observations confirm that AzTEC2 splits into two components (namely AzTEC2-A and AzTEC2-B) for which we detect [C II] and ¹²CO(5 → 4) line emission, implying a redshift of 4.626 ± 0.001 (4.633 ± 0.001) for AzTEC2-A (AzTEC2-B) and ruling out previous associations with a galaxy at $z\sim 1$ . We use the ¹²CO(5 → 4) line emission and adopt typical SMG-like gas excitation conditions to estimate the molecular gas mass, which is ${M}_{\mathrm{gas}}({\alpha }_{\mathrm{CO}}/2.5)=2.1\pm 0.4\,\times \,{10}^{11}\,{M}_{\odot }$ for AzTEC2-A, and a factor four lower for AzTEC2-B. With the infrared-derived star formation rate of AzTEC2-A ( $1920\pm 100\,{M}_{\odot }$ yr^-1) and AzTEC2-B ( $710\pm 35\,{M}_{\odot }$ yr^-1), they both will consume their current gas reservoir within (30-200) Myr. We find evidence of a rotation-dominated [C II] disk in AzTEC2-A, with a deprojected rotational velocity of ${V}_{\mathrm{rot}}(I=39^\circ )=660\pm 130$ km s^-1, velocity dispersion $\lesssim 100$ km s^-1, and dynamical mass of ${M}_{\mathrm{dyn}}(I=39^\circ )={2.6}_{-0.9}^{+1.2}\times {10}^{11}\,{M}_{\odot }$ . We propose that an elevated gas accretion rate from the cosmic web might be the main driver of the intense levels of star formation in AzTEC2-A, which might be further enhanced by gravitational torques induced by its minor companion (AzTEC2-B). These results strengthen the picture whereby the population of single-dish selected SMGs is rather heterogeneous, including a population of pairs of massive, highly active galaxies in a pre-coalescence phase.