We combine observations from the Atacama Large Millimeter/submillimeter Array ( ALMA ) and the NOrthern Extended Millimeter Array ( NOEMA ) to assess the redshift and to study the star formation conditions in AzTEC2 : one of the brightest sub-millimeter galaxies ( SMGs ) in the COSMOS field ( S _ { 1.1 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 ^ { 12 } CO ( 5 \to 4 ) line emission , implying a redshift of 4.626 \pm 0.001 ( 4.633 \pm 0.001 ) for AzTEC2-A ( AzTEC2-B ) and ruling out previous associations with a galaxy at z \sim 1 .
We use the ^ { 12 } CO ( 5 \to 4 ) line emission and adopt typical SMG-like gas excitation conditions to estimate the molecular gas mass , which is M _ { gas } ( \alpha _ { 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 de-projected rotational velocity of v _ { rot } ( i = 39 ^ { \circ } ) = 660 \pm 130 km s ^ { -1 } , velocity dispersion \lesssim 100 km s ^ { -1 } , and dynamical mass of M _ { dyn } ( i = 39 ^ { \circ } ) = 2.6 ^ { +1.2 } _ { -0.9 } \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 .