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 ($S_{ \rm 1.1mm}=10.5\pm1.4$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 $4.626\pm0.001$ ($4.633\pm0.001$) for AzTEC2-A (AzTEC2-B) and ruling out previous associations with a galaxy at $z\sim1$. 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 $M_{ \rm gas}(\alpha_{ \rm CO}/2.5)=2.1\pm0.4 \times10^{ 11}{ \rm M}_\odot$ for AzTEC2-A, and a factor four lower for AzTEC2-B. With the infrared-derived star formation rate of AzTEC2-A ($1920\pm100 \,M_\odot{ \rm \, yr}^{ -1}$) and AzTEC2-B ($710\pm 35\,M_\odot{ \rm \,yr}^{ -1}$), 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 $v_{ \rm rot}(i=39^\circ)=660\pm130{ \rm \,km\,s}^{ -1}$, velocity dispersion $\lesssim100{ \rm \,km\,s}^{ -1}$, and dynamical mass of $M_{ \rm dyn}(i=39^\circ)=2.6^{ +1.2}_{ -0.9}\times10^{ 11}\,M_\odot$. 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.
Categories: astro-ph.GA