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
(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.
