The redshift and star formation mode of AzTEC2: a pair of massive galaxies at $z=4.63$

Abstract

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 ($S_{1.1mm}=10.5\pm 1.4$mJy). Our high-resolution observations confirm that AzTEC2 splits into two components (namely AzTEC2-A and AzTEC2-B) for which we detect [CII] 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 $M_{\mathrm{gas}}({\alpha }_{\mathrm{CO}}/2.5)=2.1\pm 0.4\times 10^{11}{\mathrm{M}}_{\odot }$ for AzTEC2-A, and a factor four lower for AzTEC2-B. With the infrared-derived star formation rate of AzTEC2-A ($1920\pm 100M_{\odot }{\mathrm{yr}}^{-1}$) and AzTEC2-B ($710\pm 35M_{\odot }{\mathrm{yr}}^{-1}$), they both will consume their current gas reservoir within $(30-200)$Myr. We find evidence of a rotation-dominated [CII] disk in AzTEC2-A, with a de-projected rotational velocity of $v_{\mathrm{rot}}(i=39^{\circ })=660\pm 130{\mathrm{km}\mathrm{s}}^{-1}$, velocity dispersion $\lesssim 100{\mathrm{km}\mathrm{s}}^{-1}$, 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.

19 pages, 6 figures, accepted for publication in ApJ

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