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 ± 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→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 ∼ 1. We use the 12 CO(5→4) line emission and adopt typical SMG-like gas excitation conditions to estimate the molecular gas mass, which is M gas (α CO /2.5) = 2.1 ± 0.4 × 10 11 M for AzTEC2-A, and a factor four lower for AzTEC2-B. With the infrared-derived star formation rate of AzTEC2-A (1920 ± 100 M yr −1 ) and AzTEC2-B (710 ± 35 M 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 • ) = 660 ± 130 km s −1 , velocity dispersion 100 km s −1 , and dynamical mass of M dyn (i = 39 • ) = 2.6 +1.2 −0.9 × 10 11 M . 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.

ALMA 1.3 mm Survey of Lensed Submillimeter Galaxies (SMGs) Selected by Herschel: Discovery of Spatially Extended SMGs and Implications

“…Based on the companion search, the co-centered distribution of dust/stellar continua and the evolutionary trends presented in this work, we conclude that SMGs could be triggered by a variety of mechanisms including major merger, minor merger and secular burst, consistent with the conclusions in Fujimoto et al (2017), Rujopakarn et al (2019), Lang et al (2019) and Jiménez-Andrade et al (2020). Such a diversity of triggering mechanism is different from what we have seen in galaxies of comparable infrared luminosities in the local Universe, i.e., ULIRGs, in which the major merger is the prevalent mode of igniting a circumnuclear starburst (e.g., Sanders et al 1988).…”

An ALMA Spectroscopic Survey of the Brightest Submillimeter Galaxies in the SCUBA-2-COSMOS field (AS2COSPEC): Survey Description and First Results

“…To compare to the previous large samples of ALMAidentified SMGs from follow-up observations of singledish detected submillimeter sources, in Figure 1 we show that there are only two SMGs that are similarly bright as our sample in the AS2UDS sample (Stach et al 2018) and there is none in the ALESS sample (Hodge et al 2013). Note that a subset of the sample was observed in millimeter by NOEMA and other ALMA datasets, and their measured redshifts were reported by Jiménez-Andrade et al (2020), Simpson et al (2020), andBirkin et al (2021), which are consistent with our results.…”

ALMA 1.3 mm Survey of Lensed Submillimeter Galaxies Selected by Herschel: Discovery of Spatially Extended SMGs and Implications

“…In both panels, the solid magenta line shows the track of the two-component model discussed in Section 5.2. Jiménez-Andrade et al (2020). Such a diversity of triggering mechanisms is different from what we have seen in galaxies of comparable infrared luminosities in the local universe, i.e., ULIRGs, in which the major merger is the prevalent mode of igniting a circumnuclear starburst (e.g., Sanders et al 1988).…”