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

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Jiménez-Andrade, E. F.; Zavala, J. A.;  Magnelli, B.; Casey, C. M.;  Liu, D.; Romano-Díaz, E.;  Schinnerer, E.; Harrington, K.; Aretxaga, I.; Karim, A.; Staguhn, J.; Burnham, A. D.; Montaña, A.; Smolčić, V.; Yun, M.; Bertoldi, F.; Hughes, D.

Accepted for publication in Astrophysical Journal (arXiv: 2001.06997)

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 (S1.1mm = 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 12CO(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 12CO(5→4) line emission and adopt typical SMG-like gas excitation conditions to estimate the molecular gas mass, which is Mgas(αCO/2.5) =2.1 ± 0.4 × 1011 M sun for AzTEC2-A, and a factor four lower for AzTEC2-B. With the infrared-derived star formation rate of AzTEC2-A (1920±100 Msun yr−1) and AzTEC2-B (710±35 M sun 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 vrot(i = 39◦) = 660 ± 130 km s−1, velocity dispersion 100 km s−1, and dynamical mass of Mdyn(i = 39◦) = 2.6+1.2−0.9 × 1011 Msun . 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.

AzTEC2
Multi-wavelength view towards the AzTEC2 source. Left panel: AzTEC/JCMT 1.1mm (white Scott et al. 2008), ALMA Band 7/887um (green; this study) and VLA Band S/10 cm (blue; Smolcic et al. 2017) contours overlaid on top the HST/ACS F814W image. All contour levels are above three times the noise r.m.s. The zoomed-in image (right panel) shows two foreground sources: a massive elliptical galaxy at z = 0.34 and a SFG at z = 1.12. ALMA and VLA imaging at sub-arcsec resolution reveal two optically-undetected components at z ~ 4.63, labeled as AzTEC2-A and AzTEC2-B.
  • [EN] An explanation of this paper for the general public can be found in the post  «What is AzTEC2?
  • [ES] La explicación a todo público de este artículo se encuentra en la entrada «¿Qué es AzTEC2»
  • [EU] Publiko orokorrarentzako azalpena post honetan: «Zer da AzTEC2?»

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