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Molecular gas in AzTEC/C159 : a star-forming disk galaxy 1.3 Gyr after the Big Bang.

Jiménez-Andrade, E. F. and Magnelli, B. and Karim, A. and Jones, G. C. and Carilli, C. L. and Romano-Díaz, E. and Gómez-Guijarro, C. and Toft, S. and Bertoldi, F. and Riechers, D. A. and Schinnerer, E. and Sargent, M. and Michałowski, M. J. and Fraternali, F. and Staguhn, J. G. and Smolčić, V. and Aravena, M. and Harrington, K. C. and Sheth, K. and Capak, P. L. and Koekemoer, A. M. and van Kampen, E. and Swinbank, M. and Zirm, A. and Magdis, G. E. and Navarrete, F. (2018) 'Molecular gas in AzTEC/C159 : a star-forming disk galaxy 1.3 Gyr after the Big Bang.', Astronomy & astrophysics., 615 . A25.


We studied the molecular gas properties of AzTEC/C159, a star-forming disk galaxy at z = 4.567, in order to better constrain the nature of the high-redshift end of the submillimeter-selected galaxy (SMG) population. We secured 12CO molecular line detections for the J = 2 →1 and J = 5 →4 transitions using the Karl G. Jansky Very Large Array (VLA) and the NOrthern Extended Millimeter Array (NOEMA) interferometer. The broad (FWHM ~ 750 km s−1) and tentative double-peaked profiles of the two 12CO lines are consistent with an extended molecular gas reservoir, which is distributed in a rotating disk, as previously revealed from [CII] 158 μm line observations. Based on the 12CO(2 →1) emission line, we derived L′CO=(3.4±0.6)×1010 K km s−1 pc2, which yields a molecular gas mass of MH2(αCO/4.3)=(1.5±0.3)×1011 M⊙ and unveils a gas-rich system with μgas(αCO/4.3)≡MH2/M⋆=3.3±0.7. The extreme star formation efficiency of AzTEC/C159, parametrized by the ratio LIR/L′CO=(216±80) L⊙ (K km s−1 pc2)−1, is comparable to merger-driven starbursts such as local ultra-luminous infrared galaxies and SMGs. Likewise, the 12CO(5 →4)/CO(2 →1) line brightness temperature ratio of r52 = 0.55 ± 0.15 is consistent with high-excitation conditions as observed in SMGs. Based on mass budget considerations, we constrained the value for the L′CO – H2 mass conversion factor in AzTEC/C159, that is, αCO=3.9−1.3+2.7 M⊙ K−1 km−1 s pc−2, which is consistent with a self-gravitating molecular gas distribution as observed in local star-forming disk galaxies. Cold gas streams from cosmological filaments might be fueling a gravitationally unstable gas-rich disk in AzTEC/C159, which breaks into giant clumps and forms stars as efficiently as in merger-driven systems and generates high gas excitation. These results support the evolutionary connection between AzTEC/C159-like systems and massive quiescent disk galaxies at z ~ 2.

Item Type:Article
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Publisher statement:Reproduced with permission from Astronomy & Astrophysics, © ESO 2018
Date accepted:09 February 2018
Date deposited:20 July 2018
Date of first online publication:05 July 2018
Date first made open access:20 July 2018

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