Molecular gas in AzTEC/C159: a star-forming disk galaxy 1.3 Gyr after the Big Bang
Magnelli, B.; Smolčić, V.; van Kampen, E.; Gómez-Guijarro, C.; Magdis, G. E.; Jiménez-Andrade, E. F.; Toft, S.; Koekemoer, A. M.; Schinnerer, E.; Riechers, D. A.; Michałowski, M. J.; Harrington, K. C.; Karim, A.; Sargent, M.; Bertoldi, F.; Aravena, M.; Fraternali, F.; Capak, P. L.; Carilli, C. L.; Staguhn, J. G.; Zirm, A.; Sheth, K.; Jones, G. C.; Navarrete, F.; Romano-Díaz, E.; Swinbank, M.
Germany, United States, United Kingdom, Denmark, Poland, Italy, Netherlands, Croatia, Chile, Greece
Abstract
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.