摘要：We studied the molecular gas properties of AzTEC/C159, a star-forming disk galaxy atz= 4.567, in order to better constrain the nature of the high-redshift end of the submillimeter-selected galaxy (SMG) population. We secured12CO molecular line detections for theJ= 2 →1 andJ= 5 →4 transitions using theKarl G. JanskyVery 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 two12CO lines are consistent with an extended molecular gas reservoir, which is distributed in a rotating disk, as previously revealed from [CII] 158μm line observations. Based on the12CO(2 →1) emission line, we derivedL′CO=(3.4±0.6)×1010 K km s−1 pc2, which yields a molecular gas mass ofMH2(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 ratioLIR/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, the12CO(5 →4)/CO(2 →1) line brightness temperature ratio ofr52= 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– H2mass 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 atz~ 2.
