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

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 sub-mm 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 both ^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) × 10^10 K km s^⁻1pc^2, that yields a molecular gas mass of MH2 (αCO/4.3) = (1.5 ± 0.3) × 10^11 M and unveils a gas-rich system with µgas(αCO/4.3) ≡ MH₂ /M∗ = 3.3 ± 0.7. The extreme star formation efficiency (SFE) of AzTEC/C159, parametrized by the ratio L_IR/L’_CO = (216 ± 80) L⊙ (K km s^⁻1pc^2) ⁻1, is comparable to merger-driven starbursts such as local ultra-luminous infrared galaxies (ULIRGs) and SMGs. Likewise, the ^12CO(5→4)/CO(2→1) line brightness temperature ratio of r_52 = 0.55 ± 0.15 is consistent with high excitation conditions, similar to that observed in SMGs. Based on mass budget considerations we constrained the value for the L 0 CO – H2 mass conversion factor in AzTEC/C159, i.e. αCO = 3.9 ^+2.7 _−1.3 M⊙K^ ⁻1 km s^⁻1pc^2, that 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 forming stars as efficiently as in merger-driven systems and generate high gas excitation. These results support the evolutionary connection between AzTEC/C159-like systems and massive quiescent disk galaxies at z ∼ 2.