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

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 ^(12)CO 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 ^(12)CO 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^(−1) pc^2, which yields a molecular gas mass of M_(H2)(α_(CO)/4.3)=(1.5±0.3)×10^(11) M⊙ and unveils a gas-rich system with μ_(gas)(α_(CO)/4.3)≡M_(H2)/M⋆=3.3±0.7. The extreme star formation efficiency of AzTEC/C159, parametrized by the ratio L_(IR)/L′_(CO)=(216±80) L⊙ (K km s^(−1) pc^2)^(−1), is comparable to merger-driven starbursts such as local ultra-luminous infrared galaxies and SMGs. Likewise, the ^(12)CO(5 →4)/CO(2 →1) line brightness temperature ratio of r_(52)= 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) – H_2 mass conversion factor in AzTEC/C159, that is, α_(CO)=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.

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