2018A&A...615A..25J

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∼750km/s) 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)x10¹⁰K(km/s)pc², which yields a molecular gas mass of M_H2(α_CO/4.3)=(1.5±0.3)x10¹¹M_☉ and unveils a gas-rich system with µ_gas(α_CO/4.3)∼M_H2/M*=3.3±0.7µ_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)pc²), 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, α_CO=3.9_–1.3^+2.7M_☉/K(km/s)pc², 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.