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

We studied the molecular gas properties of AzTEC/C159, a star-forming disk galaxy at $z=4.567$. We secured ${}^{12}$CO molecular line detections for the $J=2\to 1$ and $J=5\to 4$ transitions using the Karl G. Jansky VLA and the NOEMA interferometer. The broad (FWHM$\sim 750{\mathrm{k}\mathrm{m}\mathrm{s}}^{-1}$) and tentative double-peaked profiles of both ${}^{12}$CO lines are consistent with an extended molecular gas reservoir, which is distributed in a rotating disk as previously revealed from [CII] 158$\mu$m line observations. Based on the ${}^{12}$CO(2$\to$1) emission line we derived $L_{\mathrm{C}\mathrm{O}}^{\mathrm{\prime }}=(3.4\pm 0.6)\times {10}^{10}{\mathrm{K}\mathrm{k}\mathrm{m}\mathrm{s}}^{-1}{\mathrm{p}\mathrm{c}}^2$, that yields a molecular gas mass of $M_{\rm H_2 }(\alpha_{\rm CO}/4.3)=(1.5\pm0.3)\times 10^{11}{\rm M}\odot$ and unveils a gas-rich system with ${\mu }_{\mathrm{g}\mathrm{a}\mathrm{s}}({\alpha }_{\mathrm{C}\mathrm{O}}\mathrm{/}4.3)\equiv M_{{\mathrm{H}}_2}\mathrm{/}{M}_{\star }=3.3\pm 0.7$. The extreme star formation efficiency (SFE) of AzTEC/C159, parametrized by the ratio $L{\rm{IR}}/L’{\rm{CO}}=(216\pm80), {\rm L}{\odot}{\rm ,(K,km,s}^{-1}{\rm ,pc}^{2})^{-1}$, is comparable to merger-driven starbursts such as local ultra-luminous infrared galaxies (ULIRGs) and SMGs. Likewise, the ${}^{12}$CO(5$\to$4)/CO(2$\to$1) line brightness temperature ratio of $r_{52}=0.55\pm 0.15$ is consistent with high excitation conditions, similar to that observed in SMGs. We constrained the value for the $L_{\text{CO}}^{\mathrm{\prime }}-{\mathrm{H}}_2$ mass conversion factor in AzTEC/C159, i.e. $\alpha_{$\text{CO}$}=3.9^{+2.7}{-1.3}{\rm ,M}{\odot}{\rm ,K}^{-1}{\rm ,km}^{-1}{\rm ,s,pc}^{-2}$, 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.