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

Abstract

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$\sim 750{\mathrm{km}\mathrm{s}}^{-1}$) 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$\hat{I}¼$m line observations. Based on the ${}^{12}$CO(2$\to$1) emission line we derived $L_{\mathrm{CO}}^{\prime }=(3.4\pm 0.6)\times 10^{10}{\mathrm{K}\mathrm{km}\mathrm{s}}^{-1}{\mathrm{pc}}^2$, that yields a molecular gas mass of $M_{{\mathrm{H}}_2}(\hat{I}{\pm }_{\mathrm{CO}}/4.3)=(1.5\pm 0.3)\times 10^{11}{\mathrm{M}}_{\odot }$ and unveils a gas-rich system with $\hat{I}{¼}_{\mathrm{gas}}(\hat{I}{\pm }_{\mathrm{CO}}/4.3)\equiv M_{{\mathrm{H}}_2}/M_{\star }=3.3\pm 0.7$. The extreme star formation efficiency (SFE) of AzTEC/C159, parametrized by the ratio $L_{\mathrm{IR}}/L_{\mathrm{CO}}^{\prime }=(216\pm 80){\mathrm{L}}_{\odot }{(\mathrm{K}\mathrm{km}\mathrm{s}}^{-1}{\mathrm{pc}}^2{)}^{-1}$, 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 $L_{\text{CO}}^{\prime }-{\mathrm{H}}_2$ mass conversion factor in AzTEC/C159, i.e. $\hat{I}{\pm }_{\text{CO}}=3.9_{-1.3}^{+2.7}{\mathrm{M}}_{\odot }{\mathrm{K}}^{-1}{\mathrm{km}}^{-1}{\mathrm{s}\mathrm{pc}}^{-2}$, 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.

10 pages, 4 figures, Accepted for publication in A&A