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Observations of low order CO transitions represent the most direct way tostudy galaxies' cold molecular gas, the fuel of star formation. Here we presentthe first detection of CO(2-1) in a galaxy lying on the main-sequence ofstar-forming galaxies at z>6. Our target, G09-83808 at z=6.03, has a shortdepletion time-scale of T_dep~50Myr and a relatively low gas fraction ofM_gas/M_star=0.30 that contrasts with those measured for lower redshiftmain-sequence galaxies. We conclude that this galaxy is undergoing a starburstepisode with a high star formation efficiency that might be the result of gascompression within its compact rotating disk. Its starburst-like nature isfurther supported by its high star formation rate surface density, thusfavoring the use of the Kennicutt-Schmidt relation as a more precise diagnosticdiagram for starbursts. Without further significant gas accretion, this galaxywould become a compact, massive quiescent galaxy at z~5.5. In addition, we findthat the calibration for estimating ISM masses from dust continuum emissionsatisfactorily reproduces the gas mass derived from the CO(2-1) transition(within a factor of ~2). This is in line with previous studies claiming a smallredshift evolution in the gas-to-dust ratio of massive, metal-rich galaxies. Inthe absence of gravitational amplification, this detection would have requiredof order ~1000h of observing time. The detection of cold molecular gas inunlensed star-forming galaxies at high redshifts is thus prohibitive withcurrent facilities and requires a ten-fold improvement in sensitivity, such asthat envisaged for the ngVLA.

Probing cold gas in a massive, compact star-forming galaxy at z=6