Probing Cold Gas in a Massive, Compact Star-forming Galaxy at z = 6

Observations of low-order 12 C 16 O transitions represent the most direct way to study galaxies’ cold molecular gas, the fuel of star formation. Here we present the first detection of CO( J = 2 → 1) in a galaxy lying on the main-sequence of star-forming galaxies at z > 6. Our target, G09-83808 at z = 6.03, has a short depletion timescale of τ dep ≈ 50 Myr and a relatively low gas fraction of M gas / M ⋆ ≈ 0.30 that contrasts with those measured for lower-redshift main-sequence galaxies. We conclude that this galaxy is undergoing a starburst episode with a high star formation efficiency that might be the result of gas compression within its compact rotating disk. Its starburst-like nature is further supported by its high star formation rate surface density, thus favoring the use of the Kennicutt–Schmidt relation as a more precise diagnostic diagram. Without further significant gas accretion, this galaxy would become a compact, massive quiescent galaxy at z ∼ 5.5. In addition, we find that the calibration for estimating interstellar medium masses from dust continuum emission satisfactorily 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 small redshift evolution in the gas-to-dust ratio of massive, metal-rich galaxies. In the absence of gravitational amplification, this detection would have required of order 1000 hr of observing time. The detection of cold molecular gas in unlensed star-forming galaxies at high redshifts is thus prohibitive with current facilities and requires a tenfold improvement in sensitivity, such as that envisaged for the Next-Generation Very Large Array .