2026MNRAS.545f1897A

In recent years, observations with the James Webb Space Telescope (JWST) have started to map out the rapid metal enrichment of the early Universe, while (sub)millimetre observations have simultaneously begun to reveal the ubiquity of dust beyond $z\gtrsim 6$. However, the pathways that led to the assembly of early dust reservoirs remain poorly quantified, and require pushing our understanding of key scaling relations between dust, gas, and metals into the early Universe. We investigate the dust build-up in twelve $6.5 \lesssim z \lesssim 7.7$ galaxies drawn from the Reionization Era Bright Emission Line Survey (REBELS) that benefit from (i) JWST/NIRSpec strong-line metallicity measurements, (ii) Atacama Large Millimetre/submillimetre Array (ALMA) [C II]-based redshifts and gas masses, and (iii) dust masses from single- or multi-band ALMA continuum observations. Combining these measurements, we investigate the dust-to-gas (DtG), dust-to-metal (DtM), and dust-to-stellar mass (DtS) ratios of our sample as a function of metallicity. While our analysis is limited by systematic uncertainties related to the [C II]-to-H$_2$ conversion factor and dust temperature, we explore a wide range of possible values, and carefully assess their impact on our results. Under a fiducial set of assumptions, we find an average $\log (\mathrm{DtG}) = -3.02 \pm 0.23$, only slightly below that of local metal-rich galaxies. On the other hand, at fixed metallicity our average $\log (\mathrm{DtS}) = -2.15 \pm 0.42$ is significantly larger than that of low-redshift galaxies. Finally, through a comparison to various theoretical models of high-redshift dust production, we find that assembling the dust reservoirs in massive galaxies at $z\approx 7$ likely requires the combination of rapid supernova enrichment and efficient interstellar medium dust growth.