From stardust to interstellar grain growth in the first galaxies: a cosmological transition in dust evolution near z 8.9

When and how did dust begin to shape galaxies? Motivated by the identification of an apparent redshift break in galaxy dust masses, suggesting substantially lower dust masses at z 9, we investigate dust enrichment during the first billion years of cosmic history. We aim to determine whether the observed evolution marks a transition in the dominant dust-production mechanism and to identify the physical conditions under which such a transition is expected to occur. Using JWST, ALMA, and NOEMA observations, we measure ultraviolet dust attenuation and dust masses. We apply a censored change-point analysis and compare the observations with dust evolution modelling. The analysis identifies a preferred transition near redshift z 8.9, corresponding to about 570 Myr after the Big Bang. The evidence for a break is strongest in dust mass and dust-to-stellar mass ratio, mostly estimated from JWST NIRSpec spectrophotometric fitting but also partly from sub-mm data. The ultraviolet attenuation measurements are consistent with a transition at the same epoch but do not independently require one. The models are consistent with the onset of efficient interstellar grain growth above a characteristic metallicity. We interpret the transition near z 8.9 as the emergence of grain-growth-dominated dust evolution from an earlier regime dominated by supernova-produced grains. Population III enrichment can modify the earliest chemical-enrichment history but leaves the timing of the dust transition nearly unchanged and is not required for its emergence.