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Recent ALMA observations of high-redshift normal galaxies have been providinga great opportunity to clarify the general origin of dust in the Universe, notbiased to very bright special objects even at $z>6$. To clarify what constraintwe can get for the dust enrichment in normal galaxies detected by ALMA, we usea theoretical model that includes major processes driving dust evolution in agalaxy; that is, dust condensation in stellar ejecta, dust growth by theaccretion of gas-phase metals, and supernova destruction. Using the dustemission fluxes detected in two normal galaxies at $z>6$ by ALMA as aconstraint, we can get the range of the time-scales (or efficiencies) of theabove mentioned processes. We find that if we assume extremely highcondensation efficiency in stellar ejecta ($f_{\mathrm{in}} \ga 0.5$), rapiddust enrichment by stellar sources in the early phase may be enough to explainthe observed ALMA flux, unless dust destruction by supernovae in those galaxiesis stronger than that in nearby galaxies. If we assume a condensationefficiency expected from theoretical calculations ($f_{\mathrm{in}} \la 0.1$),strong dust growth (even stronger than assumed for nearby galaxies if they aremetal-poor galaxies) is required. These results indicate that the normalgalaxies detected by ALMA at $z>6$ are biased to objects (i) with high dustcondensation efficiency in stellar ejecta, (ii) with strong dust growth in verydense molecular clouds, or (iii) with efficient dust growth because of fastmetal enrichment up to solar metallicity. A measurement of metallicity iscrucial to distinguish among these possibilities.

Constraint on dust evolution processes in normal galaxies at $z>6$ detected by ALMA