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The dust properties in high-redshift galaxies provide clues to the origin ofdust in the Universe. Although dust has been detected in galaxies at redshift$z>7$, it is difficult to constrain the dominant dust sources only from thetotal dust amount. Thus, we calculate the evolution of grain size distribution,expecting that different dust sources predict different grain sizedistributions. Using the star formation time-scale and the total baryonic massconstrained by the data in the literature, we calculate the evolution of grainsize distribution. To explain the total dust masses in ALMA-detected $z>7$galaxies, the following two solutions are possible: (i) high dust condensationefficiency in stellar ejecta, and (ii) efficient accretion (dust growth byaccreting the gas-phase metals in the interstellar medium). We find that thesetwo scenarios predict significantly different grain size distributions: in (i),the dust is dominated by large grains ($a\gtrsim 0.1~\mu$m, where $a$ is thegrain radius), while in (ii), the small-grain ($a\lesssim 0.01~\mu$m) abundanceis significantly enhanced by accretion. Accordingly, extinction curves areexpected to be much steeper in (ii) than in (i). Thus, we conclude thatextinction curves provide a viable way to distinguish the dominant dust sourcesin the early phase of galaxy evolution.

Expected dust grain size distributions in galaxies detected by ALMA at $z>7$