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

Abstract

ABSTRACT The dust properties in high-redshift galaxies provide clues to the origin of dust 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 the total dust amount. Thus, we calculate the evolution of grain-size distribution, expecting that different dust sources predict different grain-size distributions. Using the star formation time-scale and the total Baryonic mass constrained by the data in the literature, we calculate the evolution of grain-size distribution. To explain the total dust masses in ALMA-detected z > 7 galaxies, the following two solutions are possible: (i) high dust condensation efficiency in stellar ejecta and (ii) efficient accretion (dust growth by accreting the gas-phase metals in the interstellar medium). We find that these two 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 the grain radius), while in (ii), the small-grain ($a\lesssim 0.01\,{\mu m}$) abundance is significantly enhanced by accretion. Accordingly, extinction curves are expected to be much steeper in (ii) than in (i). Thus, we conclude that extinction curves provide a viable way to distinguish the dominant dust sources in the early phase of galaxy evolution.