Origin of dust in the early Universe - Dusty quasars

Recent development of observational facilities has enabled us to see the dust in distant Universe, where we can observe the Universe in the past. In very distant quasars at the epoch when the age of the Universe is only 1/10 of the current age, a large amount of dust has been detected by observing radio (submillimeter) emission from dust. Such a large amount of dust has been a mystery since even the sum of all the dust supply from stars failed to explain the large dust amount. In other words, the Universe is too young for stars to supply and accumulate dust in these quasars.

We tackled this mystery (Kuo & Hirashita 2012) and found that, if we consider dust growth in the dense medium, we can naturally explain the large dust content in those quasars (Figure). Moreover, they also show that for the rapid dust growth, dust grains much smaller than 0.1 μm should exist. These results indicate that the rapid dust increase by dust growth is essential in explaining the large dust abundance in distant quasars and that the existence of small grains is crucial to activate dust growth earlier.

So dust grains are not static solid particles. Even at the early epoch of the Universe, dust grains are dynamically growing and at the same time, small grains are continuously being supplied. Recent studies also revealed that such small grains are supplied as a result of grain disruption after two dust grains collide with each other. It is interesting that in dark clouds, which appear to be a quiet environment at a glance, dust has evolved through such dynamical growth or disruption. Remember, too, that the Earth was formed from those dust grains. We expect that further studies of dust by astronomers will lead to a complete understanding of the formation of the Earth.

The evolution of dust abundance relative to the total gas content calculated by the theoretical models developed by Tzu-Ming Kuo and me (Kuo & Hirashita 2012, MNRAS, 432, 637). The horizontal axis shows the time sequence. The vertical axis show the dust abundance. We examine two cases: without and with small (< 0.1 μm) dust grains. Without small grains (i.e., only with large grains), the rapid increase of dust abundance due to dust growth comes later, failing to explain the observationally acceptable range (blue box) for the distant quasars. With small grains, the rapid increase of the dust abundance by dust growth occurs earlier, and the line successfully pass through the observational range (blue box), which means that we can successfully explain the dust abundance observed in the quasars.

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