The discoveries of huge amounts of dust and unusual extinction curves in high-redshift quasars ( z \ga 4 ) cast challenging issues on the origin and properties of dust in the early universe .
In this Letter , we investigate the evolutions of dust content and extinction curve in a high- z quasar , based on the dust evolution model taking account of grain size distribution .
First , we show that the Milky-Way extinction curve is reproduced by introducing a moderate fraction ( \simeq 0.2 ) of dense molecular-cloud phases in the interstellar medium for a graphite-silicate dust model .
Then we show that the peculier extinction curves in high- z quasars can be explained by taking a much higher molecular-cloud fraction ( \ga 0.5 ) , which leads to more efficient grain growth and coagulation , and by assuming amorphous carbon instead of graphite .
The large dust content in high- z quasar hosts is also found to be a natural consequence of the enhanced dust growth .
These results indicate that grain growth and coagulation in molecular clouds are key processes that can increase the dust mass and change the size distribution of dust in galaxies , and that , along with a different dust composition , can contribute to shape the extinction curve .