Evolution of grain size distribution in high-redshift dusty quasars: Integrating large amounts of dust and unusual extinction curves

TL;DR

This study models dust evolution in high-redshift quasars, showing that grain growth, coagulation, and dust composition changes in molecular clouds explain their large dust content and unusual extinction curves.

Contribution

It introduces a dust evolution model incorporating grain size distribution and molecular cloud effects to explain high-z quasar dust properties.

Findings

Milky Way extinction curve reproduced with ~0.2 dense molecular-cloud fraction.

High molecular-cloud fraction (>0.5) explains peculiar high-z extinction curves.

Enhanced dust growth accounts for large dust content in high-z quasar hosts.

Abstract

The discoveries of huge amounts of dust and unusual extinction curves in high-redshift quasars (z > 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 (~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 (>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.