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

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.

Dense molecular cloud cores as a source of micrometer-sized grains in galaxies

Coreshine in dense molecular cloud cores (dense cores) is interpreted as evidence for micrometer-sized grains (referred to as very large grains, VLGs). VLGs may have a significant influence on the total dust amount and the extinction curve. We estimate the total abundance of VLGs in the Galaxy, assuming that dense cores are the site of VLG formation. We find that the VLG abundance relative to the total dust mass is roughly $phi_mathrm{VLG}sim 0.01(1-epsilon )/epsilon (tau_mathrm{SF}/5times 10^9~mathrm{yr})^{-1} (f_mathrm{VLG}/0.5)(t_mathrm{shat}/10^8~mathrm{yr})$, where $epsilon$ is the star formation efficiency in dense cores, $tau_mathrm{SF}$ the timescale of gas consumption by star formation, $f_mathrm{VLG}$ the fraction of dust mass eventually coagulated into VLGs in dense cores, and $t_mathrm{shat}$ the lifetime of VLGs (determined by shattering). Adopting their typical values for the Galaxy, we obtain $phi_mathrm{VLG}sim 0.02$--0.09. This abundance is well below the value detected in the heliosphere by Ulysses and Galileo, which means that local enhancement of VLG abundance in the solar neighborhood is required if the VLGs originate from dense cores. We also show that the effects of VLGs on the extinction curve are negligible even with the upper value of the above range, $phi_mathrm{VLG}sim 0.09$. If we adopt an extreme value, $phi_mathrm{VLG}sim 0.5$, close to that inferred from the above spacecraft data, the extinction curve is still in the range of the variation in Galactic extinction curves, but is not typical of the diffuse ISM.