The evolution of grain mantles and silicate dust growth at high redshift

TL;DR

This study investigates how grain mantle composition and silicate dust growth vary in high-redshift galaxies, revealing that mantle thickness decreases with redshift and silicate formation remains unlikely due to minor silicon presence.

Contribution

It provides a numerical analysis of grain mantle evolution and silicate growth potential in high-zeta galaxies, highlighting environmental effects on dust composition.

Findings

Mantle thickness decreases from 120 to 20 layers as redshift increases.

Mantle composition varies significantly with redshift, affecting molecule prevalence.

Silicate formation is unlikely due to the minor presence of silicon-bearing species.

Abstract

In dense molecular clouds, interstellar grains are covered by mantles of iced molecules. The formation of the grain mantles has two important consequences: it removes species from the gas phase and promotes the synthesis of new molecules on the grain surfaces. The composition of the mantle is a strong function of the environment which the cloud belongs to. Therefore, clouds in high-zeta galaxies, where conditions -like temperature, metallicity and cosmic rays flux- are different from those in the Milky Way, will have different grain mantles. In the last years, several authors have suggested that silicate grains might grow by accretion of silicon bearing species on smaller seeds. This would occur simultaneously to the formation of the iced mantles and be greatly affected by its composition as a function of time. In this work, we present a numerical study of the grain mantle formation in high-zeta galaxies and we quantitatively address the possibility of silicate growth. We find that the mantle thickness decreases with increasing redshift, from about 120 to 20 layers for z varying from 0 to 8. Furthermore, the mantle composition is also a strong function of the cloud redshift, with the relative importance of CO, CO2, ammonia, methane and methanol highly varying with z. Finally, being Si-bearing species always a very minor component of the mantle, the formation of silicates in molecular clouds is practically impossible.