Silicon and iron dust in gamma-ray burst host galaxy absorbers

TL;DR

This study analyzes the dust composition in high-redshift gamma-ray burst host galaxies by examining silicon and iron depletion, revealing the significant role of silicates in interstellar extinction and the complexity of far-UV extinction sources.

Contribution

It provides new insights into the relationship between dust-phase element depletion and extinction curve parameters in GRB host galaxies, emphasizing silicates' role in extinction.

Findings

Strong correlation between R_V and Si/Fe dust-phase column densities.

Silicates are crucial in causing interstellar extinction across wavelengths.

Far-UV extinction likely results from diverse small dust grains.

Abstract

Depletion studies provide a way to understand the chemical composition of interstellar dust grains. We here examine 23 gamma-ray bursts (GRB) optical afterglow spectra (spanning 0.6<z<5.0) and compare their silicon and iron dust-phase column densities with different extinction curve parameters to study the composition of the interstellar dust grains in these high-redshift GRB host galaxies. The majority of our sample (87%) show featureless extinction curves and only vary in shape. We observe strong correlations (with >96% significance) between the total-to-selective extinction, R_V, and the dust-phase column densities of Si and Fe. Since a large fraction of interstellar iron is locked in silicate grains, this indicates that high Si and Fe depletion leads to an increase in the fraction of large silicate grains and vice versa. This suggests that silicates play a vital role to induce the entire extinction at any wavelength. On the other hand, the far-UV extinction is usually attributed to the presence of small silicates. However, we find no trend between the far-UV parameter of the extinction curve, c_4, and the abundance of Si and Fe in the dust phase. We, therefore, propose that the far-UV extinction could be a combined effect of small (probably nanoparticles) dust grains from various species.