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

Depletion studies provide a way to understand the chemical composition of interstellar dust grains. We here examine 23 gamma-ray bursts (GRBs) 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 per cent) show featureless extinction curves and only vary in shape. We observe strong correlations (with > 96 per cent 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-ultraviolet (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, c₄, 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.