[Paper Review] Dust-depletion sequences in damped Ly-{\alpha} absorbers II. The composition of cosmic dust, from low-metallicity systems to the Galaxy

Lars Mattsson, A. De+2 more|arXiv (Cornell University)|Jan 15, 2019

Astrophysics and Star Formation Studies50 references4 citations

TL;DR

This study uses observed dust depletion patterns in damped Lyman-alpha absorbers (DLAs) and the Milky Way to computationally infer the composition of non-carbonaceous cosmic dust. By applying Monte Carlo simulations to elemental abundances (O, Si, Mg, S, Fe), it identifies metallic iron and iron oxides—particularly wüstite (FeO)—as major mass components, each constituting ~25% of dust, while silicates are a mixed population of iron-poor and iron-rich olivine and pyroxene, with no single dominant type.

ABSTRACT

We aim at assessing what are the most dominant dust species or types, including silicate and iron oxide grains present in the ISM, by using recent observations of dust depletion of galaxies at various evolutionary stages. We use the observed elemental abundances in dust of several metals (O, S, Si, Mg, and Fe) in different environments, considering systems with different metallicities and dust content, namely damped Lyman-{\alpha} absorbers (DLAs) towards quasars and the Galaxy. We derive a possible dust composition by computationally finding the statistically expected elemental abundances in dust assuming a set of key dust species with the iron content as a free parameter. Carbonaceous dust is not considered in the present study. Metallic iron (likely in the form of inclusions in silicate grains) and iron oxides is an important component of the mass composition of carbon-free dust. Iron oxides make up a significant mass fraction (~1/4 in some cases) of the oxygen-bearing dust and there are good reasons to believe that metallic iron constitutes a similar mass fraction of dust. W\"ustite (FeO) could be a simple explanation for the depletion of iron and oxygen because it is easily formed. There appears to be no silicate species clearly dominating the silicate mass, but rather a mix of iron-poor as well as iron-rich olivine and pyroxene. To what extent sulphur depletion is due to sulfides remains unclear. In general, there seems to be little evolution of the dust composition (not considering carbonaceous dust) from low-metallicity systems to the Galaxy.

Motivation & Objective

• To determine the dominant dust species in the interstellar medium (ISM) across a range of metallicities, from low-metallicity DLAs to the Milky Way.
• To investigate whether dust composition evolves with metallicity and dust content in different galactic environments.
• To assess the role of iron-rich phases—such as metallic iron and iron oxides—in explaining observed elemental depletions.
• To test the validity of the standard silicate-graphite model (SGM) against observed depletion patterns.
• To constrain the mass fractions of key dust components using statistical modeling of elemental abundances in dust.

Proposed method

• Uses observed elemental abundances (relative to hydrogen) in the gas phase of DLAs and the Galaxy to infer dust depletion levels.
• Applies Monte Carlo stochastic simulations to explore a range of possible dust compositions with free parameters for iron content in silicates and metallic iron abundance.
• Considers key dust species: iron-poor and iron-rich olivine and pyroxene, iron oxides (e.g., FeO), sulfides, and metallic iron.
• Imposes stoichiometric constraints on elemental mass fractions and evaluates statistical consistency with observed depletions.
• Analyzes degeneracies in the solution space, particularly when iron content in silicates approaches critical ratios (e.g., xpy = 2xol).
• Excludes carbonaceous dust from the model to isolate the composition of non-carbonaceous dust.

Experimental results

Research questions

• RQ1 What are the dominant non-carbonaceous dust species responsible for observed metal depletions in the ISM?
• RQ2 How does the composition of cosmic dust—specifically silicates, iron oxides, and metallic iron—vary with metallicity and dust content?
• RQ3 To what extent can observed depletion patterns in DLAs and the Galaxy be explained by a mixture of silicates and iron-bearing phases?
• RQ4 Is the standard silicate-graphite model (SGM) consistent with observed depletion trends, particularly for iron and oxygen?
• RQ5 What mass fractions do metallic iron and iron oxides contribute to the total dust mass in the ISM?

Key findings

• Metallic iron and iron oxides each constitute approximately 25% of the mass of non-carbonaceous dust, based on statistical modeling of depletion patterns.
• Wüstite (FeO) is a likely candidate for the dominant iron oxide due to its ease of formation and ability to explain iron and oxygen depletion.
• No single silicate type (e.g., olivine or pyroxene) dominates the dust mass; instead, a mixture of iron-poor and iron-rich silicates is required.
• The overall composition of non-carbonaceous dust does not show significant evolution with metallicity or dust content from low-metallicity DLAs to the Milky Way.
• The standard silicate-graphite model (SGM) is inconsistent with the observed depletion patterns, particularly for iron, due to insufficient iron content in silicates alone.
• Even with uncertainties, the model strongly favors a significant fraction of iron not bound in silicates or oxides, pointing to metallic iron inclusions as a necessary component.