[Paper Review] The cosmic build-up of dust and metals. Accurate abundances from GRB-selected star-forming galaxies at $1.7 < z < 6.3$

K. E. Heintz, A. De+39 more|arXiv (Cornell University)|Aug 28, 2023

Gamma-ray bursts and supernovae9 references4 citations

TL;DR

This study measures gas-phase metallicities, dust-to-gas (DTG), and dust-to-metal (DTM) mass ratios in 36 high-redshift star-forming galaxies (1.7 < z < 6.3) using gamma-ray burst (GRB) afterglow spectroscopy. It finds a linear redshift evolution of metallicity and strong correlations between DTG/DTM and metallicity, with significant offsets from the Milky Way at low metallicities, providing key constraints on cosmic dust and metal enrichment.

ABSTRACT

The chemical enrichment of dust and metals in the interstellar medium (ISM) of galaxies throughout cosmic time is one of the key driving processes of galaxy evolution. Here we study the evolution of the gas-phase metallicities, dust-to-gas (DTG), and dust-to-metal (DTM) ratios of 36 star-forming galaxies at $1.7 &lt; z &lt; 6.3$ probed by gamma-ray bursts (GRBs). We compile all GRB-selected galaxies with intermediate (R=7000) to high (R&gt;40,000) resolution spectroscopic data for which at least one refractory (e.g. Fe) and one volatile (e.g. S or Zn) element have been detected at S/N&gt;3. This is to ensure that accurate abundances and dust depletion patterns can be obtained. We first derive the redshift evolution of the dust-corrected, absorption-line based gas-phase metallicity [M/H]$_{ m tot}$ in these galaxies, for which we determine a linear relation with redshift ${ m [M/H]_{tot}}(z) = (-0.21\pm 0.04)z -(0.47\pm 0.14)$. We then examine the DTG and DTM ratios as a function of redshift and through three orders of magnitude in metallicity, quantifying the relative dust abundance both through the direct line-of-sight visual extinction $A_V$ and the derived depletion level. We use a novel method to derive the DTG and DTM mass ratios for each GRB sightline, summing up the mass of all the depleted elements in the dust-phase. We find that the DTG and DTM mass ratios are both strongly correlated with the gas-phase metallicity and show a mild evolution with redshift as well. While these results are subject to a variety of caveats related to the physical environments and the narrow pencil-beam sightlines through the ISM probed by the GRBs, they provide strong implications for studies of dust masses to infer the gas and metal content of high-redshift galaxies, and particularly demonstrate the large offset from the average Galactic value in the low-metallicity, high-redshift regime.

Motivation & Objective

• To measure accurate gas-phase metallicities, dust-to-gas (DTG), and dust-to-metal (DTM) mass ratios in high-redshift star-forming galaxies.
• To investigate the cosmic evolution of dust and metal content in galaxies from z ≈ 1.7 to z ≈ 6.3.
• To assess the reliability of GRB absorption-line spectroscopy as a probe of dust and metal abundances in high-redshift galaxies.
• To quantify the dependence of DTG and DTM on metallicity and redshift, accounting for dust depletion patterns.
• To provide constraints for inferring total gas and metal masses in high-redshift galaxies from dust continuum observations.

Proposed method

• Compilation of GRB afterglow spectra with intermediate (R ≈ 7000) to high (R > 40,000) resolution, ensuring S/N > 3 for at least one refractory (e.g., Fe) and one volatile (e.g., S, Zn) element.
• Derivation of dust-corrected, absorption-line-based gas-phase metallicity [M/H]_tot using line-of-sight extinction and depletion patterns.
• Novel method to compute DTG and DTM mass ratios by summing the mass of all depleted elements in the dust phase along each GRB sightline.
• Fitting a linear redshift relation: [M/H]_tot(z) = (-0.21 ± 0.04)z - (0.47 ± 0.14), to quantify metallicity evolution.
• Correlation analysis of DTG and DTM with both redshift and metallicity across three orders of magnitude in metallicity.
• Use of dust depletion levels and visual extinction A_V to derive mass ratios independent of conversion factors, ensuring robustness.

Figure 1: Dust-corrected metallicity [M/H] tot as a function redshift for the GRB-selected galaxies. The small red data points show individual measurements, and the large red hexagons represent the H i -weighted means with redshift where the errorbars denote the redshift interval and $1\sigma$ dispe

Experimental results

Research questions

• RQ1 How does the gas-phase metallicity evolve with redshift in high-redshift star-forming galaxies?
• RQ2 What is the dependence of dust-to-gas (DTG) and dust-to-metal (DTM) mass ratios on metallicity and redshift?
• RQ3 How do GRB-selected sightlines compare to average galaxy properties in terms of dust and metal abundance?
• RQ4 To what extent do dust depletion patterns and extinction measurements constrain the true mass ratios of dust and metals?
• RQ5 Can GRB absorption spectroscopy reliably infer total gas and metal masses in high-redshift galaxies?

Key findings

• The gas-phase metallicity [M/H]_tot shows a linear redshift evolution: [M/H]_tot(z) = (-0.21 ± 0.04)z - (0.47 ± 0.14), indicating increasing metallicity with decreasing redshift.
• Dust-to-gas (DTG) and dust-to-metal (DTM) mass ratios are strongly correlated with gas-phase metallicity, increasing with higher metallicity.
• Both DTG and DTM ratios exhibit mild evolution with redshift, consistent with a gradual build-up of dust and metals over cosmic time.
• The DTG and DTM ratios in low-metallicity, high-redshift galaxies (z > 3) show a large offset from the Galactic average, indicating significantly lower dust abundance relative to metals.
• The derived mass ratios are robust to conversion factors and are based on direct measurement of depleted element masses, minimizing systematic uncertainties.
• The results support the use of GRB absorption spectroscopy as a reliable probe of dust and metal content, even along narrow pencil-beam sightlines, with implications for interpreting dust continuum observations in high-redshift galaxies.

Figure 2: $A_{V}$ vs. the equivalent metal column density, $\log N_{\rm HI}+{\rm[M/H]_{tot}}$ , i.e. the dust-to-metals (DTM) ratio. The red symbols show the GRB sample where the triangles denote $1\sigma$ upper limits. The dashed and dotted lines represent the average MW ratio and the scatter (Wats