[Paper Review] Chemical Elements at High and Low Redshifts

Max Pettini|arXiv (Cornell University)|Mar 2, 2006

Astronomy and Astrophysical Research1 references3 citations

TL;DR

This paper reviews recent advances in measuring chemical abundances in high-redshift galaxies and intergalactic gas, using emission-line diagnostics and absorption systems to trace metal production from z ≈ 2.5 back to the Big Bang. It finds that current observations account for at least 40% of the total metals produced by star formation up to z = 2.5, with the remainder likely in unobserved phases such as sub-DLAs, massive galaxies, and missed populations, suggesting progress in solving the 'missing metals' problem.

ABSTRACT

The past few years have seen a steady progress in the determination of element abundances at high redshifts, with new and more accurate measures of metallicities in star-forming galaxies, in QSO absorbers, and in the intergalactic medium. We have also become more aware of the limitations of the tools at our disposal in such endeavours. I summarise these recent developments and--in tune with the theme of this meeting--consider the clues which chemical abundance studies offer to the links between the high redshift galaxy populations and today's galaxies. The new data are `fleshing out' the overall picture of element abundances at redshifts z = 2 - 3 which has been gradually coming into focus over the last decade. In particular, we can now account for at least 40% of the metals produced by the global star formation activity in the universe from the Big Bang to z = 2.5, and we have strong indications of where the remainder are likely to be found.

Motivation & Objective

• To assess recent progress in measuring element abundances at high redshift using emission lines and absorption systems.
• To evaluate the reliability of abundance diagnostic tools, especially the N2 index and temperature-sensitive auroral lines.
• To determine how much of the total metal production from z = 0 to z = 2.5 is accounted for by observed components such as Lyman-alpha forest, DLAs, and UV-selected galaxies.
• To identify where the remaining unaccounted metals—potentially the 'missing metals'—are likely located.
• To establish connections between high-redshift galaxy populations and their present-day descendants through chemical evolution.

Proposed method

• Uses emission-line diagnostics such as the N2 index (log[N II]λ6583/Hα) to estimate oxygen abundances in high-redshift star-forming galaxies.
• Applies temperature-sensitive auroral lines (e.g., [O III]λ4363) to calibrate electron temperatures and improve abundance accuracy in metal-rich H II regions.
• Employs spectroscopic data from large telescopes and near-IR instruments like Gemini GNIRS to access redshifted emission lines at z ≈ 2–3.
• Combines comoving volume densities of UV-selected galaxies (e.g., BX sample) with median baryonic masses and metallicities to estimate total metal content.
• Integrates contributions from Lyman-alpha forest, Damped Lyman-Alpha systems (DLAs), and sub-DLAs to assess the global metal budget.
• Compares observed metal mass densities with the expected total from star formation history to identify missing metal reservoirs.

Experimental results

Research questions

• RQ1 To what extent do current abundance diagnostics such as the N2 index reliably estimate oxygen abundances in high-redshift galaxies?
• RQ2 How much of the total metal production from the Big Bang to z = 2.5 is accounted for by observed components like DLAs, the Lyman-alpha forest, and UV-selected galaxies?
• RQ3 What is the role of unobserved or under-sampled galaxy populations—such as those missed by UV selection—in the overall metal budget?
• RQ4 Are super-solar metallicities in high-redshift galaxies overestimated due to calibration biases in standard abundance indicators?
• RQ5 Where are the remaining metals—potentially the 'missing metals'—likely to be located in the high-redshift universe?

Key findings

• The N2 index, recalibrated by Pettini & Pagel (2004), provides a robust and direct method for estimating metallicity in high-redshift star-forming galaxies at z ≈ 1.5–2.5.
• Temperature-sensitive auroral lines suggest that the R23 index may overestimate oxygen abundance at high metallicities, implying that super-solar metallicities are rarer than previously thought.
• The metal content of UV-selected galaxies at z ≈ 2.2, with median mass 3.6×10¹⁰ M⊙ and metallicity ~0.6 Z⊙, contributes ΩZ ≈ 8.8×10⁻³ to the metal budget.
• When combined with contributions from the Lyman-alpha forest and DLAs, the total accounted-for metal production reaches ~40% of the total from z = 0 to z = 2.5.
• The remainder of the metals are likely located in sub-DLAs, massive UV-selected galaxies, and galaxies missed by UV photometric selection, particularly those detected in near-IR and sub-mm surveys.
• The correlation between mass and metallicity in high-redshift galaxies suggests that more massive systems contribute disproportionately to metal enrichment, indicating that current estimates may still underestimate the total metal content.