[Paper Review] Measuring Chemical Abundances in Extragalactic Metal-Rich HII Regions

Fabio Bresolin|arXiv (Cornell University)|Aug 19, 2006

Astrophysics and Star Formation Studies3 citations

TL;DR

This paper reviews challenges in measuring chemical abundances in metal-rich extragalactic H II regions, emphasizing that direct abundance determinations via temperature-sensitive collisionally excited lines are compromised by temperature fluctuations. It proposes that metal recombination lines—virtually insensitive to temperature structure—offer a robust calibration for strong-line abundance diagnostics, with new data from M101 showing oxygen abundances 0.3 dex higher than standard methods, resolving long-standing discrepancies in high-metallicity environments.

ABSTRACT

The analysis of metal-rich HII regions has a profound impact on the calibration of abundance diagnostics widely used to measure the chemical content of star-forming galaxies, both locally and at high redshift. I review the main difficulties that affect direct abundance determinations from temperature-sensitive collisionally excited lines, and briefly discuss strong-line methods, in particular their empirical calibration. In the near future it will be possible to calibrate strong-line methods using metal recombination lines, providing abundances that are virtually insensitive to uncertainties on the nebular temperature structure.

Motivation & Objective

• To address the critical challenge of temperature fluctuations in metal-rich H II regions that bias direct abundance measurements using collisionally excited lines.
• To advocate for metal recombination lines as a superior, temperature-insensitive alternative for calibrating strong-line abundance diagnostics.
• To improve the accuracy of chemical abundance determinations in high-metallicity galaxies, essential for understanding galaxy evolution and cosmological abundance gradients.
• To extend the sample of extragalactic H II regions with recombination line measurements to higher metallicities using 8–10m telescopes.
• To reconcile discrepancies between collisionally excited line and recombination line abundance determinations by accounting for temperature fluctuations.

Proposed method

• Uses high-resolution spectroscopy to detect metal recombination lines, such as C II λ4267 and O II multiplet at 4651 Å, in extragalactic H II regions.
• Applies the Peimbert (1967) and Peimbert & Costero (1969) method to derive abundances from recombination lines, assuming a constant ionization structure.
• Compares recombination line abundances with those derived from collisionally excited lines (e.g., [O III] λ4363) to assess temperature fluctuation effects.
• Employs photoionization models to test the consistency of strong-line diagnostics like R23 with recombination line abundances under varying temperature structures.
• Uses the ratio of auroral to nebular lines (e.g., [O III] λ4363 / [O III] λ5007) to infer electron temperature, though with known limitations due to temperature fluctuations.
• Analyzes the mean square temperature fluctuation t² to quantify the impact of non-uniform temperature distributions on abundance determinations.

Experimental results

Research questions

• RQ1 How do temperature fluctuations in metal-rich H II regions affect the accuracy of direct abundance measurements from collisionally excited lines?
• RQ2 To what extent do metal recombination lines provide a more reliable abundance indicator than collisionally excited lines in high-metallicity environments?
• RQ3 Can recombination line abundances be used to calibrate strong-line diagnostics such as R23 in metal-rich galaxies?
• RQ4 What is the magnitude of the systematic offset between abundances derived from collisionally excited lines and those from recombination lines in high-metallicity H II regions?
• RQ5 How do the results from recombination line measurements in M101's H1013 region compare with standard abundance diagnostics and photoionization model predictions?

Key findings

• The oxygen abundance in the metal-rich H II region H1013 in M101, derived from C II λ4267 recombination lines, is 12 + log(O/H) = 8.9, which is 0.3 dex higher than the value obtained without accounting for temperature fluctuations.
• The abundance derived from recombination lines is in good agreement with the R23 calibration derived from photoionization models, validating its use for high-metallicity calibration.
• A mean square temperature fluctuation t² = 0.06 is derived for H1013, which reconciles the discrepancy between recombination line and collisionally excited line abundances.
• The systematic offset of 0.2–0.3 dex between recombination line and collisionally excited line abundances is attributed to temperature fluctuations in H II regions.
• Metal recombination lines are virtually insensitive to temperature structure, making them ideal for calibrating strong-line abundance diagnostics in high-metallicity galaxies.
• The extension of recombination line measurements to higher metallicities is essential for resolving discrepancies in abundance determinations and improving cosmological abundance evolution models.