The O3N2 and N2 abundance indicators revisited: improved calibrations based on CALIFA and Te-based literature data

Abstract: The use of IFS is since recently allowing to measure the emission line fluxes of an increasingly large number of star-forming galaxies both locally and at high redshift. The main goal of this study is to review the most widely used empirical oxygen calibrations, O3N2 and N2, by using new direct abundance measurements. We pay special attention to the expected uncertainty of these calibrations as a function of the index value or abundance derived and the presence of possible systematic offsets. This is possible thanks to the analysis of the most ambitious compilation of Te-based HII regions to date. This new dataset compiles the Te-based abundances of 603 HII regions extracted from the literature but also includes new measurements from the CALIFA survey. Besides providing new and improved empirical calibrations for the gas abundance, we also present here a comparison between our revisited calibrations with a total of 3423 additional CALIFA HII complexes with abundances derived using the ONS calibration by Pilyugin et al. (2010). The combined analysis of Te-based and ONS abundances allows us to derive their most accurate calibration to date for both the O3N2 and N2 single-ratio indicators, in terms of all statistical significance, quality and coverage of the space of parameters. In particular, we infer that these indicators show shallower abundance dependencies and statistically-significant offsets compared to those of Pettini and Pagel (2004), Nagao et al. (2006) and P\'erez-Montero and Contini (2009). The O3N2 and N2 indicators can be empirically applied to derive oxygen abundances calibrations from either direct abundance determinations with random errors of 0.18 and 0.16, respectively, or from indirect ones (but based on a large amount of data) reaching an average precision of 0.08 and 0.09 dex (random) and 0.02 and 0.08 dex (systematic; compared to the direct estimations),respectively.

• The paper refines O3N2 and N2 calibrations by integrating over 600 Tₑ-based measurements and thousands of CALIFA survey data points.
• It achieves improved precision with average uncertainties of 0.08 dex for O3N2 and 0.09 dex for N2, underscoring a significant methodological advancement.
• These enhanced calibrations reduce systematic errors in metallicity measurements and provide a robust tool for probing galactic chemical evolution.

Review of "The O3N2 and N2 Abundance Indicators Revisited: Improved Calibrations Based on CALIFA and T$_{e}$-Based Literature Data"

The paper "The O3N2 and N2 Abundance Indicators Revisited: Improved Calibrations Based on CALIFA and T$_{e}$-Based Literature Data" by Marino et al. provides an in-depth analysis of revised calibrations for the oxygen abundance indicators O3N2 and N2. By leveraging data from the CALIFA survey and direct T$_e$-based abundance measurements, this study offers a detailed recalibration of these commonly used indicators, which are essential for determining the metallicity of \ion{H}{ii} regions in star-forming galaxies. The paper is particularly significant for researchers working on galactic chemical evolution and the development of empirical metallicity calibrations.

Methodology and Data Compilation

The authors employed integral field spectroscopy to collect emission line fluxes across a wide array of galaxies, both locally and at high redshift. They focused on using the strong-line methods, recognized for their application in deriving gas-phase metallicities from emission lines. However, previous calibrations were primarily based on a limited set of T$_e$-based measurements, which could lead to systematic errors, particularly at high redshifts.

To address these shortcomings, the authors revisit the O3N2 and N2 calibrations by integrating data from 603 \ion{H}{ii} regions with direct T$_e$-based abundance measurements from the literature, alongside thousands of measurements from the CALIFA survey. This comprehensive dataset enables them to assess the expected uncertainties of the calibrations and identify any systematic offsets.

Results and Recalibration

The paper presents new empirical calibrations for the O3N2 and N2 abundance indicators. Both indicators are revisited to identify shallower abundance dependencies and statistically significant offsets compared to previously established calibrations, such as those by Pettini & Pagel (2004), Nagao et al. (2006), and Pérez-Montero & Contini (2009). Notably, they find that the O3N2 and N2 indicators can achieve an average precision of 0.08 and 0.09 dex, respectively, when compared to indirect methods based on large datasets, and maintain random errors of 0.18 and 0.16, respectively, when based on direct abundance determinations.

The new calibrations exhibit improved statistical significance, quality, and parameter space coverage. This advancement is crucial for reducing systematic errors in metallicity determinations and enhancing the understanding of chemical evolution across different cosmic epochs.

Implications and Future Directions

The revised calibrations have important implications for the study of galactic chemical evolution and the interpretation of high-redshift galaxy observations. The availability of more accurate metallicity indicators enables refined constraints on the history of star formation and the lifecycle of elements within galaxies. Additionally, it provides a critical tool for investigating the mass-metallicity relation at various cosmic times—a key factor in assessing galaxy evolution.

Future studies could build upon this work by employing the updated calibrations in various galactic environments, potentially exploring further refinements with newer datasets as integral field spectroscopy becomes more widespread. The broader application of these indicators might offer insights into the interplay between metallicity, ionization conditions, and star formation in diverse galactic contexts.

In conclusion, this paper significantly contributes to the field of astrophysics by enhancing the reliability of strong-line indicators for metallicity determinations. It encourages future exploration and validation of these calibrations in larger and more varied samples of galaxies.