An international team, with the participation of researchers from the Instituto de Astrofísica de Canarias (IAC), has developed a new generation of calibrations that make it possible to estimate the chemical composition (metallicity) of both nearby galaxies and the most distant and early galaxies in the Universe observed by the James Webb Space Telescope. The study is based on the analysis of the largest collection of high-quality spectroscopic observations gathered to date, the result of more than a decade of compilation and analysis work.

To understand the origin and evolution of galaxies, astrophysicists study their chemical composition through the spectra of the clouds of ionized gas they contain, known as HII regions. The emission from these nebulae comes from the excitation produced by the ultraviolet light of the massive, newly formed stars inside them. HII regions, like any other cosmic object, contain a variety of chemical elements synthesized by previous generations of stars and released into space at the end of their lives. For this reason, the metallicity of a galaxy acts as a kind of fossil record of its evolution over time: the higher the metallicity, the more extensively the gas has been processed by previous generations of stars, allowing researchers to reconstruct the history of galaxy formation and evolution across cosmic time.

However, determining the chemical composition of an HII region from its spectrum is not straightforward. It requires precise knowledge of the density and temperature of the gas, as well as the characteristics of the radiation it receives from stars. Measuring these properties requires high-quality observations, with long exposure times on some of the world’s largest telescopes, capable of detecting even the emission from nebulae in faint, distant galaxies.

The research team, led by Fabián F. Rosales-Ortega (INAOE, Mexico), used the DESIRED database (DEep Spectra of Ionized REgions Database), the most extensive collection of high-quality spectra of ionized nebulae in both our Milky Way and other galaxies in the Universe. In total, the study used nearly 2,400 observations, an unprecedented amount of information for this type of analysis. “We can now revisit the methods used to measure the chemical composition of galaxies on a much firmer footing than before,” says Rosales-Ortega, first author of the paper.

27 calibrations for measuring metallicity

Using this dataset and recalculating all the necessary parameters in a homogeneous way, the researchers derived new formulations for the 23 calibrations that astrophysicists have historically used to estimate the metallicity of galaxies (parametrized by the oxygen abundance, the ratio of the number of oxygen particles to hydrogen) when the temperature of the gas cannot be determined from their spectra. These calibrations are built from the intensity of the brightest spectral lines of HII regions, are usually known as “strong-line methods”, and use different combinations of oxygen, nitrogen, sulphur, argon and neon lines. 

With the addition of four new methods, the study provides precise formulations for 27 calibrations in total, extending the range of physical conditions to which they can be applied and making them the most complete and robust empirical reference available for estimating the metallicity of galaxies. “These calibrations are essential for distant galaxies, where the faint lines cannot be detected, and using such a large, high-quality sample makes the metallicities derived from them more reliable,” explains Jorge García-Rojas, researcher at the IAC and co-author of the paper.

There is also a fundamental problem that has affected the determination of the chemical composition of nebulae for almost eighty years: temperature fluctuations within a nebula can introduce biases into the calculation. In this new work, strong-line calibrations are formulated for the first time under two different physical scenarios: the traditional one, which assumes a homogeneous temperature in the objects, and a new one that accounts for these temperature fluctuations. When they are included, metallicities are on average about 35% higher than those obtained with traditional methods, providing a quantitative measure of the effect of the thermal structure of the gas on the metallicity scale. “Including the effect of temperature fluctuations is a novelty in this type of study, and we can now tell how the metallicity scale changes depending on the scenario adopted,” says César Esteban, Associate Professor at the Universidad de La Laguna, researcher at the IAC and co-author of the paper.

Application to the Early Universe

The team also tested the newly proposed scales on spectral data from the most distant galaxies known, formed only a few hundred million years after the Big Bang and revealed by the James Webb Space Telescope (JWST). 

The analysis shows that many of the calibrations built from nearby galaxies can also provide reliable estimates in the early Universe, at large cosmological distances. Where differences arise, they can be explained by the more intense stellar ultraviolet radiation fields and/or the different nitrogen abundance in the earliest galaxies. Ultimately, this work makes it possible to use a common scale to study the chemical evolution of galaxies across a large fraction of cosmic history, from our local cosmic environment to epochs when the Universe was only a fraction of its present age.

Ultimately, this work makes it possible to use a common scale to study the chemical evolution of galaxies across a large fraction of cosmic history, from our local cosmic environment to epochs when the Universe was only a fraction of its present age.

The study has benefited from the contributions of several researchers from the IAC: César Esteban, Jorge García-Rojas, Maialen Orte-García, Elena Reyes-Rodríguez, and Sebastián F. Sánchez.

This work is the first paper in a series dedicated to the development of new strong-line calibrations within the international DESIRED collaboration. The new calibrations are freely available to the entire astronomical community. The article was published in the scientific journal *Monthly Notices of the Royal Astronomical Society* (MNRAS), one of the leading international publications in astronomy and astrophysics.

Article: F. F. Rosales-Ortega, J. E. Méndez-Delgado, J. U. Guerrero-González, C. Esteban, J. García-Rojas, K. Z. Arellano-Córdova, A. Z. Lugo-Aranda, O. Espíndola-Camacho, J. C. López-Gutiérrez, L. E. Martínez-Rivero, C. Morisset, M. Orte-García, E. Reyes-Rodríguez, L. Toribio San Cipriano, K. Kreckel, O. Egorov, I. A. Zinchenko, S. F. Sánchez, and J. M. Vílchez. “The DESIRED strong-line calibrations: I. New empirical metallicity relations for the local and high-redshift universe”, Monthly Notices of the Royal Astronomical Society, 2026. DOI: https://doi.org/10.1093/mnras/stag1963

Contact at the iac:
César Esteban, ces [at] iac.es (ces[at]iac[dot]es)
Jorge García-Rojas, jogarcia [at] iac.es (jogarcia[at]iac[dot]es)