Echoes of the First Light: A Mysterious Metal-Poor Galaxy at Cosmic Dawn

Title: The JWST EXCELS survey: an extremely metal-poor galaxy at z = 8.271

hosting an unusual population of massive stars

Authors: F. Cullen, A. C. Carnall, D. Scholte, D. J. McLeod, R. J. McLure , K. Z. Arellano-Córdova, et al.

First Author’s Institute: Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh.

Status: Preprint, Submitted to MNRAS.

The James Webb Space Telescope (JWST) is at it again—uncovering some of the most extreme galaxies in the early universe. This time, it’s found EXCELS-63107, a tiny but mighty star-forming galaxy at redshift z = 8.271, which means we’re seeing it as it was less than 600 million years after the Big Bang. What makes this little galaxy stand out? It’s running on cosmic fumes, with barely any metals—just 1.6% of the oxygen found in the Sun. Even more intriguing, its spectrum hints at a population of unusual, massive stars that don’t quite fit into our current models.

A Window into the First Generations of Stars

Metal-poor galaxies like EXCELS-63107 are like time capsules, offering a glimpse of what the first galaxies might have been like. Unlike most galaxies we see today, which have been enriched by billions of years of star formation, extremely metal-poor galaxies (EMPGs) are still in their early stages, retaining the raw ingredients of the universe’s first stars. Previous JWST observations suggested that even at high redshifts, galaxies were already somewhat metal-enriched—but this one is different. EXCELS-63107 is one of the most metal-poor galaxies ever observed, making it a rare find.

JWST’s NIRSpec instrument captured a detailed spectrum of the galaxy, covering a crucial range of wavelengths (Figure 1). The researchers analyzed its emission lines to measure temperature, ionization state, and chemical composition. Ionization refers to how much energy is needed to strip electrons from atoms, which tells us about the strength of radiation from young stars. The more ionizing radiation a star emits, the more electrons are stripped from surrounding atoms, leading to a higher ionization state in the galaxy’s gas. The intense radiation from massive stars in EXCELS-63107 suggests an exceptionally energetic stellar population. NIRCam images also revealed that EXCELS-63107 is compact, resembling local star-forming clusters rather than typical high-redshift galaxies.

Metal-Poor Outlier with a Fiery Core

So, what makes EXCELS-63107 so unusual?

• It’s severely lacking in metals: With an oxygen abundance of 12 + log(O/H) = 6.89, it’s one of the most metal-poor galaxies ever found. For comparison, the Sun has an oxygen abundance of 12 + log(O/H) ≈ 8.69, meaning EXCELS-63107 contains only about 1.6% of the Sun’s oxygen content. While some of the most metal-poor galaxies, such as Peekaboo (6.99), have similar values, EXCELS-63107 is at the lower end, making it one of the least enriched galaxies observed to date.
• It’s burning hot: The strong [O III] λ4363 auroral line signals an electron temperature of T_e ≃ 34,000 K—way higher than what’s expected in metal-poor star-forming regions (Figure 2).
• Its stars don’t play by the rules: Standard stellar population models can’t explain the observed spectrum. The authors suggest that this galaxy might be packed with an unusually high number of massive stars (M > 50 M⊙), pointing to a top-heavy initial mass function (IMF). The IMF describes how many small versus large stars a galaxy forms, and a “top-heavy” IMF means more massive stars than expected. Some even speculate that this galaxy could contain remnants of Population III stars—the first generation of stars that ever formed in the universe.

What This Means for Cosmic Evolution

Finding a galaxy like EXCELS-63107 at z ≃ 8 is a big deal. If its top-heavy IMF is real, it means that massive stars were more common in early galaxies than we thought. This has huge implications for reionization—the era when the first galaxies flooded the universe with ionizing radiation. Alternatively, if this galaxy has conditions similar to those of Population III stars, it could be one of the closest observational links to these elusive first-generation stars.

EXCELS-63107 is also incredibly compact, measuring less than 200 light-years across—far smaller than our Milky Way, which spans about 100,000 light-years. Despite its tiny size, it’s forming stars at a rapid pace, producing about 8 solar masses per year. That’s an impressive rate for such a small system, packing intense star formation into a minuscule volume.

Another key clue about its extreme nature is its unusually blue light (Figure 3). Astronomers measure this using the UV continuum slope (β), which reveals how much dust is blocking starlight. For EXCELS-63107, β = -3.3, one of the bluest slopes ever measured, indicating that the galaxy is almost dust-free and full of extremely hot, young stars.

Alternative Explanations for EXCELS-63107’s Properties

While a top-heavy IMF is one possibility, there are other ways to explain EXCELS-63107’s extreme ionization and high temperatures. Some astronomers propose that unusual stellar evolution pathways, such as binary interactions or rapidly rotating massive stars, could contribute to the observed spectral features. Others suggest that the galaxy may contain an active galactic nucleus (AGN), though current data do not show definitive signs of one. Figure 4 shows how different stellar population models compare to the observed spectral features of EXCELS-63107.

The Search for More Cosmic Oddballs

JWST’s discovery of EXCELS-63107 shows we’ve only begun uncovering the universe’s most primitive galaxies. These galaxies hold clues to early star formation, chemical enrichment, and reionization. Future JWST and ground-based spectroscopy will determine if EXCELS-63107 is unique or part of a broader population.

Astrobite edited by Brandon Pries

Featured image credit: ESA/Webb, NASA, CSA, A. Hirschauer, M. Meixner et al.