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A spectroscopically confirmed, strongly lensed, metal-poor Type II supernova at z = 5.13

This paper reports the discovery of "SN Eos," the farthest spectroscopically confirmed Type II supernova at z = 5.13, which was magnified by gravitational lensing to reveal a metal-poor progenitor in the early Universe and provide direct evidence of massive star formation shortly after reionization.

Original authors: David A. Coulter, Conor Larison, Justin D. R. Pierel, Seiji Fujimoto, Vasily Kokorev, Joseph F. V. Allingham, Takashi J. Moriya, Matthew Siebert, Yoshihisa Asada, Rachel Bezanson, Maruša Bradač, Gabri

Published 2026-01-22

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CC BY 4.0

Original authors: David A. Coulter, Conor Larison, Justin D. R. Pierel, Seiji Fujimoto, Vasily Kokorev, Joseph F. V. Allingham, Takashi J. Moriya, Matthew Siebert, Yoshihisa Asada, Rachel Bezanson, Maruša Bradač, Gabriel Brammer, John Chisholm, Dan Coe, Pratika Dayal, Michael Engesser, Steven L. Finkelstein, Ori D. Fox, Lukas J. Furtak, Anton M. Koekemoer, Thomas Moore, Minami Nakane, Masami Ouchi, Richard Pan, Robert Quimby, Armin Rest, Johan Richard, Luke Robbins, Louis-Gregory Strolger, Fengwu Sun, Tommaso Treu, Hiroto Yanagisawa, Abdurro'uf, Aadya Agrawal, Ricardo Amorín, Joseph P. Anderson, Rodrigo Angulo, Hakim Atek, Franz E. Bauer, Larry D. Bradley, Volker Bromm, Mateusz Bronikowski, Christopher J. Conselice, Christa DeCoursey, James M. DerKacy, Guillaume Desprez, Suhail Dhawan, Jose M. Diego, Eiichi Egami, Andreas Faisst, Brenda Frye, Sebastian Gomez, Mauro González-Otero, Massimo Griggio, Yuichi Harikane, Kohei Inayoshi, Saurabh W. Jha, Yolanda Jiménez-Teja, Jeyhan S. Kartaltepe, Patrick L. Kelly, Lindsey A. Kwok, Zachary G. Lane, Xiaolong Li, Ivo Lobbe, Paulo A. A. Lopes, Ray A. Lucas, Georgios E. Magdis, Nicholas S. Martis, Jorryt Matthee, Ashish K. Meena, Rohan P. Naidu, Gaël Noirot, Masamune Oguri, Estefania Padilla Gonzalez, Massimo Pascale, Tanja Petrushevska, Massimo Ricotti, Daniel Schaerer, Stefan Schuldt, Melissa Shahbandeh, William Sheu, Koji Shukawa, Akiyoshi Tsujita, Eros Vanzella, Qinan Wang, John Weaver, Robert Williams, Rogier Windhorst, Yi Xu, Yossef Zenati, Adi Zitrin

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, dark ocean. For the first billion years of its life, this ocean was filled with a thick, invisible fog that blocked out the light of the very first stars. Then, something happened: the fog cleared, and the universe became transparent. This era is called the "Epoch of Reionization."

For a long time, astronomers wanted to see the individual stars that lived and died during this clearing of the fog, but they were too far away and too dim to see with even our most powerful telescopes. It was like trying to spot a single firefly in a stadium from a hundred miles away.

The Cosmic Magnifying Glass
To solve this, the scientists in this paper used a trick called "gravitational lensing." They found a massive cluster of galaxies acting like a giant, natural magnifying glass in space. Just as a glass lens bends light to make a small object look huge, this galaxy cluster bent the light from a distant explosion, making it appear dozens of times brighter.

The Discovery: SN Eos
Using this cosmic magnifying glass and the James Webb Space Telescope (JWST), the team discovered a supernova named "SN Eos."

What is it? It is a Type II supernova, which is the spectacular, explosive death of a massive star.
How far away? It exploded when the universe was only about 1 billion years old. Because light takes time to travel, we are seeing this explosion as it happened billions of years ago.
The "Ghost" Images: Because of the lensing effect, the team didn't just see one explosion; they saw two images of the same event, appearing side-by-side in the sky. It's like looking at a reflection in a funhouse mirror, where you see the same object twice. One image arrived about a day later than the other.

The "Metal" Mystery
In astronomy, "metals" don't just mean iron or gold; they mean any element heavier than hydrogen and helium. Our sun and Earth are full of these heavy elements because they were forged in previous generations of stars.

The Finding: When the scientists analyzed the light from SN Eos, they found it was extremely metal-poor. The star that exploded was born in an environment where the "heavy stuff" was less than 10% of what we find in our own solar neighborhood.
Why it matters: This is like finding a recipe for a cake that uses almost no sugar or flour, just the basic ingredients. It proves that the first stars were born in a "pristine" universe, before many generations of stars had time to pollute the gas clouds with heavy elements. This provides direct evidence of how the universe chemically evolved from a simple beginning to the complex, element-rich world we live in today.

The Early Warning System
The team also got lucky with timing. Before the JWST could look at SN Eos, the Hubble Space Telescope had taken pictures of the same spot years earlier.

The Flash: Hubble caught a faint, blue glow just a few days after the star exploded. This is called "shock breakout." Imagine a firecracker popping; the initial flash happens before the main boom. Hubble caught that initial flash, which is incredibly rare to see for such a distant object.
The Aftermath: By combining the early flash from Hubble with the detailed "autopsy" of the explosion from JWST, the scientists could build a model of what the star looked like before it died. They found it was likely a red supergiant (a huge, cool star) that had a thick shell of gas around it, which helped create that early flash.

The Big Picture
This paper is a milestone because it is the first time we have clearly identified a supernova from this early era of the universe and measured its chemical makeup. It confirms that the first stars were indeed born in metal-poor environments and helps us understand how the building blocks of life (like carbon and oxygen) were first scattered into the cosmos.

In short, SN Eos is a time capsule. By using a cosmic magnifying glass, astronomers were able to peek inside a star that died when the universe was a toddler, confirming that the early universe was a very different, much "cleaner" place than the one we see today.

Technical Summary: A Spectroscopically Confirmed, Strongly Lensed, Metal-Poor Type II Supernova at z = 5.13

Problem and Context
The Epoch of Reionization (EoR), occurring roughly 400–500 million years after the Big Bang, marks the transition of the Universe from a neutral to an ionized state. Understanding the chemical evolution of the Universe during this era requires direct observations of early stellar populations and their environments. However, individual stars and galaxies at these redshifts ( $z > 5$ ) are typically too faint for detailed spectroscopic study. Core-collapse supernovae (CC SNe), particularly Type II, offer a unique opportunity to probe these early environments because they are intrinsically luminous and their progenitors (massive stars) have short lifetimes, tracing instantaneous star formation. While theoretical models suggest early SNe formed in low-metallicity environments, direct spectroscopic confirmation of metallicity in SNe at $z > 5$ has remained elusive.

Methodology
The authors present the discovery and analysis of "SN Eos," a transient detected by the James Webb Space Telescope (JWST) as part of the VENUS (Vast Exploration for Nascent, Unexplored Sources) treasury program. The discovery leverages the gravitational lensing effect of the MACS 1931.8-2635 galaxy cluster ( $z=0.35$ ), which magnifies background sources.

Discovery and Imaging: SN Eos was identified as a pair of images (101.1 and 101.2) in NIRCam imaging taken on September 1, 2025. Archival Hubble Space Telescope (HST) data from 2024 revealed the transient in rest-frame far-ultraviolet (FUV) filters (F814W and F110W), while 2012 HST data provided non-detections, constraining the explosion time.
Spectroscopy: Follow-up NIRSpec PRISM spectroscopy was obtained on October 8, 2025. The spectra were analyzed to classify the transient and measure spectral features.
Lens Modeling: A parametric lens model for MACS 1931.8-2635 was constructed using 51 multiple images of 19 background sources. This model predicted five images for the SN Eos system, estimating magnifications ( $\mu$ ) and time delays ( $\tau$ ) between them.
Metallicity Analysis: The authors measured the pseudo-equivalent width (pEW) of Fe II $\lambda$ 5018 and $\lambda$ 5169 absorption features. These features are known tracers of gas-phase metallicity in Type II supernovae. The measurements were compared against a grid of local low-metallicity SNe (e.g., SN 2015bs, SN 2017ivv, SN 2023ufx) and solar-metallicity templates (SN 1992H).
Light Curve Modeling: Hydrodynamical models were fitted to the photometry (HST and JWST) to constrain progenitor properties, including explosion energy, nickel mass, and circumstellar material (CSM) interaction.

Key Results

Discovery and Classification: SN Eos is a Type II supernova at a spectroscopic redshift of $z = 5.133 \pm 0.001$ , making it the farthest spectroscopically confirmed supernova discovered to date. The spectra clearly show hydrogen Balmer P-Cygni profiles (H $\alpha$ , H $\beta$ , H $\gamma$ , H $\delta$ ), confirming its classification as a hydrogen-rich core-collapse event.
Lens Properties: The lens model predicts five images for the system. Two images (101.1 and 101.2) were detected in JWST data with high magnification factors of $\mu \approx 25.5$ and $\mu \approx 29.9$ , respectively. The time delay between these two images is predicted to be approximately 1 day (observer frame), with image 101.2 arriving last.
Metallicity Constraints: The Fe II pEW measurements for SN Eos are consistent with those of local, extremely metal-poor SNe ( $Z \lesssim 0.1 Z_{\odot}$ ). The authors infer that the progenitor formed in an environment with a metallicity of $\lesssim 10\%$ of the solar abundance. This is supported by the host galaxy's identification as a faint Lyman- $\alpha$ emitter (LAE) with an absolute UV magnitude of $M_{UV} \approx -14.4$ , which, via the mass-metallicity relationship, suggests a low gas-phase metallicity.
Progenitor and Explosion Physics: Light curve modeling suggests SN Eos is consistent with a Type IIP supernova at the end of its plateau phase. The best-fit model involves a $16 M_{\odot}$ red supergiant progenitor with a hydrogen-rich envelope, an explosion energy of $1.5 \times 10^{51}$ erg, and a synthesized $^{56}$ Ni mass of $0.1 M_{\odot}$ . The early FUV emission detected by HST is modeled as shock breakout or interaction with confined circumstellar material, though the first two UV epochs show excess flux potentially indicating more complex CSM structures.
Spectral Diversity: While SN Eos shares Fe II features with low-metallicity local analogs, its H $\alpha$ and Na I profiles more closely resemble the solar-metallicity SN 1992H. This highlights the intrinsic diversity in SN II properties even at similar metallicities, suggesting that factors beyond metallicity (e.g., progenitor structure, rotation, binary interaction) significantly influence observables.

Significance and Claims
The paper claims that SN Eos provides the first "strong, independent, and direct evidence for the chemical evolution of the Universe" at $z > 5$ . By confirming that a supernova exploded in a metal-poor environment ( $Z \lesssim 0.1 Z_{\odot}$ ) shortly after the EoR, the study validates theoretical expectations that early stellar populations formed from pristine or near-pristine gas.

The authors emphasize that the discovery demonstrates the efficacy of combining strong gravitational lensing with JWST capabilities to probe the faint end of the galaxy luminosity function and the chemical state of the early Universe. They note that SN Eos serves as a critical testbed for local, low-metallicity SNe as analogs for high-redshift events, finding that while metallicity correlates with specific spectral features (like Fe II pEW), it does not solely dictate all spectrophotometric properties. The work opens a new frontier for precision high-redshift transient science, offering a method to constrain the rate of chemical enrichment and the initial mass function in the infant Universe.

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