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

Abstract: Observing supernovae (SNe) in the early Universe (z > 3) provides a window into how both galaxies and individual stars have evolved over cosmic time, yet a detailed study of high-redshift stars and SNe has remained difficult due to their extreme distances and cosmological redshifting. To overcome the former, searches for gravitationally lensed sources allow for the discovery of magnified SNe that appear as multiple images - further providing the opportunity for efficient follow-up. Here we present the discovery of "SN Eos": a strongly lensed, multiply-imaged, SN II at a spectroscopic redshift of z = 5.133 +/- 0.001. SN Eos exploded in a Lyman-α emitting galaxy when the Universe was only ~1 billion years old, shortly after it reionized and became transparent to ultraviolet radiation. A year prior to our discovery in JWST data, archival HST imaging of SN Eos reveals rest-frame far ultraviolet (~1,300Å) emission, indicative of shock breakout or interaction with circumstellar material in the first few (rest-frame) days after explosion. The JWST spectroscopy of SN Eos, now the farthest spectroscopically confirmed SN ever discovered, shows that SN Eos's progenitor star likely formed in a metal-poor environment (<= 0.1 Z_{\odot}), providing the first direct evidence of massive star formation in the metal-poor, early Universe. SN Eos would not have been detectable without the extreme lensing magnification of the system, highlighting the potential of such discoveries to eventually place constraints on the faint end of the cosmic star-formation rate density in the very early Universe.

• The paper confirms SN Eos as a lensed, metal-poor Type II supernova at z=5.13, marking the farthest spectroscopically identified event of its kind.
• It employs high-resolution JWST, HST, and VLT/MUSE spectroscopy to measure metal diagnostics and constrain explosion parameters with precision.
• The findings provide critical insights into early Universe star formation, progenitor characteristics, and the chemical enrichment process during the epoch of reionization.

Strong Lensing Discovery of a Metal-Poor Type II Supernova at $z=5.13$

Introduction and Scientific Context

This study presents the detection and spectroscopic confirmation of SN Eos, a strongly lensed, multiply-imaged Type II (core-collapse) supernova at $z=5.133\pm0.001$ (2601.04156). This event represents the farthest spectroscopically confirmed supernova to date and provides direct constraints on the properties of massive stellar explosions in the early Universe. The supernova is located in a faint, Lyman-$\alpha$ emitting galaxy (LAE), when the Universe was approximately 1 Gyr old, closely following the completion of reionization.

Gravitational lensing by the MACS galaxy cluster yielded a magnification of $\mu\sim30$ at the SN positions, enabling the detection of a system that would otherwise be inaccessible with current instrumentation. The spatial and temporal lensing pattern produced five predicted images with measurable time delays, two of which (101.1, 101.2) display the SN Eos event.

Figure 1: JWST discovery imaging of MACS including multiple, highly magnified images of SN~Eos near the cluster critical curve; insets display detected images 101.1 and 101.2.

SN Eos resides in an environment with direct spectroscopic and photometric evidence for extremely low metallicity, inferred to be $\lesssim0.1\,Z_\odot$. This work demonstrates the value of combining cluster lensing, high-resolution JWST and HST photometry, NIR spectroscopy, and archival IFU data to probe the high-redshift transient universe and gather constraints on early cosmic star formation, metal enrichment, and stellar evolution.

Multiband Observations and Host Galaxy Characterization

The detection strategy leverages continuous time-domain monitoring by JWST and key archival data from HST (rest-frame FUV). Multiple imaging and spectroscopic epochs confirm SN Eos as a hydrogen-rich Type II SN based on prominent Balmer P-Cygni profiles, and unambiguously tie the transient to its $z=5.133$ host.

Archival MUSE/IFU observations reveal the host galaxy’s Ly$\alpha$ emission at $z_{spec}=5.133\pm0.001$, consistent with the SN redshift.

Figure 2: VLT/MUSE observations yield detection of Ly$\alpha$ emission from multiple host images, confirming redshift for both galaxy and SN.

The host LAE is extremely faint ($M_{UV}\approx -14.4$ after magnification correction) and compact, with photometric and line data indicative of vigorous star formation and low metallicity, as is typical for LAEs in the EoR. This is in line with recent JWST studies which highlight the prevalence of low-mass, low-metallicity galaxies at high $z$ [Saxena2024AAP, Willott2025ApJ, vanzella_2024].

Spectroscopy and Metallicity Diagnostics

Spectroscopic data from JWST/NIRSpec for both 101.1 and 101.2 confirm the SN II classification, via clear Balmer P-Cygni features, and exhibit additional metallic and alpha-element lines (O, Na, Ca). The high signal-to-noise and the spatially resolved nature of the data allow comparison of the two images, showing negligible temporal evolution within the measured $\sim1$-day lensing delay.

Comparison to local well-studied SNe II—IIP subtypes including the metal-poor SN 2015bs, SN 2017ivv, SN 2023ufx ($Z\lesssim 0.1~Z_\odot$), and solar-metallicity SN 1992H—demonstrates spectral similarity in Fe-group and Balmer lines, despite diversity in other features.

Figure 3: Combined JWST/NIRSpec spectra of SN Eos compared to local SNe~II at varying metallicities, demonstrating that the Fe~II complex in SN~Eos is best matched by $Z\lesssim0.1~Z_\odot$ events.

Pseudo-equivalent width (pEW) measurements of Fe~II~$\lambda5018$ and $\lambda5169$ are consistent with those of local low-$Z$ SNe IIP. Phase-corrected values yield an upper limit on progenitor metallicity $Z\lesssim 0.1\,Z_\odot$, supporting the expectation that massive stars formed in the early Universe in pristine environments.

Figure 4: Evolution of Fe~II~$\lambda5018$ pEW for SN~Eos and comparison SNe illustrating the correspondence of SN~Eos to the lowest metallicity regime.

Light Curve Modeling and Explosion Properties

The rest-frame FUV rise in archival HST photometry is interpreted as shock breakout or circumstellar interaction within days of explosion, rare for SNe II due to rapid UV decline. Hydrodynamical light curve modeling (using MESA+STELLA) with low-$Z$ red supergiant progenitors, supplemented with finite confined CSM, successfully reproduces both the early UV excess and subsequent plateau, constraining explosion time to within five rest-frame days of earliest detection. The model requires a 16 $M_\odot$ RSG, explosion energy $1.5 \times 10^{51}$ erg, and $0.1\,M_\odot$ of $^{56}$Ni with envelope masses and CSM consistent with other luminous SNe IIP.

Figure 5: Archival HST rest-FUV imaging shows the early (within days) UV detection of SN Eos, establishing tight constraints on explosion timing.

Figure 6: HST+JWST photometry and hydrodynamical model fits for SN~Eos, indicating a high-luminosity, long plateau typical of SNe~IIP in metal-poor environments.

Implications for Population Synthesis, IMF Evolution, and Chemical Enrichment

The discovery of SN Eos in such a low-metallicity host directly supports hierarchical galaxy formation models and the predicted prevalence of massive star formation in metal-poor environments at high $z$ [karlsson_2013, hutter_imf_2025]. The event’s properties suggest little metallicity dependence for most SNe II spectrophotometric observables, with the exception of Fe~II pEW, in accordance with [Anderson2016_SNeIIP_metallicity_probes].

SN Eos’ confirmation at $z=5.13$ yields the first true test of the analogy between local metal-poor SNe and their high-$z$ counterparts, implying that rare low-$Z$ SNe in the local universe are indeed viable proxies. These data furnish a direct probe of the faint-end star formation rate and chemical enrichment history around the EoR.

The utility of strong gravitational lensing in combination with JWST observing strategy is manifest: events such as SN Eos provide unique leverage for exploring the elusive population of low-luminosity, metal-poor, high-$z$ galaxies and their transient phenomena.

Conclusions

This work establishes several key results:

• Confirmation of a Type II SN at $z=5.133$, the highest redshift spectroscopically characterized SN to date, with direct metallicity measurement via Fe~II pEW;
• The environment of SN Eos is an ultra-faint, LAE galaxy with $M_{UV}\approx -14.4$, supporting models of early low-metallicity galaxy populations;
• High S/N JWST/NIRSpec spectra definitively match SN Eos to local, low-metallicity SNe~IIP, and its photometric/spectroscopic diversity underscores complex dependencies on progenitor and explosion physics at fixed $Z$;
• The discovery enables robust inferences regarding the early Universe star formation, the shape of the IMF at low $Z$, and the time-dependent metal enrichment of the IGM;
• The study demonstrates the power of lensing magnification for future high-$z$ transient studies, advancing the prospects for constraining cosmic star-formation and chemical evolution across the first gigayear.

This event paves the way for statistical studies of SNe and hosts at $z>5$ with upcoming lensing surveys and JWST/ELT-class follow-up, with the potential to calibrate SNe II as metallicity probes and further unravel the nature of the earliest stars and galaxies.