Hunting for the First Explosions at the High-Redshift Frontier

TL;DR

The paper investigates whether hyper-energetic Pop III PISNe could appear as transients at extreme redshifts ($z\sim30$) within JWST surveys. Using cosmological simulations of an overdense region capable of forming Pop III stars and PISNe, it estimates PISN rates from the Pop III IMF and star formation history, and evaluates observability with JWST across realistic survey volumes and durations. The results suggest that, while rare, such PISNe could occur with non-negligible probability in biased regions, potentially yielding about 0.1 observable events in the tail of the distribution, and bright PISN phases could be detectable by JWST in deep fields for roughly two decades in the observer frame. This work highlights the exciting possibility of probing the epoch of first star formation through transient events and underscores the need for coordinated, deep, and time-domain JWST observations, complemented by future wide-field surveys to maximize discovery potential.

Abstract

The James Webb Space Telescope (JWST) has spectroscopically confirmed galaxies up to $z\sim14$, 300 Myr after the Big Bang, and several candidates have been discovered at $z\sim15-25$, with one candidate as high as $z\sim30$, only 100 Myr after the Big Bang. Such objects are unexpected, since theoretical studies have not predicted the existence of detectable galaxies at $z\sim30$. While any $z\sim30$ candidates may be contaminants at lower redshifts, we explore whether such extreme redshift sources could be consistent with hyper-energetic transient events linked to the formation of the first, metal-free, stars. Specifically, we consider pair-instability supernovae (PISNe), a predicted class of extreme thermonuclear explosions that leave no remnant behind. Using cosmological simulations, we investigate an overdense cosmic region, where star formation and subsequent PISNe occur at $z\sim30-40$, even within standard cosmology. Assessing the likelihood of such a region, the corresponding number of PISNe at $z\gtrsim20$, and their observed flux, we find that JWST has a non-negligible chance to detect a PISN event at extremely high redshifts. If a transient event were confirmed at $z\sim30$, this would provide a direct glimpse into the epoch of first star formation, dramatically extending the empirical reach of astronomy.

Figures (4)

• Figure 1: Biased, overdense simulation volume. Left: The most massive dark matter halo in our simulation with $M_h\simeq1.2\times10^8$ M$_\odot$ at $z=30.4$ and the corresponding peak height $\nu\simeq5$. The corresponding number densities are also shown, giving $8\times10^{-7}$ Mpc$^{-3}$ dex$^{-1}$ for the most massive halo, based on the GUREFT halo mass function Yung2024. A similar number density of $\sim10^{-6}$ Mpc$^{-3}$ dex$^{-1}$ is derived when using the Sheth1999 halo mass function. The overdense region is unlikely but not impossible to encounter in the early Universe. Right: The projected gas density distribution of our simulation volume at $z\sim30.4$. The length is in physical units. Stellar particles are marked as black dots and the most massive halo as an orange circle, with sizes not to scale, showing that SF and the subsequent transient events can occur in such highly biased regions within the early Universe.
• Figure 2: PISN rate across redshift inferred from our simulation. The shaded region reflects the range due to Pop III IMF variation. We compare to predictions from previous work Weinmann2005Hummel2012, scaled to our simulation volume. Hummel2012 consider an average-density region of the Universe, resulting in rates that are lower than for the biased case considered here. Weinmann2005 predict a much higher rate, but with substantial uncertainties in their model assumptions. Across the whole redshift range spanned by the simulations $(99<z<22)$ and accounting for visibility time, cumulatively we expect $\sim10^{-1}$ events (see Section \ref{['sec:eventprob']}). Thus, JWST could detect a PISN event as observations continue, at the tail-end of the probability distribution.
• Figure 3: Model PISN spectra from a source at $z=30.4$. We reproduce spectra from Kasen2011, corresponding to their B250 model, a PISN from a 250 M$_\odot$ metal-free blue supergiant, and the R250 model, a PISN from a 250 M$_\odot$ low-metallicity ($10^{-4}$ Z$_\odot$) red supergiant. We show the situation at breakout, when the explosion shock wave first reaches the surface, and at subsequent times, when the expanding and cooling ejecta produce bright emission. We display the spectra at peak luminosity following the breakout (17 days after breakout for the red and 383 days for the blue supergiant), and for the red supergiant case 100 and 300 days after the explosion. We compare the model spectra with the observed photometry of Capotauro, proposed to be at $z\sim32$Gandolfi2025. The observed photometry and upper limits are comparable to the predicted PISN spectra. Thus, if PISN events do occur at extremely high redshifts, they could be bright enough to be observable.
• Figure 4: Brightness evolution of the model PISN spectra from Kasen2011 at $z=30.4$ across restframe days/observed years after the PISN explosion for the red and blue supergiant cases. We measure magnitudes around 1500 and 4000 Å restframe, corresponding to wavelengths observable by JWST's NIRCam (1500 Å) and MIRI (4000 Å) instruments. We further show the magnitude limits of different JWST surveys. At their peak, PISN magnitudes are around 28-29, lasting for $\sim200$ days in the restframe and $\sim20$ years in the observed frame, which existing JWST surveys like CEERS, JADES, and the MIDIS have reached Finkelstein2025Ostlin2025Eisenstein2023. Therefore, PISNe at extremely high redshifts may be observable with JWST, at least at peak brightness in their lightcurves.