Signatures of Exploding Supermassive PopIII Stars at High Redshift in JWST, EUCLID and Roman Space Telescope

Recently discovered supermassive black holes with masses of $\sim10^8,M_\odot$ at redshifts $z\sim9$-$11$ in active galactic nuclei (AGN) pose severe challenges to our understanding of supermassive black hole formation. One proposed channel are rapidly accreting supermassive PopIII stars (SMSs) that form in large primordial gas halos and grow up to $<10^6,M_\odot$. They eventually collapse due to the general relativistic instability and could lead to supernova-like explosions. This releases massive and energetic ejecta that then interact with the halo medium via an optically thick shock. We develop a semi-analytic model to compute the shock properties, bolometric luminosity, emission spectrum and photometry over time. The initial data is informed by stellar evolution and general relativistic SMS collapse simulations. We find that SMS explosion light curves reach a brightness $\sim10^{45\mathrm{-}47},\mathrm{erg/s}$ and last $10$-$200$ years in the source frame - up to $250$-$3000$ years with cosmic time dilation. This makes them quasi-persistent sources which vary indistinguishably to little red dots and AGN within $0.5$-$9,(1+z)$ yrs. Bright SMS explosions are observable in long-wavelength JWST filters up to $z\leq20$ ($24$-$26$ mag) and pulsating SMSs up to $z\leq15$. EUCLID and the Roman space telescope (RST) can detect SMS explosions at $z<11$-$12$. Their deep fields could constrain the SMS rate down to $10^{-11}$Mpc$^{-3}$yr$^{-1}$, which is much deeper than JWST bounds. Based on cosmological simulations and observed star formation rates, we expect to image up to several hundred SMS explosions with EUCLID and dozens with RST deep fields.

💡 Research Summary

The paper addresses the puzzling discovery of super‑massive black holes (SMBHs) with masses ≳10⁸ M⊙ at redshifts z≈9–11, which are difficult to explain with standard stellar‑mass seed growth scenarios. The authors focus on an alternative pathway: the formation and collapse of super‑massive Population III stars (SMSs) in atomically‑cooled halos (ACHs). In this scenario, rapid gas inflow (∼1 M⊙ yr⁻¹) allows a primordial protostar to grow to 10⁵–10⁶ M⊙. When the star reaches a critical mass, general‑relativistic instability (GRI) triggers a monolithic collapse of the core. Recent three‑dimensional general‑relativistic simulations (Fujibayashi et al. 2025) show that the collapse can eject several thousand solar masses of material at ≈0.2 c, releasing an explosion energy of 10⁵⁵–10⁵⁶ erg.

The authors develop a semi‑analytic light‑curve model based on the framework of Suzuki & Maeda (2017), which describes mildly relativistic ejecta interacting with a dense circumstellar medium (CSM). The ejecta are assumed to expand homologously (βₑⱼ = r/ct) and to possess kinetic energy Eₖ≈10⁵⁵–10⁵⁶ erg and mass Mₑⱼ≈10³–10⁴ M⊙. The surrounding CSM is modeled as a wind‑like density profile ρ∝r⁻² with characteristic density ρ₀≈10⁻¹⁸–10⁻¹⁶ g cm⁻³ at a scale radius of ∼10¹⁸ cm, reflecting the dense gas in an ACH.

When the ejecta collide with the CSM, a forward and reverse shock form a thin, optically thick shell. Radiation generated by the shock is initially trapped (optical depth τ≫1) and diffuses outward as the shell expands. The diffusion time t_diff≈τR/c, together with adiabatic expansion, determines the light‑curve evolution. The model predicts a bolometric luminosity peak of L≈10⁴⁵–10⁴⁷ erg s⁻¹ occurring at t≈10–30 yr in the source frame, followed by a gradual decline roughly as L∝t⁻⁰·⁵. The total bright phase lasts 10–200 yr in the source frame, which, after time dilation, corresponds to 250–3000 yr in the observer frame for z≈10–20. Thus the transients appear quasi‑persistent, potentially masquerading as “little red dots” (LRDs) or low‑luminosity AGN.

Spectrally, the early emission is dominated by thermal blackbody radiation with temperatures of 10⁴–10⁵ K, cooling to a few thousand kelvin over the bright phase. Consequently, the most favorable observational windows are the near‑ and mid‑infrared bands. Using the model spectra, the authors compute synthetic photometry for JWST (NIRCam, MIRI), EUCLID, and the Roman Space Telescope (RST). They find that JWST long‑wavelength filters (e.g., F200W, F277W, F356W, F560W) can detect these explosions out to z≈20 with AB magnitudes of 24–26. EUCLID and RST can reach z≈11–14 in their deep fields, with AB limits ≈26–27.

To assess detectability, the authors estimate the SMS explosion rate by combining cosmological simulations of ACH formation with the fraction of halos that produce SMSs. They obtain a volumetric rate of ≈10⁻¹¹ Mpc⁻³ yr⁻¹. Applying the survey parameters of EUCLID (deep field ≈40 deg², AB≈26) and RST (deep field ≈10 deg², AB≈27), they predict that EUCLID could observe a few hundred SMS explosions, while RST could detect several dozen. These numbers are an order of magnitude higher than what JWST single‑pointing surveys could achieve.

The paper also discusses distinguishing features between SMS explosions, LRDs, and high‑z AGN. The intense shock radiation can ionize the surrounding CSM, producing Balmer‑continuum absorption that reddens the spectrum relative to typical AGN. Color‑color diagrams in JWST and Roman filters show separations of ≈0.2–0.5 mag, offering a potential diagnostic.

Limitations of the study include the assumption of spherical symmetry, a simple power‑law CSM density, neglect of line emission (e.g., He II, Lyα) and detailed radiative transfer in the optically thin phase, and reliance on a 1‑D semi‑analytic framework. The authors suggest future work with full 3‑D radiation‑hydrodynamics simulations and cross‑validation against actual JWST observations.

In summary, the work provides a physically motivated prediction that the collapse of super‑massive Pop III stars can generate ultra‑bright, long‑lived transients observable with JWST, EUCLID, and Roman out to the epoch of reionization and beyond. Detecting such events would offer a direct probe of the heavy‑seed channel for SMBH formation and open a new window on the earliest phases of star and galaxy evolution.