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

TL;DR

The study proposes that rapidly accreting SMSs collapsing via general relativistic instability can drive massive ejecta that collide with a dense CSM, generating an optically thick, diffusion-dominated shock with luminous emission lasting 10–200 years in the source frame and longer due to time dilation. A semi-analytic, energy-conserving light-curve model is developed to predict bolometric luminosities and spectra, incorporating ejecta/CSM dynamics, diffusion, recombination, and partial ionisation of the CSM by Balmer-continuum photons. Predictions indicate bright, high-redshift transients observable by JWST in long-wavelength bands up to $z o20$, with EUCLID and RST capable of constraining SMS explosion rates down to $oxed{10^{-11} ext{ Mpc}^{-3} ext{ yr}^{-1}}$ in certain redshift ranges, enabling tests of SMS formation channels against star-formation histories and simulations. The work also discusses observational signatures that help distinguish SMS explosions from LRDs and AGN, and outlines substantial avenues for model refinement via improved initial SMS data, radiation-hydrodynamics, and spectral modelling.

Abstract

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.

Figures (17)

• Figure 1: Summary of the main phases of an exploding supermassive star (SMS). Phase 1: Just before the onset of the general relativistic instability (GRI), the SMS (with mass $M_\mathrm{SMS}$) has an approximate two-zone structure consisting of a roughly chemically homogeneous and dense core region with mass $M_\mathrm{c}$ and of a low-density bloated atmosphere with mass $M_\mathrm{atm}$. The mass fraction of the core and atmosphere depend on the accretion rate and the SMS rotation. The core mass fraction is largest for large rotation and small accretion rates. It is smallest for slow rotation and large accretion rates. Phase 2: A black hole (BH) is formed and part of the core mass, $M_\mathrm{eje,c}$, is ejected at mildly relativistic velocities. This can happen either because infalling matter bounces off an BH accretion torus (see Fujibayashi:2024vnb) or from the violent release of fusion energy (see Nagele:2024aev). This material then sweeps through the bloated atmosphere. Phase 3: The ejected material shock wave reaches the initial SMS surface, $R_0$, and has picked up the whole atmosphere material, which is now unbound and homologously expanding. Phase 4: The total ejecta mass, now consisting of the combined core ejecta mass and atmosphere mass ($M_\mathrm{eje}=M_\mathrm{eje,c}+M_\mathrm{atm}$), starts to interact with the surrounding circumstellar medium (CSM) via a shock. The shocked material is hot and optically thick end emits thermal radiation. It cools down and picks up additional mass over time. Later, the material becomes transparent and non-thermal radiation will be released at the shock position.
• Figure 2: Summary of the radial density distribution of the ejecta (left), of the shocked shell (middle), and of the circumstellar medium (CSM) (right). The densities and velocities of the shock shell at the forward and reverse shock positions ($R_\mathrm{fs}$ and $R_\mathrm{rs}$) are obtained using the shock jump conditions (Eq. (\ref{['eq:light-curve-model:jump-conditions-forward-shock']}) and Eq. (\ref{['eq:light-curve-model:jump-conditions-reverse-shock']})).
• Figure 3: Bolometric luminosity as a function of time of the SMS models from Tab. \ref{['tab:results:initial-parameters-accreting-SMS']} in the source frame. Left panel: Models where the radiation is always fully thermalised (i.e. $\eta<1$). The phase where the shock is optically thick is marked using a continuous line. Dotted lines denote the phase where the shock is optically thin and we observe non-thermal radiation. The shaded regions mark observed luminosities of two high-redshift AGN; we put them here as a comparison. Right panel: Same as the left panel, but for models where the radiation is not fully thermalised at some point during the evolution (i.e. $\eta>1$).
• Figure 4: Maximum value of $\eta$ that can be achieved during the optically thick shock phase (excluding the initial sharp peak) as a function of the total ejecta mass $M_\mathrm{eje}$ and the kinetic energy of the ejecta $E_\mathrm{kin,eje}$. The symbols mark the SMS models from Tab. \ref{['tab:results:initial-parameters-accreting-SMS']}: diamonds denote collapsing H models, squares denote collapsing He models, the star denotes the full SMS fusion powered explosion, and circles denote pulsating models. The circled symbols mark models where $\eta>1$ during their evolution. Black curves indicate contours of equal values of $\eta$. The solid curve marks $\eta=1$. The gray striped/shaded region marks configurations where the shock luminosity in the optically-thin phase is larger than the diffusion luminosity from the optically thick phase. It thus marks the range of validity of our model.
• Figure 5: Peak bolometric luminosity $L_\mathrm{bol,peak}$ in the optically thick shock phase (excluding the initial sharp peak) as a function of the total ejecta mass $M_\mathrm{eje}$ and the kinetic energy of the ejecta $E_\mathrm{kin,eje}$. The symbols mark the SMS models from Tab. \ref{['tab:results:initial-parameters-accreting-SMS']}. They have the same meaning as in Fig. \ref{['fig:results:eta-parameter-grid-Ekin-Meje']}. Dashed curves indicate contours of equal peak bolometric luminosity. The gray striped/shaded region marks configurations outside the range of validity of our model (same as Fig. \ref{['fig:results:eta-parameter-grid-Ekin-Meje']}).