Supermassive Star Explosions in Primordial Halos Produce Detectable Luminosity Signatures

The rapid emergence of supermassive black holes in the early universe presents a significant puzzle for astronomers, and a new study explores a potential origin story involving exploding, supermassive Population III stars. Cédric Jockel from the Max Planck Institute for Gravitational Physics, Kyohei Kawaguchi from the University of Tokyo, and Sho Fujibayashi from Tohoku University, alongside their colleagues, investigate whether these primordial stars, growing to immense sizes, could have collapsed and exploded as supernovae, seeding the first black holes. Their research demonstrates that these explosions would appear as unusually persistent, bright sources detectable by current and upcoming telescopes, including the James Webb Space Telescope, Euclid, and the Roman Space Telescope. Crucially, the deep-field observations from these instruments could not only identify these events, but also place strong constraints on how frequently such supermassive stars exploded in the early universe, potentially resolving the mystery of rapid black hole formation.

Recently discovered supermassive black holes with masses of approximately 10⁸ M_☉ at redshifts z∼9, 11 in active galactic nuclei (AGN) present significant challenges to our understanding of supermassive black hole formation. One proposed pathway involves the rapid accretion of gas onto supermassive Population III stars (SMSs) forming within large primordial gas halos. These Population III stars, the first stars to form in the universe, are theorised to have been significantly more massive than stars observed today, lacking the heavier elements that limit stellar growth in later epochs. The formation of these SMSs, and their subsequent collapse into seed black holes, remains a key area of research in cosmology and astrophysics, as it attempts to explain the unexpectedly early appearance of supermassive black holes.

SMS Explosions Produce Prolonged Light Bursts

Recent research investigates the possibility of detecting extremely massive stars, known as supermassive stars (SMSs), that existed in the early universe. These stars, far larger than any observed today, are theorised to have formed through the rapid accumulation of gas in primordial halos. The process, known as direct collapse, circumvents the typical fragmentation that limits the mass of stars in metal-rich environments. The study focuses on the bright, energetic explosions that would have marked the end of these stars’ lives, and whether these events could be observable with modern telescopes. Unlike typical supernovae, the death of an SMS is predicted to be a pair-instability supernova, where the core collapses directly into a black hole without a neutron star phase, releasing immense energy. The research team developed a detailed model to simulate the aftermath of an SMS explosion, tracking how the ejected material interacts with the surrounding gas.

This model predicts that these explosions would produce a prolonged burst of light, lasting for thousands to millions of years, appearing as a quasi-persistent source rather than a fleeting event. This extended duration arises from the continuous emission of energy as the expanding ejecta collide with and heat the surrounding primordial gas. The luminosity of these explosions is significantly higher than typical supernovae, reaching peak values exceeding 10⁴⁵ erg/s. The simulations indicate that these SMS explosions could reach exceptional luminosities, bright enough to be detected by the James Webb Space Telescope (JWST) at distances of up to 10 billion light-years. Furthermore, future telescopes like Euclid and the Roman Space Telescope are predicted to be even more sensitive, capable of detecting these explosions at even greater distances and in larger numbers. Specifically, researchers estimate that Euclid could image hundreds of these events, while the Roman Space Telescope could detect dozens, allowing for a detailed census of SMSs in the early universe. The ability to observe these ancient explosions would provide crucial insights into the formation of supermassive black holes and the reionization of the early universe, offering a unique window into the cosmos’s formative years. Detecting these events would also help constrain the parameters of SMS formation, such as the mass of the progenitor star and the density of the surrounding gas.

Supermassive Star Explosions Mimic Distant Galaxies

This research investigates the potential signatures of exploding supermassive stars (SMSs) at very high redshifts, offering a possible explanation for recently discovered, faint objects resembling low-redshift active galactic nuclei. These high-redshift objects, detected by JWST, are unexpectedly bright for their redshift, challenging existing models of galaxy formation. The team developed a model to predict the light curves produced by the energetic explosions of these SMSs as their ejecta interact with surrounding gas. The model incorporates radiative transfer calculations to accurately simulate the emission of light from the expanding ejecta and the surrounding gas. The results demonstrate that these explosions can be remarkably bright, reaching luminosities comparable to observed high-redshift objects, and can remain visible for up to 250 years when accounting for cosmic time dilation. Cosmic time dilation, a consequence of the expanding universe, stretches the duration of events observed at high redshifts, making them appear longer than they would at lower redshifts.

The study suggests that Euclid and the Roman Space Telescope could potentially image hundreds of these SMS explosions, allowing astronomers to constrain their frequency and better understand the formation of supermassive black holes. The team also explored how the interaction of the explosion with the surrounding gas affects the emitted light, finding that certain conditions could lead to a bluer spectrum and significant ionisation of the surrounding material. Ionisation occurs when energetic photons strip electrons from atoms, creating a plasma. The model predicts that the spectra of these SMS explosions will be distinct from those of typical galaxies or quasars, allowing for their identification. While the current model simplifies some aspects of this complex environment, such as the detailed chemical composition of the primordial gas, it provides a crucial framework for interpreting observations and distinguishing SMS explosions from other potential sources at high redshifts. Future work will focus on incorporating more realistic physics into the model, including the effects of magnetic fields and turbulence.