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A rapidly growing black hole in the first billion years
A team of astronomers has identified a supermassive black hole in the quasar host galaxy RACS J0320-35 that appears to be accreting matter at roughly 2.4 times the theoretical Eddington limit. Located about 920 million years after the Big Bang, this object offers rare observational evidence of so-called super-Eddington accretion — a short-lived, extreme feeding phase that may explain how the first supermassive black holes reached masses of millions to billions of solar masses so early in cosmic history.
Discovery and multiwavelength observations
RACS J0320-35 was first flagged in deep X-ray observations from NASA’s Chandra X-ray Observatory in 2023, where it stood out as unusually bright for an object within the first billion years after the Big Bang. The initial X-ray detection prompted follow-up radio observations using the Giant Metrewave Radio Telescope (GMRT), the Australia Telescope Compact Array (ATCA), and the Australian Long Baseline Array (LBA). Combining X-ray and radio data with other available photometry allowed researchers to reconstruct the source’s spectral energy distribution (SED) across the electromagnetic spectrum.
An artist's impression of RACS J0320-35 blazing with light. (NASA/CXC/SAO/M. Weiss)
Detailed SED modelling showed a close match to templates for super-Eddington accretion discs. The lead author, astrophysicist Luca Ighina (Harvard & Smithsonian Center for Astrophysics), and collaborators compared the observed emission across radio, optical/infrared and X-ray bands against theoretical predictions for accretion rates that exceed the Eddington limit, finding consistency with a rate about 2.4 times higher than the classical maximum.
What is the Eddington limit and why breaking it matters
The Eddington limit defines the maximum steady luminosity (and therefore mass-accretion rate) at which outward radiation pressure from infalling gas balances the inward pull of gravity. In simple terms, if a black hole’s accretion disk shines too brightly, radiation pressure can blow material away and halt further growth. Super-Eddington accretion describes transient phases where infall temporarily outpaces radiation feedback, enabling rapid mass gain.
Super-Eddington episodes are a leading theoretical solution to a long-standing puzzle in cosmology: how did supermassive black holes form so quickly when steady, Eddington-limited growth would require far longer than the age of the early universe allows? Observational confirmation of super-Eddington accretion, even for individual objects, strengthens models in which early black holes grow in intense, short bursts or form from massive initial seeds.
Implications for black hole formation and future observations
If the RACS J0320-35 measurements hold up under further scrutiny, the result will provide a valuable calibrator for formation scenarios of primordial supermassive black holes. By estimating the current mass and instantaneous growth rate, researchers can extrapolate backward to constrain plausible seed masses and formation channels — for example, whether seeds formed from direct collapse of massive gas clouds or from the remnants of the first generations of stars.
Co-author Alberto Moretti (INAF-Osservatorio Astronomico di Brera) notes that measuring both mass and growth rate for objects like RACS J0320-35 enables meaningful tests of competing formation models. Thomas Connor (Harvard & Smithsonian Center for Astrophysics) adds that individual extreme quasars provide critical boundary conditions for simulations of early structure formation.
Future work will require deeper spectroscopy, higher-resolution imaging, and additional multiwavelength monitoring to confirm the accretion geometry, rule out alternate explanations (for example, lensing or beaming effects), and measure the black hole mass more precisely. Instruments such as the James Webb Space Telescope, next-generation X-ray observatories, and very long baseline interferometry at radio wavelengths are poised to refine the picture.
Conclusion
RACS J0320-35 stands as a promising observational case of super-Eddington growth in the early universe. If validated, it helps bridge the gap between theory and observation by demonstrating a viable rapid-growth pathway for the earliest supermassive black holes, and it will guide future searches for similar extreme accretors in the young cosmos.
Source: sciencealert
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