Even black holes take a nap between meals. An international team of scientists, led by the University of Cambridge, has discovered an ancient supermassive black hole ‘sleeping’ in a compact, relatively dormant galaxy that we see as it was almost 13 billion years ago. The black hole, described in a paper published today in the journal Nature, has a mass 400 million times that of the Sun and dates back less than 800 million years after the Big Bang, making it one of the oldest and most massive objects ever detected.

This mammoth object is also the first non-active supermassive black hole, in terms of matter accretion, observed during the epoch of reionisation, a transitional phase in the early universe during which intergalactic gas was ionised by radiation from the first cosmic sources. It probably represents just the tip of the iceberg of a whole population of ‘resting’ black holes yet to be observed in this distant epoch. The discovery, in which researchers from the Istituto Nazionale di Astrofisica (National Institute of Astrophysics - INAF), Scuola Normale Superiore di Pisa and Sapienza University of Rome are also participating, is based on data collected by the James Webb Space Telescope (JWST) as part of the JADES (JWST Advanced Deep Extragalactic Survey) programme.

How is the black hole ‘dormant’? Using these data, the research team established that, despite its colossal size, this black hole is accreting surrounding matter at a very low rate in contrast to those of similar mass observed at the same time (so-called quasars) - about 100 times lower than the theoretical maximum limit - making it practically inactive.

Another peculiarity of this high redshift black hole (i.e. located in the primordial universe) is its relationship with the host galaxy: its mass represents 40 per cent of the total stellar mass, a thousand times higher than that of black holes normally observed in the nearby universe. Alessandro Trinca, a post-doc researcher now working at the University of Insubria but previously a post-doc at the INAF in Rome for a year, explains: “This imbalance suggests that the black hole had a very rapid growth phase, subtracting gas from the galaxy's star formation. It stole all the gas it had available before going dormant, leaving the stellar component high and dry”. Rosa Valiante, an INAF researcher involved in the international team and co-author of the article, adds: “Understanding the nature of black holes has always been a subject that fascinates the collective imagination: they are apparently mysterious objects that challenge “famous” scientific theories such as those of Einstein and Hawking. The need to observe and understand black holes, from when they form to when they become as massive as billions of times our Sun, drives not only scientific research forward, but also technological advancement”.

Supermassive black holes as old as the one described in the Nature article are a mystery in astrophysics.The rapidity with which these objects grew in the early stages of the Universe's history defies traditional models, which are unable to explain the formation of black holes on such a scale. Under normal conditions, black holes accrete matter up to a theoretical limit, called the ‘Eddington limit’, beyond which the radiation pressure generated by accretion counteracts further flows of material towards the black hole. The discovery of this primordial black hole supports the hypothesis that short but intense phases of accretion known as “super-Eddington” are essential to explain the existence of these “cosmic giants” in the early universe. These are phases during which black holes would be able to ingest matter at a much higher rate, temporarily escaping this limitation, interspersed with periods of dormancy.

“If growth took place at a rate below the Eddington limit, the black hole would have to grow gas continuously over time to hope to reach the observed mass. It would therefore be very unlikely to observe it in a dormant phase”, says Raffaella Schneider, professor at the Department of Physics at Sapienza University.

Scientists speculate that similar black holes are much more common than previously thought, but objects in such a dormant state emit very little light, making them particularly difficult to detect, even with extremely advanced instruments such as the Webb Space Telescope. So how do we find them? Although they cannot be observed directly, their presence is revealed by the glow of an accretion disc forming around them. With the JWST, a telescope of the American (NASA), European (ESA) and Canadian (CSA) space agencies designed to observe extremely dim and distant objects, it will be possible to explore new frontiers in the study of early galactic structures.

Stefano Carniani, researcher at the Scuola Normale Superiore di Pisa and member of the JADES team comments “This discovery opens a new chapter in the study of distant black holes. Thanks to the James Webb images, we will be able to investigate the properties of dormant black holes, which have remained invisible until now. These observations provide the missing pieces to complete the puzzle of galaxy formation and evolution in the early universe”.

The discovery represents just the beginning of a new phase of investigation. The JWST will now be used to detect other similar dormant black holes, helping to unravel new mysteries about the evolution of cosmic structures in the early universe. The observations used in this work were obtained as part of the JWST Advanced Deep Extragalactic Survey (JADES), a collaboration between the Near-Infrared Camera (NIRCam) and Near-Infrared Spectrograph (NIRSpec) instrument development teams, with a contribution also from the US Mid-Infrared Instrument (MIRI) team

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A dormant, overmassive black hole in the early Universe - Ignas Juodžbalis, Roberto Maiolino, William M. Baker, Sandro Tacchella, Jan Scholtz, Francesco D’Eugenio, Raffaella Schneider, Alessandro Trinca, Rosa Valiante, Christa DeCoursey, Mirko Curti, Stefano Carniani, Jacopo Chevallard, Anna de Graaff, Santiago Arribas, Jake S. Bennett, Martin A. Bourne, Andrew J. Bunker, Stephane Charlot, Brian Jiang, Sophie Koudmani, Michele Perna, Brant Robertson, Debora Sijacki, Hannah Ubler, Christina C. Williams, Chris Willott, Joris Witstok – Nature 2024  DOI: 10.1038/s41586-024-08210-5