A dormant, overmassive black hole in the early Universe
Recent observations made with the NASA/ESA/CSA James Webb Space Telescope (JWST) have found a large number of supermassive black holes already in place in the first few hundred million years after Big Bang. The channels of formation and growth of these early, massive black holes are not clear, with scenarios ranging from heavy seeds to light seeds experiencing bursts of high accretion rate.
An international team of astronomers, led by the University of Cambridge, with participation of Italian scientists from Sapienza University, INAF, Insubria University, and the Scuola Normale Superiore, used JWST to detect a black hole in a galaxy at z=6.68, just 800 million years after the Big Bang. The results of this study have been published in Nature.
The black hole mass is estimated to be 400 million times the mass of our Sun (≈ 4 10^8 Msun), making it one of the most massive black holes discovered by Webb at this redshift. The black hole is “over-massive” as its mass makes up roughly 40% of the total mass of its host galaxy: in comparison, most black holes in the local universe are roughly 0.1% of their host galaxy mass. Interestingly, the black hole appears to be “dormant” as it is accreting mass at a very low rate of only 0.02 times the Eddington limit, the critical rate at which radiation pressure on matter overcomes the gravitational pull of the black hole. Such a dormant, overmassive black hole challenges existing models of how black holes develop.
According to standard models, black holes form from the collapsed remnants of massive or super-massive stars and accrete matter up to the Eddington limit. However, the sheer size of this black hole suggests that standard models may not adequately explain how it forms and grows. Using black hole evolutionary models developed by a former PhD student at Sapienza University, Alessandro Trinca, now postdoc at the Insubria University, under the supervision of Raffaella Schneider (Sapienza University) and Rosa Valiante (INAF), we modelled how this dormant black hole could have grown to such a massive size so early in the universe. We have found that the most likely scenario is that black holes can exceed the Eddington limit for short periods, during which they grow very rapidly, followed by long periods of inactivity: black holes such as this one likely accrete at super-Eddington rates for a few million years, and go inactive for about 100 million years. The short super-Eddington bursts make the black hole reach high masses while allowing it to be dormant for long periods, hence making it more probable to be observed in such a dormant phase.
Due to their low luminosities, dormant black holes are more challenging to detect, but theoretical models suggest that this black hole is almost certainly the tip of a much larger iceberg, if black holes in the early universe spend most of their time in a dormant state.
Authors: 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