Since the James Webb Space Telescope became operational, astronomers have been able to look further and further back into the early days of the cosmos. Now a team has managed to track down the most distant and oldest black hole to date. It is the central black hole of a galaxy that existed 400 million years after the Big Bang. The intense radiation from its center indicates that there lies a matter-devouring black hole of around a million solar masses. This active galactic nucleus also appears to be sucking in more mass than the so-called Eddington limit dictates. This could explain how such black holes were able to grow so large so quickly after the Big Bang.
Supermassive black holes at the heart of galaxies can range from hundreds of millions to billions of solar masses. But how such gravity giants came into being is not clearly understood. According to a common theory, such supermassive giants arise from stellar black holes that gradually grow larger by devouring matter and merging. But observations from the early cosmos raise doubts about this. Astronomers have discovered several quasars that weighed more than a billion solar masses less than a billion years after the Big Bang. Because the growth rate of a black hole is limited by the so-called Eddington limit, these early giants could not actually have grown through slow accretion. To explain this, alternative mechanisms are being discussed, including the merger of several early “hole seedlings” or the collapse of massive gas clouds directly into precursors of supermassive black holes weighing 10,000 to 100,000 solar masses. Accretion beyond the Eddington limit is also discussed.
An early galaxy with an active core
Astronomers hope to find answers to what mechanism caused the cosmic gravity giants to grow so quickly by searching for “hole seedlings” in the early cosmos. If they catch them “in the act,” so to speak, it could reveal how the early black holes grew. The most important tool for these investigations is the James Webb telescope with its high-resolution infrared spectrometer NIRSpec. This is because it can capture and break down the spectral signatures generated by the activity of the black hole. It was not until November 2023 that astronomers managed to detect the most distant black hole to date. It lies in the galaxy UHZ1, which existed 470 million years after the Big Bang and appeared unusually large and massive for its rather slender galaxy. This led to speculation as to whether this black hole might have been formed by the direct collapse of a gas cloud.
But now astronomers led by Roberto Maiolino from the University of Cambridge have discovered an even older black hole. For their study, they used the NIRSpec spectrometer to examine the distant but unusually bright galaxy GN-z11 in more detail. Initial observations with the Hubble Space Telescope and the James Webb Telescope had already shown that this galaxy has a bright core region with a surrounding, less luminous disk. However, it remained unclear whether the glow was caused by strong star formation or by an active black hole. The new, high-resolution spectral analyzes have now brought more clarity. They show several spectral lines, including a double neon IV line, typical of active galactic nuclei (AGN). “NeIV is a clear AGN tracer because this line requires photons with energies of more than 63.5 electron volts,” explain Maiolino and his colleagues. She was also able to identify a carbon line typical of AGN.
Accretion beyond the Eddington limit?
The astronomers conclude that the galaxy GN-z11 hosts an active black hole – the oldest known to date. The galaxy and black hole therefore existed 400 million years after the Big Bang. “The AGN scenario also provides a natural explanation for the exceptional luminosity of GN-z11,” the team said. The strong emission of radiation from the black hole, which is rapidly devouring matter, could be the reason for the unusual brightness of this and other early galaxies. Based on their observations, Maiolino and his team estimate the mass of this active black hole to be around one million solar masses. Although that wouldn’t be much for a galaxy nucleus in today’s universe, it is for this era: “It’s still very early for a black hole this massive,” says Maiolino. It couldn’t have grown through normal accretion from a stellar black hole – there hasn’t been enough time for that since the Big Bang. “We therefore have to consider how other ways it could have formed,” said the astronomer.
A possible indication of this is provided by the radiation that the black hole emits. As the team determined, the luminosity corresponds to the enormous energy output of 10 to the power of 45 ergs per second. “Such a luminosity would be a factor of five higher than the Eddington limit,” they report. If this is confirmed, this early black hole would have to devour more matter five times faster than the theoretically postulated upper limit for accretion. “Such a super-Eddington accretion is one of the scenarios proposed for fast-growing supermassive black holes in the early Universe,” explain Maiolino and his colleagues. Accordingly, it could have been possible that the black holes of that time exceeded the Eddington limit, at least for a certain time. Under these circumstances, they could absorb enough matter to grow from a stellar black hole to the observed size, even within just a few hundred million years of the Big Bang.
However, this would have lasting consequences for the host galaxies of such “overactive” galaxy nuclei: the intense radiation from the black hole blows a large part of the interstellar medium out of the galaxy – and with it the gas supply from which new stars would otherwise form. As a result, star formation would largely come to a standstill and the galaxies would remain correspondingly small.
Source: Roberto Maiolino (University of Cambridge) et al., Nature, doi: 10.1038/s41586-024-07052-5