Since the James Webb Space Telescope became operational, astronomers have been able to look further and further into the early days of the universe. Now the team has managed to track down the farthest and oldest black hole to date. It is the central black hole of a galaxy that existed 400 million years after the Big Bang. Intense radiation from its center indicates the presence of a matter-devouring black hole with a mass of about one million solar masses. This AGN also appears to absorb more mass than dictated by the so-called Eddington limit. This could explain how such black holes were able to grow so large so quickly after the Big Bang.
Supermassive black holes at the core of galaxies can range in mass from hundreds of millions to billions of solar masses. But how these gravitational giants arise is not clearly understood. According to a popular theory, such super-massive giants arise from stellar black holes that gradually grow larger by devouring and merging matter. But observations from the early universe raise doubts about this. Astronomers have discovered several quasars weighing 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 have grown through slow accretion. To explain this, alternative mechanisms are discussed, including the merging of multiple early “hole seedlings” or the collapse of massive gas clouds directly into precursors of supermassive black holes of 10,000 to 100,000 solar masses. Accumulation beyond the Eddington limit is also discussed.
An early galaxy with an active nucleus
Astronomers hope to find answers to the mechanism that caused cosmic gravitational giants to grow so quickly by searching for “crater seedlings” in the early universe. If they catch them “red-handed,” so to speak, it could reveal how early black holes grew. The most important instrument for these investigations is the James Webb Telescope with its high-resolution NIRSpec infrared spectrometer. This is because it can pick up and deconstruct the spectral signatures generated by black hole activity. It was not until November 2023 that astronomers were able to discover the most distant black hole so far. It is located in the UHZ1 galaxy, which existed 470 million years after the Big Bang, and appeared unusually large and massive for its rather slender galaxy. This led to speculation about whether this black hole was formed as a result of 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. In their study, they used the NIRSpec spectrometer to examine the distant but unusually bright galaxy GN-z11 in more detail. Preliminary observations using the Hubble Space Telescope and the James Webb Telescope have already shown that this galaxy has a bright central region with a less luminous surrounding disk. However, it remained unclear whether the glow was caused by strong star formation or an active black hole. New high-resolution spectroscopy has now brought greater clarity. Some spectral lines appear, including the double neon IV line, which is typical of active galactic nuclei (AGN). “NeIV is an obvious tracer of active galactic nuclei, because this line requires photons with energies above 63.5 MeV,” Maiolino and his colleagues explain. She was also able to identify the carbon line typical of AGN.
Accumulation exceeds the Eddington limit?
Astronomers concluded that the galaxy GN-z11 hosts an active black hole, the oldest known to date. Thus the galaxy and the black hole exist 400 million years after the Big Bang. “The AGN scenario also provides a natural explanation for the exceptional brightness of GN-z11,” the team said. The strong emission of radiation from the black hole, which is rapidly devouring matter, could be the reason behind 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 about one million solar masses. Although this would not be much for a galactic core in today's world, it is also true of this era. “It's still very early for a black hole this massive,” Maiolino says. It couldn't have grown through normal accretion from a stellar black hole, and there hasn't been enough time for that since the Big Bang. “So we have to think about other ways it could have formed,” the astronomer said.
A possible indication of this is through the radiation that the black hole emits. As the team determined, the brightness corresponds to a massive power output of 10 to the power of 45 ergs per second. “Such a luminosity would be a factor five times higher than the Eddington limit,” they said. If confirmed, this early black hole would have to gobble up more matter five times faster than the theoretically assumed upper limit for accretion. “Such Eddington superaccretion is one of the proposed scenarios for rapidly growing supermassive black holes in the early universe,” Maiolino and his colleagues explain. Therefore, it is possible that black holes at that time exceeded the Eddington limit, at least for a certain period. Under these conditions, it could absorb enough matter to grow from a stellar black hole to the observed size, even within a few hundred million years of the Big Bang.
However, this will have lasting consequences for the galaxies hosting such “hyperactive” galactic nuclei: the intense radiation from the black hole pushes a significant portion of the interstellar medium out of the galaxy – and with it the supply of gas from which new stars will emerge. another shape. As a result, star formation would largely cease and galaxies would remain accordingly small.
Source: Roberto Maiolino (University of Cambridge) et al., Nature, doi: 10.1038/s41586-024-07052-5
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