An early black hole reveals secrets

Artist’s impression of a so-called quasar: A black hole surrounded by a bright accretion disk and a dust torus. © T. Müller / MPIA

But there was no particularly intensive feeding in the early cosmos: The analytical look at an extremely distant black hole contradicts the explanation that the gravitational giants of the early cosmos grew so quickly due to excessive “voracity”. The observations show that the supermassive black hole now observed grew in a similar way to today’s examples around 770 million years after the Big Bang. This suggests that the early black holes were created with high masses from the start, say the researchers.

It all started with the Big Bang: According to astronomical research, our universe was created around 13.8 billion years ago by the expansion of primordial matter from a cosmological center. As a result, the first celestial bodies and galaxies formed. Over the course of billions of years, they continued to change: the galaxies grew by absorbing more and more gas from their surroundings, or particularly large ones were formed through mergers. For a long time, it was assumed that in the course of these developments, the supermassive black holes in the centers of the galaxies also grew to their sometimes gigantic dimensions. By absorbing more and more matter, they built up to several billion solar masses.

This process is noticeable in the strong illumination of the so-called accretion disk around a black hole. These active galactic nuclei with enormous radiance are also known as quasars. Based on certain characteristics, they allow conclusions to be drawn about the mass of the central black hole. Astronomers are particularly interested in the quasars of the early days of the cosmos. The distance makes it possible to look into the past, as the light from an object takes time to reach us. The distance and thus the depth of the retrospective can be determined based on its spectral redshift. The quasars with the furthest known distance therefore appear to us as they looked less than a billion years after the Big Bang.

Young and particularly voracious?

At first, astronomers had actually expected that the very young quasar specimens were significantly less massive than the more mature versions, since they had not been able to consume as much matter in the short time since the Big Bang. But in the last twenty years, black holes from the time of the cosmic twilight have been found to have up to ten billion solar masses. This surprising mass is difficult to explain using astronomical models. However, one plausible possibility has so far been that early black holes were fed by matter even more efficiently than later ones. Possible measurement errors have also been under debate: it previously seemed possible that dust accumulations influence the observations of young quasars in such a way that the masses of the black holes are overestimated.

To help solve the mystery, researchers led by Sarah Bosman from the Max Planck Institute for Astronomy (MPIA) in Heidelberg have now taken another look at one of the most distant known quasars: J1120+0641 appears in the constellation Leo, and the redshift of its light shows that we see this quasar in a state around 770 million years after the Big Bang. The team has now examined it with the MIRI spectrometer on the James Webb Space Telescope. The spectral signature of the light in the mid-infrared range was recorded. As the researchers explain, the data reflects information about the properties of the quasar’s accretion disk and the dust torus further out. These structures guide matter to the central black hole – they are, as it were, its feeding structures.

Surprisingly inconspicuous

As the team reports, the analytical look at the early quasar revealed that the mass of the black hole at the center of J1120+0641 could initially be narrowed down to about 1.5 billion solar masses based on the data. In principle, the spectral analyses of the accretion disk provided no evidence that the light of the quasar is influenced by an above-average amount of dust. It therefore seems unlikely that the masses of early black holes have simply been overestimated, say the researchers. However, they also found no evidence for the theory of particularly intensive feeding. The spectral data, which allow conclusions to be drawn about the accretion disk and the dust torus of the early quasar, showed that the rate at which this quasar consumes matter differs little from counterparts from later times.

J1120+0641 shows no unusual features in almost all properties that can be deduced from the spectrum, the researchers conclude. “Our observations make the puzzle a little more puzzling. Early quasars were surprisingly normal. Regardless of the wavelengths at which we observe them, quasars are apparently almost identical in all epochs of the universe,” says Bosman. The study results thus suggest that not only the supermassive black holes themselves, but also their feeding mechanisms were already “mature” in the youth of the universe.

But how can the surprisingly large masses of early black holes be explained? According to the researchers, the findings suggest that they probably had this characteristic from the very beginning and that the gradual accumulation of matter is therefore less important. Specifically, supermassive black holes could not have been formed from the remains of early stars and further inflows. Instead, they could have formed from the collapse of massive early gas clouds with masses of at least a hundred thousand solar masses, according to a possible explanation.

Source: Max Planck Institute for Astronomy, scientific article: Nature Astronomy, doi: 10.1038/s41550-024-02273-0

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