A team of researchers has succeeded in directly calculating the mass of the central black hole in a protogalactic system in the early Universe. The results of the study show that the black hole predates its tiny “galaxy,” that it must have “formed” from the very beginning with a very large mass, and that it accounts for at least two-thirds of the total mass of the system to which it belongs. These significant findings will help astrophysicists gain a deeper understanding of the origin of at least some of the supermassive black holes, which are found at the cores of nearly all galaxies in the Universe.
Which one is formed first: A supermassive black hole or the galaxy that hosts it? Scientists used to believe that galaxies form first and that the giant black holes at their cores form hierarchically from an “initial seed” of stellar black holes, which continuously increase in mass. This is achieved in two ways: by accreting matter from the accretion disk that forms around them, as well as through repeated collisions and mergers with other black holes. More recently, scientists have come to believe that supermassive black holes and their host galaxies “co-evolve,” in the sense that they form in parallel and simultaneously, with each influencing the other’s development. To date, however, the major cosmological problem of the origin of supermassive black holes has not yet been definitively solved.
In recent years, however, these more “classic” answers to our initial question have begun to be called into question with the discovery in the early Universe of more and more black holes with masses millions, even billions of times greater than that of the Sun. In fact, it is very difficult to explain exactly how the giant black holes of the early Universe managed to acquire their immense mass within just a few hundred million years after the Big Bang, if they indeed originated from an initial population of stellar black holes, which are “born” with very small masses.
Now, however, with the help of the James Webb Space Telescope, researchers have found clear evidence that at least some of them formed from the very beginning with a very large mass, that is, through a physical mechanism that must be different from the one described above. The results of the study were published in the scientific journals Nature and Monthly Notices of the Royal Astronomical Society , and are based on detailed observations of Abell2744-QSO1 (QSO1), a typical “Little Red Spot” that existed just 700 million years after the Big Bang.
Little Red Dots (LRD) are a new type of compact object discovered by the James Webb Space Telescope that existed in the early Universe, approximately 600 million to 1 billion years after the Big Bang. Initially, astronomers believed that LRDs were massive, early-stage galaxies, an interpretation that proved problematic, as it conflicted with the predictions of the Standard Model of Cosmology regarding the evolution of large-scale structures.
Although QSO1 has a diameter of just 1,300 light-years and its light traveled for more than 13 billion years before being detected by the James Webb Space Telescope, it was easier to study than other LRD objects, thanks to the phenomenon of gravitational lensing caused by the enormous mass of the Abell 2744 galaxy cluster (Pandora Cluster).
In fact, instead of estimating the black hole’s mass indirectly—as had been done until recently for such early-stage black holes— used the James Webb Space Telescope to directly measure the effect of its gravity on the gas swirling around it, as well as to map the distribution of the various elements contained within it. So when they plotted the rotational velocity of the gases as a function of their distance from the center of QSO1, they found that they follow a Keplerian orbit: In other words, they orbit around a central point in the same way that the planets in our solar system orbit around the Sun.
This finding is important because it demonstrates that most of the mass of QSO1 is concentrated in the black hole at its center. Since Keplerian motion is governed by simple laws of gravity, scientists used measurements of the gases’ velocity to directly calculate the black hole’s mass, concluding that it is on the order of 50 million solar masses. This is the first time that the mass of a supermassive black hole—which had managed to accumulate such a large mass so early in cosmological evolution—has been calculated directly.
Equally impressive is the fact that it contains at least two-thirds of the total mass of QSO1. This mass ratio between the black hole and its galaxy is thousands of times greater than in our neighboring galaxies, where supermassive black holes account for only a small fraction of the total mass of the host galaxy. Analysis of the relevant data also shows that the gases in QSO1 consist almost entirely of hydrogen and helium, with minimal impurities from heavier elements. It is precisely this low metallicity of QSO1, as well as the nearly perfect Keplerian rotation of its gas, that suggest the absence of a significant stellar population.
The low metallicity, the absence of stars and stellar remnants, and the enormous mass of the black hole in QSO1 also demonstrate that its initial seed cannot be stellar black holes, but that both it and at least some of the supermassive black holes in the early Universe must have been born with much greater mass from the outset.
The findings of these studies, in other words, point to the existence of an initial population of black holes with much greater mass, the most notable examples being primordial black holes and instantaneous black holes, which have been studied theoretically but whose existence has not been confirmed by observation.
Primordial black holes are thought to have formed at most within the first second after the Big Bang, through the immediate collapse of superdense and superhot clumps of subatomic matter. Instant-collapse black holes, on the other hand, are thought to have formed 100–250 million years later, when giant clouds of hydrogen and helium collapsed directly into black holes.
In any case, and regardless of which scenario ultimately prevails, the researchers conclude: “The fact that the black hole in QSO1 is surrounded by gas with an extremely low metal content, and that it is almost ‘bare,’ with minimal (if any) stellar mass around it, suggests that this is a massive protogalactic seed” that is in the early stages of forming a galaxy around it. This, in turn, demonstrates that, in at least some cases, the black hole may have formed first, while the galaxy formed later around it. If these studies are confirmed, scientists will have taken a significant step toward solving the major cosmological problem concerning the origin and subsequent evolution of galactic black holes.
Source: NASA’s Webb Reveals a Black Hole That Formed Before Its Galaxy - NASA Science
Photo: In this image from the James Webb Space Telescope, QSO1 is visible, magnified and with a triple image, thanks to the gravitational lensing effect caused by the Abell galaxy cluster