Years ago, astronomers began to wonder about the reasons for the considerable slowdown in the growth of the largest black holes in the universe, which has been stalling for around ten billion years. According to Live Science, we may have finally understood the reason for this astrophysical enigma: they are simply lacking gas.
Supermassive black holes (their official name) have an immense gravitational capacity which allowed them to reach masses millions, even billions of times greater than that of the Sun, at a surprising speed during the first billion years following the Big Bang. However, their growth has continued to slow since the so-called “cosmic noon” period, when the universe was less than a quarter of its current age.
Until then, the scientific community was scratching its head: was there an external force helping to slow down the growth of these black holes, and if not, how could the phenomenon be explained? A research team has finally shed credible light, publishing the results of its investigations in The Astrophysical Journal. “It turns out that each black hole consumes much less matter”summarizes Fan Zou, astronomer at the University of Michigan and co-author of the study, interviewed by Live Science.
The “cosmic afternoon”
To understand the evolution of galaxies and the birth of stars, the study of the growth of black holes is fundamental. Supermassive black holes and their host galaxies indeed evolve in a coordinated manner. We also know that their size is linked to the total mass of the stars as well as their chaotic movements in the galactic bulge, the name given to the central part of spiral galaxies, where the stars form a very dense grouping.
The measurement of the evolution of black hole growth over cosmic time is based on data from nine extragalactic surveys. These data, stratified in several layers – we speak of “wedding cake” diagrams – include superficial surveys of large, relatively close regions of the sky, as well as extremely precise observations, made using very fine beams, of more restricted fields.
These observations were made by the world’s largest X-ray space telescopes – including NASA’s Chandra Observatory, the European Space Agency’s XMM-Newton Space Telescope, and the German-Russian eROSITA telescope. In total, the researchers analyzed observations of 1.3 million galaxies and 8,000 growing, X-ray-emitting supermassive black holes to determine why the black hole growth rate dropped dramatically.
After constructing several hypotheses, the researchers came to the conclusion that the slowdown in the growth of these black holes was due to the reduction in the quantity of cold gas to be absorbed, which began at the time of “cosmic noon”, around 10 billion years ago.
The study also allowed the research team to decide that the quantity of supermassive black holes is not likely to change significantly. “We do not expect the appearance or significant growth of many supermassive black holes in the futureconcludes Fan Zou. We observed in 2024 that the number of supermassive black holes had almost stabilized 7 billion years ago and will probably remain so.”
