At the beginning of her presentation, the young astronomer, Edwige Pezzulli, shows a photograph of what the universe looked like in the very beginning, almost 14 billion years ago.
“At that time matter was surrounded by an extremely small , evenly-distributed space: wherever we looked the universe was always identical, a uniform sea of matter. But if we take a closer look at this photograph, we realize that there are small irregularities, i.e. several points where the matter is slightly denser than the rest. These red zones are extremely important, because they represent the seeds from which galaxies were formed. In fact, these high density areas attracted increasing amounts of matter, leading to the creation of the first stars and first galaxies a billion years after the Big Bang”.
Today, using the most advanced telescopes, we have been able to observe how the first galaxies host “supermassive black holes” in their centers. But how are they formed? And how exactly did they manage to grow so much in such a (relatively) short period of time?
In order to attempt to answer these questions, we need to go back a step. As Pezzulli explains: “some stars come to the end of the their lives by exploding, and a black hole can be what is left at the end of this explosion, i.e. an object that is so compact that the pull of gravity does not allow anything to escape it, even light”.
When they are formed, black holes have a similar mass to our sun, i.e. they are much smaller than those that we see inside galaxies. In order to grow, they merge with other black holes and, in particular, they devour all the matter around them.
“These two growth mechanisms of black holes go hand in hand with the galaxy in which they are found, and so for each galaxy we can put together a kind of family tree, a dynasty made up of smaller galaxies that slowly slowly create the grand finale, with the accumulation of matter from the surrounding atmosphere”.
By studying how gas cools, which is a process that leads to the formation of stars, and by following them until their death, astronomers have been able to identify the galaxies in which the ancestors of the first black holes were created and reconstruct their evolution. This is a phenomenon that is not the easiest to observe: the ancestors of black holes grow so fast that the flashes of light they give off resemble cosmic fireworks. With this violent action lots of energy is released, forming powerful winds of matter that crash into the inner and outer parts of the host galaxy. In this way the chemical elements produced by stars are spread everywhere, contaminating the entire universe.
The first supermassive black holes are without doubt one of the most extreme objects in our universe. However, as Pezzulli observes, “the most extraordinary thing about this puzzle is realizing that such incredible objects also originate from a tiny, microscopic random imperfection”.