Observing the distant stars born shortly after the Big Bang is a real challenge. Astronomers have however succeeded by using an innovative method: they use certain galaxies which deform light and which play the role of a natural telescope.
As telescopes become more sophisticated every day, they reveal ever more distant and ever more detailed objects to us. But sometimes even all that science is not enough, so you have to ask nature for a helping hand.
This is what a team of American researchers did, using a galaxy as a magnifying glass to look at an object that was behind it. Rongmon Bordoloi of the University of North Carolina, lead author of thestudy published in Nature on May 18, 2022, summarizes for Numerama: “ We used the natural phenomenon of gravitational lensing and the latest technological advances to observe damped Lyman-Alpha systems. »
Before going any further, a few explanations are necessary in view of this avalanche of complicated terms. For starters, what are these technological advances? This is the WM Keck Observatory in Hawaii, one of the largest optical telescopes in the world. It has a handy so-called field-integral spectrograph, which makes it possible to study the chemical composition of very large objects.
This is the case of what is observed here: damped Lyman-Alpha systems. Behind this barbaric name hide very diffuse gas clouds that appeared about 11 billion years ago, when the Universe was then very young. The Universe was full of them in its early youth, before these clouds ended up forming stars. In these areas, matter gathers to give rise to galaxies, which sometimes earns them the nickname of galaxy nurseries.
We are moving forward, but there remains a definition, that of the gravitational lens. This phenomenon occurs when the observer finds himself in front of an object so massive that it deflects the light rays.
It is, for example, a cluster of galaxies that forces light to deviate so much the gravitational attraction that it causes is important. Already in the 1930s, scientists relied on Einstein’s theory of general relativity to predict the existence of this phenomenon, but it took until 1979 to see it for the first time. Under these conditions, if an object located behind this cluster emits light, the photons will bypass the cluster and it will be possible to observe it with an optical effect that makes it appear a little larger, as if you were looking through a magnifying glass.
“It’s like a natural telescope”
And that’s what Rongmon Bordoloi did with his team. Taking advantage of the presence of a gravitational lens, they were able to observe the system behind it in more detail. ” It’s like a natural telescopesays the researcher. Other scientists have been using it this way for years, but we are the first to be able to observe an entire Lyman-Alpha system in this way. »
More specifically, two systems have been observed. Two gas clouds that formed about 11 billion years ago and which measure no less than 238 square kiloparsecs. That is an area of… 775,000 light-years approximately. Which is extremely large, hence the feat of having succeeded in distinguishing them as a whole.
” This is the first time that we have been able to determine the size of systems like thesesays Rongmon Bordoloi. But also the galaxies associated with it, which is particularly difficult. »
It should be emphasized here that these objects are very diffuse, very distant, and also not very luminous, because they have not yet formed stars, which makes their observation particularly complicated. This is why the use of a gravitational lens is so determining to hope to see something.
But the technique also has its drawbacks. As through a magnifying glass, observed objects may appear distorted. This was the case here: the image that came to them was stretched, and it was necessary to understand what the exact proportions of the object were. ” You have to be very careful not to make a mistakeassures Rongmon Bordoloi. We had to analyze the mass distribution of the cluster of galaxies that we were using as a lens. Once this is done, we have templates that recreate the original image. »
Explore the mysteries of the primordial Universe
These observations were instructive for the study of Lyman-Alpha systems. The researchers were able to discover billions of times the mass of the Sun as neutral gas. Gas which should then be used as fuel to form future stars. They also revealed the presence of large mass changes depending on the region of each cloud. In other words, there are internal structures inside, even if the mechanisms related to them could not be identified.
That said, Rongmon Bordoloi does not intend to stop there and is looking for his next target. ” We know a few dozen gravitational lenses. This is a fairly rare phenomenon, but we hope to be able to use it to observe other distant objects. This study is only the first of a long series of which we will see the continuation in the months or the years which come. The priority targets will be the most distant ones, that is to say those which came into being shortly after the Big Bang, which will certainly reveal the secrets of the beginnings of the Universe.