How can a galaxy serve as a telescope?

Observing the distant stars born shortly after the Big Bang is a real challenge. However, astronomers have been successful in using an innovative method: they use certain galaxies that deform light and play the role of a natural telescope.

As telescopes become more sophisticated every day, they reveal ever more distant and more detailed objects to us. But sometimes even all that science is not enough, so one has to ask nature for a helping hand.

This is what a team of American scientists did by 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 the study published in Nature May 18, 2022 summarizes for Numerama: ” We used the natural phenomenon of gravitational lenses and the latest technological advances to observe attenuated Lyman-Alpha systems. »

Before we move on, a few explanations are needed in light of this avalanche of complicated terms. First, 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 practical so-called field-integral spectrograph, which makes it possible to study the chemical composition of very large objects.

This is the case with what is observed here: attenuated Lyman-Alpha systems. Behind this barbaric name hides very diffuse gas clouds that appeared about 11 billion years ago when the universe was very young at the time. The universe was filled with them in its early youth before these clouds ended up forming stars. In these areas, matter accumulates to create galaxies, sometimes giving them the nickname galaxy nurseries.

We are moving forward, but there is still a definition, namely the gravitational lens. This phenomenon occurs when the observer is in front of an object so massive that it deflects the light rays.

Representation of a gravitational lens. Source: WM Keck Observatory

It is, for example, a cluster of galaxies that forces light to deviate as much as the gravitational pull it causes is important. As early as the 1930s, scientists relied on Einstein’s general theory of 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 placed 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. By exploiting 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 that measure no less than 238 square kiloparsecs. It is an area of ​​… 775,000 light years approx. Which is extremely large, hence the feat of having managed to distinguish them as a whole.

This is the first time 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. That is why the use of a gravity lens is so crucial to hoping to see something.

big bang universe birth astronomy space
A representation of the Big Bang. // Source: Pixabay (cropped image)

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 need to be very careful not to make a mistakeassures Rongmon Bordoloi. We had to analyze the mass distribution of the galaxy cluster that we used as a lens. Once this is done, we have templates that recreate the original image. »

Explore the mysteries of the original universe

These observations were instructive for the study of Lyman-Alpha systems. Scientists were able to detect billions of times the mass of the Sun as neutral gas. Gas, which will 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, although the mechanisms related to them could not be identified.

That said, Rongmon Bordoloi has no intention of stopping there and is looking for his next goal. ” We know a few dozen gravitational lenses. This is a rather 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 that we will see the continuation of in the coming months or years. The priority targets will be the most distant, that is, those created shortly after the Big Bang, which will surely reveal the secrets behind the beginning of the universe.

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