The James Webb Space Telescope has done it again. He has found a phenomenon in the Universe that contradicts the physics known until now. In this case, the discovery consists of a star bar in a galaxy that should not host a structure of this type. The good thing is that, properly understood, this discovery can help unravel a mystery for which there was no explanation. We will have to modify what we knew about galaxies, but in exchange we have answers to questions that we did not have before.
A stellar bar in GN20. Many star bars are known in the nearby Universe. It is even known that there are some in our Milky Way. However, they are not found at points close to the Big Bang because they are slow to form, so they could not have been born so early. Furthermore, in those early stages of the Universe there was a lot of gas in the galaxies, the movement of which is believed to inhibit the formation of stellar bars.
All this is what makes the find so rare. recently described by a team of scientists from Leiden University. And, thanks to James Webb, they have found one of these structures in GN20, a very old massive galaxy rich in gases, which formed about 1.5 billion years after the Big Bang. It is a galaxy that is too young and has too much gas to already host a formed star bar. Nothing fits.
Let’s clarify concepts. Star bars are elongated arrangements of stars found at the centers of galaxies, rotating as a rigid unit. With this rotation they drag the gas around them and lead it to the galactic nucleus as if it were a funnel. This possibly serves to feed the galaxy’s central black hole.
The detection is clear. The authors of the study have confirmed that they are looking at a star bar using three different methods. To begin with, it was carried out a technique called isofocal analysis. This consists of drawing a series of imaginary lines on a galaxy that join points with the same brightness. It is something similar to what is done on topographic maps with contour lines. Once this is done, changes in brightness can be detected that indicate the presence of specific structures.
In this case, the galaxy’s light is stretched and rotated in a way that corresponds to a star bar. But that’s not all, its existence has also been proven with an independent mathematical analysis and with observations from the NOEMA telescope. Once this structure was detected, it had to be seen as clearly as possible. That’s where James Webb comes into play, whose near-infrared camera is capable of go beyond the veil of gas and dust which makes observations in the oldest stages of the Universe difficult.
An impossible size. With all these observations, it was also possible to measure the galaxy, which extends over 7 kiloparsecs or, which is the same, 22,800 light years. It is too big for known physics. On the one hand, because of what we have already seen. To grow so much it should have started forming a long time ago and, supposedly, in the youngest stages of the Universe such a structure could not be formed. And, on the other hand, because such a large star bar should collapse according to the description of current models.
Gas to the rescue. These scientists have discovered that, curiously, this galaxy has survived so long thanks to gas. We have seen that, normally, gas makes its formation difficult. But that happens when the gas moves slowly and orderly. However, in this case, in the inner disk of the galaxy there is highly turbulent gas that would act as a shield thanks to a phenomenon known as radial shear.
Shear what? Normally, gas in galaxies moves in concentric circles, so that those in the center move faster and those outside move more slowly. This is known as differential rotation. In this case, however, there are turbulent movements, with the gas moving in a disorderly manner, in such a way that in different rings it rubs, drags and mixes. That’s radial shear. This, broadly speaking, helps the bar grow instead of hindering its formation.
Two key points. When entering with the James Webb to observe the star bar closely, two important details were seen. On the one hand, at the point where it coincides with the outer disk of the galaxy, to the south, there is a large accumulation of gas that acts as a hot spot for the formation of many stars. On the other hand, in the center the bar contributes to sweeping a lot of material into the black hole of the galactic nucleus.
What it teaches us. All of this makes us rethink the physics of star bars, but it also helps scientists understand something that until now was a mystery: inert elliptical giants. These are very large and young galaxies that They are already inactive. That is, new stars are no longer forming within it.
With everything discovered in GN20, the authors of the study that has just been published consider that the star bars could be the reason. By creating star-forming hot spots and sweeping material into the black hole, they essentially make the galaxy live very fast. Create a lot of stars very quickly and use up your fuel early. They live fast, die young and leave an enigmatic corpse that, perhaps, is no longer so enigmatic.
Image | NASA | Leindert A. Boogaard et al (2026).
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