In the chaos of the young universe about 10 billion years ago, the formation of stars and galaxies reached its peak. The environment is dense and violent collisions between galaxies result in aggregates with crude and strange shapes.
At least that's what the images from the Hubble Space Telescope suggested and confirmed the scenario favored by cosmologists. However, the scenario in question has just been shattered! Under the keen eyes of the James Webb Space Telescope (TSJW), launched two years ago, the merged galaxies in question turned out to be mostly beautiful and round as pancakes.
“Their disc-shaped structure was already present, but could not be discovered by Hubble,” explains Leonardo Ferreira, a postdoctoral researcher at the University of Victoria and lead author of the paper published in the Astrophysical Journal. This challenges our understanding of galaxy mergers: either the disk reforms more easily [après la collision] Either that's what we thought, or we don't fully understand the role of these collisions in the evolution of galaxies. These are probably both hypotheses at the same time. »
Either way, the numbers speak for themselves: of 3,956 galaxies re-examined by the international team (the largest sample examined by the TSJW to date), more than 40% were rotating disks, just like the Milky Way. . This is ten times more than expected! Instead, astronomers assumed that these so-called spiral galaxies, with their delicate arms, were widespread in a quieter universe six billion years ago.
“We continue to study these galaxies to determine how many of them show traces of violent interactions by combing through the TSJW’s tons of public data,” the astronomer continues. At the same time, we are trying to figure out how our cosmological models differ from these new observations. »
A new light
The TSJW may be brand new, but it's not the first time it's shaken up dogma. A quick reminder: The light emitted by very distant objects is “stretched” towards infrared wavelengths by the expansion of the cosmos. By detecting infrared light with unprecedented sensitivity, the TSJW instruments can observe very distant and therefore very old galaxies. The TSJW therefore sheds light on the nature of primordial galaxies dating back to 13.5 billion years ago, shortly after the Big Bang.
These first galaxies also appear in a new light. In February 2023, an Australian team described in Nature the observation of primordial galaxies that are 10 to 100 times more massive than predicted by theory! And in late September, a Danish study published in Nature Astronomy reported that the latter contained four times lower levels of heavy elements than expected, a departure from the rule that applies to almost the entire cosmos. In short, with the arrival of the TSJW, certainties begin to falter.
“The astronomical community is discovering objects that challenge some of the current ideas about how galaxies form,” concludes Leonardo Ferreira. But that doesn't mean it's all wrong: we have puzzle pieces to put together the universe's timeline, still frames with timestamps [c’est-à-dire associées à un âge précis], but we are missing a lot of intermediate information. » Information that the TSJW is well on its way to providing.