James-Webb solves the mystery of cosmology’s mysterious Lyman-α emission

The entire astronomical community undoubtedly had high hopes when the James Webb Space Telescope (JWST) was launched on December 25, 2021. It reached the Lagrange point L2, its destination, so the region of Earth’s orbit where the Planck satellite made a stunning study of fossil radiation, the oldest observable light in the cosmos, giving us information about its age, its curvature, its form and content. In Dark Matter and Energy.

Fossil radiation was emitted a few thousand years after the Big Bang, about 380,000 years. James-Webb does not make such early observations of the history of the observable universe, but it may allow us to go back at least 250 million years after the Big Bang and at least to better understand light levels. , let’s say between 400. million and a billion years, which were already accessible with Hubble but more imperfectly.

For 13.8 billion years, the universe has been evolving. Contrary to what our eyes tell us when we contemplate the sky, what makes it is far from static. Physicists make observations and simulations of the universe at different ages in which they replay its formation and evolution. It appears that dark matter played a major role from the beginning of the universe to the formation of the massive structures observed today. © CEA Research

primordial galaxies that should be invisible in Lyman-α emission

However, an article published in nature astronomy, and which can be freely accessed arXiv, JWST reports observations that solve a puzzle that has troubled cosmologists for some time. According to the standard cosmological model, based on dark matter and dark energy, the most distant galaxies should not shine much, due to the so-called Lyman-α emission from hydrogen atoms. They also become less luminous as we observe them in primitive levels of light, to the point of disappearing less than a billion years after the Big Bang.

Not so, why? Does this indicate another problem in standard cosmology, such as the value of the famous Hubble-Lemaitre constant?

To understand what it really is, we have to go back to the emission of fossil radiation. Over a few thousand years, the temperature of the universe’s plasma has dropped enough due to its expansion that during this period, the first atoms of hydrogen and helium are formed, the nuclei capturing free electrons to give neutral atoms. This is the beginning of the famous dark ages Because it would take hundreds of millions of years before a large number of stars began to appear.

Reionization occurred very early in the history of the universe, making it difficult to observe directly. Minutes after the Big Bang, the universe was still too hot for atomic nuclei to capture electrons: it was then completely ionized. Then, the universe continued to expand and cool until its temperature dropped enough for electrons to combine with nuclei and form the first atoms. This so-called “recombination” took place about 380,000 years after the Big Bang. This moment also marks another important event in the history of the universe: while light is scattered very easily by electrons when they are free, this phenomenon is much less so when they are bound to nuclei. Thus, recombination also marks the moment when the universe became transparent and when light could transmit freely through it. © HFI Planck

These stars are very hot and emit radiation in the ultraviolet, precisely according to Lyman-α emission. Even then, neutral hydrogen still exists in abundance, especially around new galaxies, and it will take millions of years for the stars in these young evolving galaxies to radiate, and perhaps even heat up the first massive black holes. Above and accordingly radiate, ionize this neutral hydrogen between galaxies that is opaque to Lyman-α emission. So the observable universe should only gradually become transparent during the so-called reionization period, which we know will end about a billion years after the Big Bang.

Are “galaxies” made up of many colliding galaxies?

Astrophysicists believe they now have the key to the puzzle of the unusual luminosity of young galaxies when reionization is not yet sufficient. So it comes to us from JWST and its NIRCam, an instrument observing in the near-infrared, which is able to look for light moving to these frequencies for distant galaxies.

NIRCam resolved images of galaxies that turned out to be in fact large galaxies, but surrounded or even colliding with nearby smaller galaxies.

The team behind the release in Nature Astronomy Then used computer simulations to reproduce the events occurring with these galaxies and, as the ESA press release explains, its members “ found that the rapid accretion of stellar mass due to galaxy mergers both leads to strong hydrogen emission and facilitates the escape of abundant neutral gas through channels cleared of this radiation. Thus, the high merger rate of small, previously unobserved galaxies presented an attractive solution to the long-standing puzzle of the inexplicable early emission of hydrogen.

The team plans follow-up observations with galaxies at various stages of merger to continue developing their understanding of how hydrogen emission is ejected from these evolving systems. Ultimately, this will allow them to improve our understanding of the evolution of galaxies “

More precisely, close collisions between many dwarf galaxies and large galaxies initially surrounded by halos of neutral hydrogen led to the ionization of this halo, allowing the formation of an ionized bubble transparent to the alpha emission of hydrogen from the formation of these young stars. galaxies