James-Webb discovers cosmic oddity in the early Universe
Example of galaxies Small red dots, with unexpected properties and seen in large numbers by the JWST. © JWST (Nasa/ESA), Matthee & al, (arXiv 2024)
The physical “laws” of Nature are such that the further we see, the further into the past we see because of the time it takes for light to reach our eyes or telescopes. It is the same phenomenon that makes us hear thunder several seconds after seeing the lightning that generated it, sound being much slower than photons.
In this way, the infrared eye of the NASA telescope James-Webb is able to go back almost to the very first galaxies, after the primordial stars lit up during theCosmic Dawn.
The physical scenario of the formation of the first galaxies seemed roughly understood, even if it called upon the dark matter of which we still know nothing, but at least the chronology seemed established. Until the James-Webb opened its golden panels like flowers, the large Darwin said they were hiding “an abominable mystery”.
These observations have reshuffled the cards. First, we have discovered galaxies that are earlier and more massive than expected, similar to the Milky Way or to Andromedabut shortly after the first glimmers of the Universe. Then came these Little Red Dots (LRDs) and their incomprehensible black holes.
PRDs (LRDs in English) only appear in the Universe at least four times younger, when it was “only” about 3 billion years old (its age is currently estimated at 13.78 billion years). What is striking is the smallness and density of these galaxies, which are only a few percent of the size of the Milky Way, but sometimes contain almost as many stars!
Imagine the population of a building in a single apartment… These bizarre galaxies exceed the density of globular clusters yet already impressive. In fact, they sometimes appear so compact that they would be on the verge of automatically collapsing into a black hole, which raises questions.
The globular cluster Omega Centaury in the Milky Way, very massive and very compact. © Nasa/Esa
Compactness diagram, namely the mass-radius ratio as a function of various astrophysical objects. If the compactness reaches the value of 1, then the object collapses into a black hole. © Adrien Coffinet (Wikipedia)
These PRDs are overwhelmingly AGNs, or active nucleus galaxies in the language of ordinary mortals. That is to say, they belong to the family of quasarsOr radio galaxieswhose central supermassive black hole is in a phase of bulimic growth, therefore of accretion of matter.
The problem is that these supermassive black holes are even more gigantic than those we know “today”: compared to the mass of their galaxy, they are “supramassive”. And that’s not all: they emit almost no X-rays, unlike the supermassive black holes that we usually observe.
Several scientific hypotheses have been put forward:
- Perhaps these galaxies are so dense that we mistake their spectral emission (the study of their light) with that of supermassive black holes and that they do not have any, but that would be another mystery…
- Their supramassive black holes are so gluttonous that they are enveloped in a cloud of “detritus” gas that masks the expected X-ray emission…
- Or, these supermassive black holes are a clue that there were indeed primordial stars at the beginning of the Universe 10,000 times more massive than those of today, and that they are the seeds of the supermassive black holes that we observe now.
Sources and recent studies on these Little Red Dots:
https://arxiv.org/html/2308.01230v4
https://arxiv.org/html/2409.13441v1
