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Hierarchical galaxy formation

How do galaxies form ? According to the standard cosmological model, ordinary matter (or baryons) collapses into dark matter halos, after decoupling from photons, 380 000 years after the Big Bang. Dark matter, on the other hand, collapsed much earlier, forming halos, or primordial dark galaxies. Initially, the first galaxies to form stars are the smallest, then merge to form more massive ones - a hierarchical formation. The mass of galaxies is assembled by interaction and fusion, but also by the accretion of gas from the filaments of the cosmic web. For over thirty years, telescopes, and in particular the Hubble Space Telescope (HST), have enabled us to look far and wide, and to go back in time to study these primordial galaxies. Because of the high redshift, images have shown us fragmented, lumpy galaxies. Since July 2022, images from the JWST, in the far infrared, have revealed more regular galaxies, disks, spirals and bars, clearly visible in the population of older stars. The number of visible galaxies has greatly increased, and these are capable of explaining the reionization of the Universe. The growth of supermassive black holes very soon after the Big Bang still poses problems.

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