About two-thirds of present-day, large galaxies are spirals such as the MilkyWay or Andromeda, but the way their thin rotating disks formed remainsuncertain. Observations have revealed that half of their progenitors, sixbillion years ago, had peculiar morphologies and/or kinematics, which excludethem from the Hubble sequence. Major mergers, i.e., fusions between galaxies ofsimilar mass, are found to be the likeliest driver for such strongpeculiarities. However, thin disks are fragile and easily destroyed by suchviolent collisions, which creates a critical tension between the observedfraction of thin disks and their survival within the L-CDM paradigm. Here weshow that the observed high occurrence of mergers amongst their progenitors isonly apparent and is resolved when using morpho-kinematic observations whichare sensitive to all the phases of the merging process. This provides anoriginal way of narrowing down observational estimates of the galaxy mergerrate and leads to a perfect match with predictions by state-of-the-art L-CDMsemi-empirical models with no particular fine-tuning needed. These resultsimply that half of local thin disks do not survive but are actually rebuiltafter a gas-rich major merger occurring in the past nine billion years, i.e.,two-thirds of the lifetime of the Universe. This emphasizes the need to studyhow thin disks can form in halos with a more active merger history thanpreviously considered, and to investigate what is the origin of the gasreservoir from which local disks would reform.
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