GALAXY DISKS DO NOT NEED TO SURVIVE IN THE ΛCDM PARADIGM: THE GALAXY MERGER RATE OUT TO<i>z</i>∼ 1.5 FROM MORPHO-KINEMATIC DATA

Abstract

About two-thirds of present-day, large galaxies are spirals such as the Milky\nWay or Andromeda, but the way their thin rotating disks formed remains\nuncertain. Observations have revealed that half of their progenitors, six\nbillion years ago, had peculiar morphologies and/or kinematics, which exclude\nthem from the Hubble sequence. Major mergers, i.e., fusions between galaxies of\nsimilar mass, are found to be the likeliest driver for such strong\npeculiarities. However, thin disks are fragile and easily destroyed by such\nviolent collisions, which creates a critical tension between the observed\nfraction of thin disks and their survival within the L-CDM paradigm. Here we\nshow that the observed high occurrence of mergers amongst their progenitors is\nonly apparent and is resolved when using morpho-kinematic observations which\nare sensitive to all the phases of the merging process. This provides an\noriginal way of narrowing down observational estimates of the galaxy merger\nrate and leads to a perfect match with predictions by state-of-the-art L-CDM\nsemi-empirical models with no particular fine-tuning needed. These results\nimply that half of local thin disks do not survive but are actually rebuilt\nafter a gas-rich major merger occurring in the past nine billion years, i.e.,\ntwo-thirds of the lifetime of the Universe. This emphasizes the need to study\nhow thin disks can form in halos with a more active merger history than\npreviously considered, and to investigate what is the origin of the gas\nreservoir from which local disks would reform.\n