The Galaxy Merger History as Evidence for Dark Matter

An important question that has arisen in the past few decades concerns the nature of dark matter, including whether dark matter exists at all or should be replaced by a form of Modified Newtonian Dynamics (MOND). Observational tests of dark matter and MOND have been extensively applied at lower redshifts, whereas the time-evolution of galaxies has largely been ignored. As we are understanding the evolution of galaxies up to z~6 in some detail, we can now compare predictions of a dark matter-based theory, typically CDM, and predictions from MOND. Using recent observations of the merger history up to z~3 measured in two different ways, we show that dynamical friction time-scales and arguments are such that dark matter is required to reproduce the evolution of galaxies. Comparing galaxies in pairs to those that have already merged, we demonstrate that this time-scale for merging is several times quicker than what would be predicted in MOND. This is further evidence for the existence of dark matter associated with galaxy halos throughout cosmic time.