The spatial distribution of mass is almost completely uniform at very early cosmic times, but gravitational instability causes minute overdensities to grow, until dark matter halos form. Small dark matter halos form first, and the most massive ones form later. On their way to becoming more massive, dark matter halos grow by accreting mass from their environments, including other dark matter halos. Sometimes similar dark matter halos merger together in a “major merger”.
To Investigate the rate of growth of dark matter halos, I’ve analyzed the Millennium and Millennium-II Simulations, which are dark matter-only simulations run by the Virgo Consortium. I’ve quantified the merger rate of dark matter halos, as well as their growth by accretion of smooth mass. The merger trees I’ve constructed as part of this study can be made available.