ON THE UNIVERSALITY OF THE BOUND ZONE PECULIAR VELOCITY PROFILE

Abstract

ABSTRACT Numerical evidence for the universality of the bound-zone peculiar velocity profile in a “Λ+Cold Dark Matter” (ΛCDM) universe is presented. Analyzing the dark matter halo catalogs from the Millennium-II simulation, we determine the average peculiar velocity profiles of the objects located in the bound zones around massive group-size halos at various redshifts and compare them to an analytic formula characterized by two parameters, the amplitude and slope of the profile. The best-fit values of the two parameters are found to be robust against the changes of the mass scales and the key cosmological parameters. It is also found that the amplitude and slope parameters of the bound-zone peculiar velocity profile are constant, but only in the limited ranges of redshifts. In the dark matter dominated epoch corresponding to z > 0.6, the two parameters have constant values. In the transition period corresponding to 0.2 ≤ z ≤ 0.6, when the density of Λ begins to exceed that of dark matter, the two parameters grow almost linearly with redshifts. At later epochs with z < 0.2, when the Λ-domination prevails, the two parameters regain constancy, settling upon higher constant values. Noting that the length of the transition period depends on the amount of Λ, and speculating that the linear evolution of the profile, with redshifts in the transition period, is a unique feature of the Λ-dominated universe, we suggest that the redshift evolution of the bound-zone peculiar velocity profile should be a powerful local discriminator of dark energy candidates.