Gravitational interactions — mergers and y-by encounters — between galaxies play a key role as the drivers of their evolution. Here we perform a cosmological N-body simulation using the tree-particle-mesh code GOTPM, and attempt to separate out the effects of mergers and y-bys between dark matter halos. Once close pair halos are identified by the halo finding algorithm PSB, they are classified into mergers (E12 < 0) and y-by encounters (E12 > 0) based on the total energy (E12) between two halos. The y-by and merger fractions as functions of redshift, halo masses, and ambient environments are calculated and the result shows the following: (1) Among Milky-way sized halos (0:33-2:0 X 1012h-1M⊙), 5:37±0:03% have experienced major y-bys and 7.98±0.04% have undergone major mergers since z ~ 1; (2) Among dwarf halos (0:1 - 0.33 X 1012h-1M⊙), 6.42 ± 0.02% went through major y-bys and 9.51 ± 0.03% experienced major mergers since z ~ 1; (3) Milky-way sized halos in the cluster environment experienced fly-bys (mergers) 4-11(1.5 - 1.7) times more frequently than those in the field since z ~ 1; and (4) Approaching z = 0, the y-by fraction decreases sharply with the merger fraction remaining constant, implying that the empirical pair/merger fractions (that decrease from z ~ 1) are in fact driven by the fly-bys, not by the mergers themselves.

We consider a late decaying dark matter model in which cold dark matter begins to decay into relativistic particles at a recent epoch (z ≤ 1). A complete set of Boltzmann equations for dark matter and other relevant particles particles is derived, which is necessary to calculate the evolution of the energy density and density perturbations. We show that the large entropy production and associated bulk viscosity from such decays leads to a recently accelerating cosmology consistent with observations. We determine the constraints on the decaying dark matter model with bulk viscosity by using a MCMC method combined with observational data of the CMB and type Ia supernovae.

We trace the dynamical evolution of dark matter (DM) content in NGC 6397, one of the native Galactic globular clusters (GCs). The relatively strong tidal field (Galactocentric radius of ~ 6 kpc) and short relaxation timescale (~ 0.3 Gyr) of the cluster can cause a significant amount of DM particles to evaporate from the cluster in the Hubble time. Thus, the cluster can initially contain a non-negligible amount of DM. Using the most advanced Fokker-Planck (FP) method, we calculate the dynamical evolution of GCs for numerous initial conditions to determine the maximum initial DM content in NGC 6397 that matches the present-day brightness and velocity dispersion profiles of the cluster. We find that the maximum allowed initial DM mass is slightly less than the initial stellar mass in the cluster. Our findings imply that NGC 6397 did not initially contain a significant amount of DM, and is similar to that of NGC 2419, the remotest and the most massive Galactic GC.

Observations of dark matter dominated dwarf and low surface brightness disk galaxies favor density profiles with a flat-density core, while cold dark matter (CDM) N-body simulations form halos with central cusps, instead. This apparent discrepancy has motivated a re-examination of the microscopic nature of the dark matter in order to explain the observed halo profiles, including the suggestion that CDM has a non-gravitational self-interaction. We study the formation and evolution of self-interacting dark matter (SIDM) halos. We find analytical, fully cosmological similarity solutions for their dynamics, which take proper account of the collisional interaction of SIDM particles, based on a fluid approximation derived from the Boltzmann equation. The SIDM particles scatter each other elastically, which results in an effective thermal conductivity that heats the halo core and flattens its density profile. These similarity solutions are relevant to galactic and cluster halo formation in the CDM model. We assume that the local density maximum which serves as the progenitor of the halo has an initial mass profile δM / M ∝ M-є, as in the familiar secondary infall model. If є = 1/6, SIDM halos will evolve self-similarly, with a cold, supersonic infall which is terminated by a strong accretion shock. Different solutions arise for different values of the dimensionless collisionality parameter, Q ≡ σpbrs, where σ is the SIDM particle scattering cross section per unit mass, pb is the cosmic mean density, and rs is the shock radius. For all these solutions, a flat-density, isothermal core is present which grows in size as a fixed fraction of rs. We find two different regimes for these solutions: 1) for Q < Qth(≃ 7.35 × 10-4), the core density decreases and core size increases as Q increases; 2) for Q > Qth, the core density increases and core size decreases as Q increases. Our similarity solutions are in good agreement with previous results of N-body simulation of SIDM halos, which correspond to the low-Q regime, for which SIDM halo profiles match the observed galactic rotation curves if Q ~ [8.4×10-4 - 4.9 × 10-2]Qth, or σ ~ [0.56 - 5.6] cm2g-1. These similarity solutions also show that, as Q → ∞, the central density acquires a singular profile, in agreement with some earlier simulation results which approximated the effects of SIDM collisionality by considering an ordinary fluid without conductivity, i.e. the limit of mean free path ⋋mfp → 0. The intermediate regime where Q ~ [18.6 - 231]Qth or σ [1.2×10 4 - 2.7×10 4] cm2-1, for which we find flat-density cores comparable to those of the low-Q solutions preferred to make SIDM halos match halo observations, has not previously been identified. Further study of this regime is warranted.