Toshiyuki Fukushige

On the Origin of Cusps in Dark Matter Halos

Observed cusps with density profiles ρ ∝ r-1 or shallower in the central regions of galaxies cannot be reproduced in the standard cold dark matter picture of hierarchical clustering. Previous claims to the contrary were based on simulations with relatively few particles and substantial softening. We present simulations with particle numbers 1 order of magnitude higher and essentially no softening, and we show that typical central density profiles are clearly steeper than ρ ∝ r-1. The observed shallower profiles may have formed through the smoothing effect of the spiral-in of central black holes in previous merger phases. In addition, we confirm the presence of a temperature inversion in the inner 5 kpc of massive galactic halos and illustrate its formation as a natural result of the merging of unequal progenitors.

Structure of Dark Matter Halos from Hierarchical Clustering

We investigate the structure of the dark matter halo formed in three different cold dark matter scenarios. We performed N-body simulations of formation of 13 cluster-sized halos. In all runs, density cusps proportional to

The time-scale of escape from star clusters

r^{-1.5}

GRAPE-6: Massively-Parallel Special-Purpose Computer for Astrophysical Particle Simulations

developed at the center. This result was independent of the cosmological models we simulated. We could not reproduce the cusp shallower than

STRUCTURE OF DARK MATTER HALOS FROM HIERARCHICAL CLUSTERING. III. SHALLOWING OF THE INNER CUSP

r^{-1.5}

GRAPE-6A: A Single-Card GRAPE-6 for Parallel PC-GRAPE Cluster Systems

, which was obtained in some of previous studies. We also found that in all runs the density structure evolves in a self-similar way, even in

GRAPE-5: A Special-Purpose Computer for N-Body Simulations

\Omega\ne 1

On the mass distribution of planetesimals in the early runaway stage

universes. These results show that the formation of structural form is a process decoupled from a background cosmology.

Structure of Dark Matter Halos from Hierarchical Clustering. II. Dependence of Cosmological Models in Cluster-sized Halos

In this paper a cluster is modelled as a smooth potential (due to the cluster stars) plus the steady tidal field of the Galaxy. In this model there is a minimum energy below which stars cannot escape. Above this energy, however, the time-scale on which a star escapes varies with the orbital parameters of the star (mainly its energy) in a way which we attempt to quantify, with both theoretical arguments and computer simulations. Within the limitations of the model we show that the time-scale is long enough to complicate the interpretation of full N-body simulations of clusters, and that stars above the escape energy may remain bound to the cluster for about a Hubble time.

GreeM: Massively Parallel TreePM Code for Large Cosmological N-body Simulations

In this paper, we describe the architecture and performance of the GRAPE-6 system, a massively-parallel special-purpose computer for astrophysical