10 research outputs found
Dark Matter Halos within Clusters
We examine the properties of dark matter halos within a rich galaxy cluster
using a high resolution simulation that captures the cosmological context of a
cold dark matter universe. The mass and force resolution permit the resolution
of 150 halos with circular velocities larger than 80 kms within the cluster's
virial radius of 2 Mpc. This enables an unprecedented study of the statistical
properties of a large sample of dark matter halos evolving in a dense
environment. The cumulative fraction of mass attached to these halos varies
from 0% at 200 kpc, to 13% at the virial radius. Even at this resolution the
overmerging problem persists; halos that pass within 200 kpc of the cluster
center are tidally disrupted. Additional substructure is lost at earlier epochs
within the massive progenitor halos. The median ratio of apocentric to
pericentric radii is 6:1; the orbital distribution is close to isotropic,
circular orbits are rare, radial orbits are common. The orbits of halos are
unbiased with respect to both position within the cluster and with the orbits
of the smooth dark matter background and no velocity bias is detected. The
tidal radii of surviving halos are generally well-fit using the simple analytic
prediction applied to their orbital pericenters. Halos within clusters have
higher concentrations than those in the field. Within the cluster, halo density
profiles can be modified by tidal forces and individual encounters with other
halos that cause significant mass loss - ``galaxy harassment''. Mergers between
halos do not occur inside the clusters virial radius.Comment: LaTeX MN style, 20 pages, 30 figures included + 1 colour plo
Dark Matter Substructure in Galactic Halos
We use numerical simulations to examine the substructure within galactic and
cluster mass halos that form within a hierarchical universe. Clusters are
easily reproduced with a steep mass spectrum of thousands of substructure
clumps that closely matches observations. However, the survival of dark matter
substructure also occurs on galactic scales, leading to the remarkable result
that galaxy halos appear as scaled versions of galaxy clusters. The model
predicts that the virialised extent of the Milky Way's halo should contain
about 500 satellites with circular velocities larger than Draco and Ursa-Minor
i.e. bound masses > 10^8Mo and tidally limited sizes > kpc. The substructure
clumps are on orbits that take a large fraction of them through the stellar
disk leading to significant resonant and impulsive heating. Their abundance and
singular density profiles has important implications for the existence of old
thin disks, cold stellar streams, gravitational lensing and indirect/direct
detection experiments.Comment: Astrophysical Journal Letters. 4 pages, latex. Simulation images and
movies at http://star-www.dur.ac.uk:80/~moore