5,364 research outputs found
Tidal Effects in Clusters of Galaxies
High-redshift clusters of galaxies show an over-abundance of spirals by a
factor of 2-3, and the corresponding under-abundance of S0 galaxies, relative
to the nearby clusters. This morphological evolution can be explained by tidal
interactions with neighboring galaxies and with the hierarchically growing
cluster halo. The efficiency of tidal interactions depends on the size and
structure of the cluster, as well as on the epoch of its formation. I simulate
the formation and evolution of Virgo-type clusters in three cosmologies: a
critical density model Omega_0=1, an open model Omega_0=0.4, and a flat model
Omega_0=0.4 with a cosmological constant. The orbits of identified halos are
traced with a high temporal resolution (~10^7 yr). Halos with low relative
velocities merge only shortly after entering the cluster; after virialization
mergers are suppressed. The dynamical evolution of galaxies is determined by
the tidal field along their trajectories. The maxima of the tidal force do not
always correspond to closest approach to the cluster center. They are produced
to a large extent by the local density structures, such as the massive galaxies
and the unvirialized remnants of infalling groups of galaxies. Collisions of
galaxies are intensified by the substructure, with about 10 encounters within
10 kpc per galaxy in the Hubble time. These very close encounters add an
important amount (10-50%) of the total heating rate. The integrated effect of
tidal interactions is insufficient to transform a spiral galaxy into an
elliptical, but can produce an S0 galaxy. Overall, tidal heating is stronger in
the low Omega_0 clusters
The Next Generation Virgo Cluster Survey. IX. Estimating the Efficiency of Galaxy Formation on the Lowest-Mass Scales
The Next Generation Virgo Cluster Survey has recently determined the
luminosity function of galaxies in the core of the Virgo cluster down to
unprecedented magnitude and surface brightness limits. Comparing simulations of
cluster formation to the derived central stellar mass function, we attempt to
estimate the stellar-to-halo-mass ratio (SHMR) for dwarf galaxies, as it would
have been before they fell into the cluster. This approach ignores several
details and complications, e.g., the contribution of ongoing star formation to
the present-day stellar mass of cluster members, and the effects of adiabatic
contraction and/or violent feedback on the subhalo and cluster potentials. The
final results are startlingly simple, however; we find that the trends in the
SHMR determined previously for bright galaxies appear to extend down in a
scale-invariant way to the faintest objects detected in the survey. These
results extend measurements of the formation efficiency of field galaxies by
two decades in halo mass, or five decades in stellar mass, down to some of the
least massive dwarf galaxies known, with stellar masses of .Comment: 18 pages, 12 figures; published in ApJ July 1st 201
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
Dark Matter Halos from the Inside Out
The balance of evidence indicates that individual galaxies and groups or
clusters of galaxies are embedded in enormous distributions of cold, weakly
interacting dark matter. These dark matter 'halos' provide the scaffolding for
all luminous structure in the universe, and their properties comprise an
essential part of the current cosmological model. I review the internal
properties of dark matter halos, focussing on the simple, universal trends
predicted by numerical simulations of structure formation. Simulations indicate
that halos should all have roughly the same spherically-averaged density
profile and kinematic structure, and predict simple distributions of shape,
formation history and substructure in density and kinematics, over an enormous
range of halo mass and for all common variants of the concordance cosmology. I
describe observational progress towards testing these predictions by measuring
masses, shapes, profiles and substructure in real halos, using baryonic tracers
or gravitational lensing. An important property of simulated halos (possibly
the most important property) is their dynamical 'age', or degree of internal
relaxation. The age of a halo may have almost as much effect as its mass in
determining the state of its baryonic contents, so halo ages are also worth
trying to measure observationally. I review recent gravitational lensing
studies of galaxy clusters which should measure substructure and relaxation in
a large sample of individual cluster halos, producing quantitative measures of
age that are well-matched to theoretical predictions. The age distributions
inferred from these studies will lead to second-generation tests of the
cosmological model, as well as an improved understanding of cluster assembly
and the evolution of galaxies within clusters.Comment: v2: additional references and minor corrections to match the
published versio
Characterizing the Cluster Lens Population
We present a detailed investigation into which properties of CDM halos make
them effective strong gravitational lenses. Strong lensing cross sections of
878 clusters from an N-body simulation are measured by ray tracing through
13,594 unique projections. We measure concentrations, axis ratios,
orientations, and the amount of substructure of each cluster, and compare the
lensing weighted distribution of each quantity to that of the cluster
population as a whole. The concentrations of lensing clusters are on average
34% larger than the typical cluster in the Universe. Despite this bias, the
anomalously high concentrations (c >14) recently measured by several groups,
appear to be inconsistent with the concentration distribution in our
simulations, which predict < 2% of lensing clusters should have concentrations
this high. No correlation is found between lensing cross section and the amount
of substructure. We introduce several types of simplified dark matter halos,
and use them to isolate which properties of CDM clusters make them effective
lenses. Projections of halo substructure onto small radii and the large scale
mass distribution of clusters do not significantly influence cross sections.
The abundance of giant arcs is primarily determined by the mass distribution
within an average overdensity of ~ 10,000. A multiple lens plane ray tracing
algorithm is used to show that projections of large scale structure increase
the giant arc abundance by a modest amount <7%. We revisit the question of
whether there is an excess of giant arcs behind high redshift clusters in the
RCS survey and find that the number of high redshift (z > 0.6) lenses is in
good agreement with LCDM, although our simulations predict more low redshift (z
< 0.6) lenses than were observed. (abridged)Comment: 19 pages, 15 figures. Submitted to Ap
Velocity and spatial biases in CDM subhalo distributions
We present a statistical study of substructure within a sample of LCDM
clusters and galaxies simulated with up to 25 million particles. With thousands
of subhalos per object we can accurately measure their spatial clustering and
velocity distribution functions and compare these with observational data. The
substructure properties of galactic halos closely resembles those of galaxy
clusters with a small scatter in the mass and circular velocity functions. The
velocity distribution function is non-Maxwellian and flat topped with a
negative kurtosis of about -0.7. Within the virial radius the velocity bias
, increasing to b > 1.3
within the halo centers. Slow subhalos are much less common, due to physical
disruption by gravitational tides early in the merging history. This leads to a
spatially anti-biased subhalo distribution that is well fitted by a cored
isothermal. Observations of cluster galaxies do not show such biases which we
interpret as a limitation of pure dark matter simulations - we estimate that we
are missing half of the halo population which has been destroyed by physical
overmerging. High resolution hydrodynamical simulations are required to study
these issues further. If CDM is correct then the cluster galaxies must survive
the tidal field, perhaps due to baryonic inflow during elliptical galaxy
formation. Spirals can never exist near the cluster centers and the elliptical
galaxies there will have little remaining dark matter. This implies that the
morphology-density relation is set {\it before} the cluster forms, rather than
a subsequent transformation of disks to S0's by virtue of the cluster
environment.Comment: MNRAS accepted version. Due to an error in the initial conditions
these simulations have a lower sigma_8 than the published value, 0.7 instead
of 0.9. We thank Mike Kuhlen who helped us finding this mistake. See the
erratum at http://www-theorie.physik.unizh.ch/~diemand/suberr.pdf . Images
and movies available at http://www-theorie.physik.unizh.ch/~diemand/clusters
A Study of the Merger History of the Galaxy Group HCG 62 Based on X-Ray Observations and SPH Simulations
We choose the bright compact group HCG 62, which was found to exhibit both
excess X-ray emission and high Fe abundance to the southwest of its core, as an
example to study the impact of mergers on chemical enrichment in the intragroup
medium. We first reanalyze the high-quality Chandra and XMM-Newton archive data
to search for the evidence for additional SN II yields, which is expected as a
direct result of the possible merger-induced starburst. We reveal that, similar
to the Fe abundance, the Mg abundance also shows a high value in both the
innermost region and the southwest substructure, forming a high-abundance
plateau, meanwhile all the SN Ia and SN II yields show rather flat
distributions in in favor of an early enrichment. Then we carry
out a series of idealized numerical simulations to model the collision of two
initially isolated galaxy groups by using the TreePM-SPH GADGET-3 code. We find
that the observed X-ray emission and metal distributions, as well as the
relative positions of the two bright central galaxies with reference to the
X-ray peak, can be well reproduced in a major merger with a mass ratio of 3
when the merger-induced starburst is assumed. The `best-match' snapshot is
pinpointed after the third pericentric passage when the southwest substructure
is formed due to gas sloshing. By following the evolution of the simulated
merging system, we conclude that the effects of such a major merger on chemical
enrichment are mostly restricted within the core region when the final relaxed
state is reached.Comment: Accepted for publication in the Astrophysical Journa
Neutrinos in IceCube/KM3NeT as probes of Dark Matter Substructures in Galaxy Clusters
Galaxy clusters are one of the most promising candidate sites for dark matter
annihilation. We focus on dark matter with mass in the range 10 GeV - 100 TeV
annihilating to muon pairs, neutrino pairs, top pairs, or two neutrino pairs,
and forecast the expected sensitivity to the annihilation cross section into
these channels by observing galaxy clusters at IceCube/KM3NeT. Optimistically,
the presence of dark matter substructures in galaxy clusters is predicted to
enhance the signal by 2-3 orders of magnitude over the contribution from the
smooth component of the dark matter distribution. Optimizing for the angular
size of the region of interest for galaxy clusters, the sensitivity to the
annihilation cross section of heavy DM with mass in the range 300 GeV - 100 TeV
will be of the order of 10^{-24} cm^3 s^{-1}, for full IceCube/KM3NeT live time
of 10 years, which is about one order of magnitude better than the best limit
that can be obtained by observing the Milky Way halo. We find that neutrinos
from cosmic ray interactions in the galaxy cluster, in addition to the
atmospheric neutrinos, are a source of background. We show that significant
improvement in the experimental sensitivity can be achieved for lower DM masses
in the range 10 GeV - 300 GeV if neutrino-induced cascades can be reconstructed
to approximately 5 degrees accuracy, as may be possible in KM3NeT. We therefore
propose that a low-energy extension "KM3NeT-Core", similar to DeepCore in
IceCube, be considered for an extended reach at low DM masses.Comment: v2: 17 pages, 5 figures. Neutrino spectra corrected, dependence on
dark matter substructure model included, references added. Results unchanged.
Accepted in PR
Numerical Simulations of the Dark Universe: State of the Art and the Next Decade
We present a review of the current state of the art of cosmological dark
matter simulations, with particular emphasis on the implications for dark
matter detection efforts and studies of dark energy. This review is intended
both for particle physicists, who may find the cosmological simulation
literature opaque or confusing, and for astro-physicists, who may not be
familiar with the role of simulations for observational and experimental probes
of dark matter and dark energy. Our work is complementary to the contribution
by M. Baldi in this issue, which focuses on the treatment of dark energy and
cosmic acceleration in dedicated N-body simulations. Truly massive dark
matter-only simulations are being conducted on national supercomputing centers,
employing from several billion to over half a trillion particles to simulate
the formation and evolution of cosmologically representative volumes (cosmic
scale) or to zoom in on individual halos (cluster and galactic scale). These
simulations cost millions of core-hours, require tens to hundreds of terabytes
of memory, and use up to petabytes of disk storage. The field is quite
internationally diverse, with top simulations having been run in China, France,
Germany, Korea, Spain, and the USA. Predictions from such simulations touch on
almost every aspect of dark matter and dark energy studies, and we give a
comprehensive overview of this connection. We also discuss the limitations of
the cold and collisionless DM-only approach, and describe in some detail
efforts to include different particle physics as well as baryonic physics in
cosmological galaxy formation simulations, including a discussion of recent
results highlighting how the distribution of dark matter in halos may be
altered. We end with an outlook for the next decade, presenting our view of how
the field can be expected to progress. (abridged)Comment: 54 pages, 4 figures, 3 tables; invited contribution to the special
issue "The next decade in Dark Matter and Dark Energy" of the new Open Access
journal "Physics of the Dark Universe". Replaced with accepted versio
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