64,794 research outputs found
Revealing the cosmic web dependent halo bias
Halo bias is the one of the key ingredients of the halo models. It was shown
at a given redshift to be only dependent, to the first order, on the halo mass.
In this study, four types of cosmic web environments: clusters, filaments,
sheets and voids are defined within a state of the art high resolution -body
simulation. Within those environments, we use both halo-dark matter
cross-correlation and halo-halo auto correlation functions to probe the
clustering properties of halos. The nature of the halo bias differs strongly
among the four different cosmic web environments we describe. With respect to
the overall population, halos in clusters have significantly lower biases in
the {} mass range. In other
environments however, halos show extremely enhanced biases up to a factor 10 in
voids for halos of mass {}. Such a strong
cosmic web environment dependence in the halo bias may play an important role
in future cosmological and galaxy formation studies. Within this cosmic web
framework, the age dependency of halo bias is found to be only significant in
clusters and filaments for relatively small halos \la 10^{12.5}\msunh.Comment: 14 pages, 14 figures, ApJ accepte
Intracluster stellar population properties from N-body cosmological simulations -- I. Constraints at
We use a high resolution collisionless simulation of a Virgo--like cluster in
a CDM cosmology to determine the velocity and clustering properties of
the diffuse stellar component in the intracluster region at the present epoch.
The simulated cluster builds up hierarchically and tidal interactions between
member galaxies and the cluster potential produce a diffuse stellar component
free-flying in the intracluster medium. Here we adopt an empirical scheme to
identify tracers of the stellar component in the simulation and hence study its
properties. We find that at the intracluster stellar light is mostly
unrelaxed in velocity space and clustered in structures whose typical
clustering radii are about 50 kpc at R=400--500 kpc from the cluster center,
and predict the radial velocity distribution expected in spectroscopic
follow-up surveys. Finally, we compare the spatial clustering in the simulation
with the properties of the Virgo intracluster stellar population, as traced by
ongoing intracluster planetary nebulae surveys in Virgo. The preliminary
results indicate a substantial agreement with the observed clustering
properties of the diffuse stellar population in Virgo.Comment: 39 pages, 10 figures, 8 tables, in press on ApJ. Bad image quality
for some figures because resizing is neede
The age dependence of halo clustering
We use a very large simulation of the concordance LCDM cosmogony to study the
clustering of dark matter haloes. For haloes less massive than about
1e13Msun/h, the amplitude of the two-point correlation function on large scales
depends strongly on halo formation time. Haloes that assembled at high redshift
are substantially more clustered than those that assembled more recently. The
effect is a smooth function of halo formation time and its amplitude increases
with decreasing halo mass. At 1e11 Msun/h the ``oldest'' 10% of haloes are more
than 5 times more strongly correlated than the ``youngest'' 10%. This
unexpected result is incompatible with the standard excursion set theory for
structure growth, and it contradicts a fundamental assumption of the halo
occupation distribution models often used to study galaxy clustering, namely
that the galaxy content of a halo of given mass is statistically independent of
its larger scale environment.Comment: 5 pages, 5 figures, MNRAS in press. Full resolution pdf file is
avaliable at http://www.mpa-garching.mpg.de/~gaoliang/GSW.pd
Primordial bound systems of superheavy particles as the source of ultra-high energy cosmic rays
Annihilation of superheavy particles in primordial bound systems is
considered as the source of Ultra High Energy Cosmic Rays (UHECR).Charge
conservation makes them to be produced in pairs, and the estimated separation
of particle and antiparticle in such pair is shown to be in some cases much
smaller than the average separation determined by the averaged number density
of considered particles. If the new U(1) charge is the source of a long range
field similar to electromagnetic field, the particle and antiparticle,
possessing that charge, can form primordial bound system with annihilation
timescale, which can satisfy the conditions, assumed for this type of UHECR
sources. These conditions severely constrain the possible properties of
considered particles. So, the proposed mechanism of UHECR origin is impossible
to realise, if the U(1) charged particles share ordinary weak, strong or
electromagnetic interactions. It makes the proposed mechanism of pairing and
binding of superheavy U(1) charged particles an effective theoretical tool in
the probes of the physics of very early Universe and of the hidden sector of
particle theory, underlying it.Comment: 12 page
Do finite size neutrally buoyant particles cluster?
We investigate the preferential concentration of particles which are
neutrally buoyant but with a diameter significantly larger than the dissipation
scale of the carrier flow. Such particles are known not to behave as flow
tracers (Qureshi et al., Phys. Re. Lett. 2007) but whether they do cluster or
not remains an open question. For this purpose, we take advantage of a new
turbulence generating apparatus, the Lagrangian Exploration Module which
produces homogeneous and isotropic turbulence in a closed water flow. The flow
is seeded with neutrally buoyant particles with diameter 700\mum, corresponding
to 4.4 to 17 times the turbulent dissipation scale when the rotation frequency
of the impellers driving the flow goes from 2 Hz to 12 Hz, and spanning a range
of Stokes numbers from 1.6 to 24.2. The spatial structuration of these
inclusions is then investigated by a Voronoi tesselation analysis, as recently
proposed by Monchaux et al. (Phys. Fluids 2010), from images of particle
concentration field taken in a laser sheet at the center of the flow. No matter
the rotating frequency and subsequently the Reynolds and Stokes numbers, the
particles are found not to cluster. The Stokes number by itself is therefore
shown to be an insufficient indicator of the clustering trend in particles
laden flows
Properties and Origin of Galaxy Velocity Bias in the Illustris Simulation
We use the hydrodynamical galaxy formation simulations from the Illustris
suite to study the origin and properties of galaxy velocity bias, i.e., the
difference between the velocity distributions of galaxies and dark matter
inside halos. We find that galaxy velocity bias is a decreasing function of the
ratio of galaxy stellar mass to host halo mass. In general, central galaxies
are not at rest with respect to dark matter halos or the core of halos, with a
velocity dispersion above 0.04 times that of the dark matter. The central
galaxy velocity bias is found to be mostly caused by the close interactions
between the central and satellite galaxies. For satellite galaxies, the
velocity bias is related to their dynamical and tidal evolution history after
being accreted onto the host halos. It depends on the time after the accretion
and their distances from the halo centers, with massive satellites generally
moving more slowly than the dark matter. The results are in broad agreements
with those inferred from modeling small-scale redshift-space galaxy clustering
data, and the study can help improve models of redshift-space galaxy
clustering.Comment: 15 pages, 11 figures. Accepted for publication in Ap
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