184 research outputs found
The Correlation Function of Clusters of Galaxies and the Amplitude of Mass Fluctuations in the Universe
We show that if a sample of galaxy clusters is complete above some mass
threshold, then hierarchical clustering theories for structure formation
predict its autocorrelation function to be determined purely by the cluster
abundance and by the spectrum of linear density fluctuations. Thus if the shape
of the initial fluctuation spectrum is known, its amplitude can be
estimated directly from the correlation length of a cluster sample in a way
which is independent of the value of . If the cluster mass
corresponding to the sample threshold is also known, it provides an independent
estimate of the quantity . Thus cluster data should
allow both and to be determined observationally. We
explore these questions using N-body simulations together with a simple but
accurate analytical model based on extensions of Press-Schechter theory.
Applying our results to currently available data we find that if the linear
fluctuation spectrum has a shape similar to that suggested by the APM galaxy
survey, then a correlation length in excess of 20\mpch for Abell
clusters would require , while r_0<15\mpch would require
. With conventional estimates of the relevant mass threshold
these imply \Omega_0\la 0.3 and \Omega_0\ga 1 respectively.Comment: Latex, 25 pages (including 8 PS figures). The PS-file of the paper is
also available via anonymous ftp at:
ftp://ibm-3.mpa-garching.mpg.de/pub/jing/xicc.ps . Submitted to MNRAS. In the
replaced version, a typo in Eq.(1a) is fixe
Establishing the relationship between galaxies and dark matter
We use two methods to establish the relationship between galaxies and dark matter halos. One is based the conditional luminosity function model, which links galaxies and dark matter halos by matching the number density and clustering properties of galaxies with those of dark matter halos in the current CDM model. The second is based on galaxy systems identified from large redshift surveys of galaxies. The galaxy - dark halo relationships established by these two methods match well, and can provide important constraints on how galaxies form and evolve in the univers
Halo Shapes, Dynamics and Environment
In the hierarchical structure formation model cosmic halos are supposed to
form by accretion of smaller units along anisotropic direction, defined by
large-scale filamentary structures. After the epoch of primary mass aggregation
(which depend on the cosmological model), violent relaxation processes will
tend to alter the halo phase-space configuration producing quasi-spherical
halos with a relatively smooth density profiles.
Here we attempt to investigate the relation between halos shapes, their
environment and their dynamical state. To this end we have run a large ( Mpc, particles) N-body simulation of a flat low-density
cold dark matter model with a matter density , Hubble constant km s
Mpc and a normalization parameter of . The particle mass
is comparable to the mass of
one single galaxy. The halos are defined using a friends-of-friend algorithm
with a linking length given by where is the mean
density. This linking length corresponds to an overdensity at the present epoch () and the total number of halos
with more than 130 particles () is 57524.Comment: To be published in "Groups Of Galaxies In The Nearby Universe", held
in Chile, December 2005, edited by I.Saviane, V.Ivanov and J.Borissova.
Springer-Verlag series "ESO Astrophysics Symposia
Dynamical Dark Energy simulations: high accuracy Power Spectra at high redshift
Accurate predictions on non--linear power spectra, at various redshift z,
will be a basic tool to interpret cosmological data from next generation mass
probes, so obtaining key information on Dark Energy nature. This calls for high
precision simulations, covering the whole functional space of w(z) state
equations and taking also into account the admitted ranges of other
cosmological parameters; surely a difficult task. A procedure was however
suggested, able to match the spectra at z=0, up to k~3, hMpc^{-1}, in
cosmologies with an (almost) arbitrary w(z), by making recourse to the results
of N-body simulations with w = const. In this paper we extend such procedure to
high redshift and test our approach through a series of N-body gravitational
simulations of various models, including a model closely fitting WMAP5 and
complementary data. Our approach detects w= const. models, whose spectra meet
the requirement within 1% at z=0 and perform even better at higher redshift,
where they are close to a permil precision. Available Halofit expressions,
extended to (constant) w \neq -1 are unfortunately unsuitable to fit the
spectra of the physical models considered here. Their extension to cover the
desired range should be however feasible, and this will enable us to match
spectra from any DE state equation.Comment: method definitely improved in semplicity and efficacy,accepted for
publication on JCA
Correlated Hybrid Fluctuations from Inflation with Thermal Dissipation
We investigate the primordial scalar perturbations in the thermal dissipative
inflation where the radiation component (thermal bath) persists and the density
fluctuations are thermally originated. The perturbation generated in this model
is hybrid, i.e. it consists of both adiabatic and isocurvature components. We
calculate the fractional power ratio () and the correlation coefficient
() between the adiabatic and the isocurvature perturbations at the
commencing of the radiation regime. Since the adiabatic/isocurvature
decomposition of hybrid perturbations generally is gauge-dependent at
super-horizon scales when there is substantial energy exchange between the
inflaton and the thermal bath, we carefully perform a proper decomposition of
the perturbations. We find that the adiabatic and the isocurvature
perturbations are correlated, even though the fluctuations of the radiation
component is considered uncorrelated with that of the inflaton. We also show
that both and depend mainly on the ratio between the
dissipation coefficient and the Hubble parameter during inflation.
The correlation is positive () for strong dissipation cases
where , and is negative for weak dissipation instances where
. Moreover, and in this model are not
independent of each other. The predicted relation between and
is consistent with the WMAP observation. Other testable predictions are also
discussed.Comment: 18 pages using revtex4, accepted for publication in PR
Effects of dark sectors' mutual interaction on the growth of structures
We present a general formalism to study the growth of dark matter
perturbations when dark energy perturbations and interactions between dark
sectors are present. We show that dynamical stability of the growth of
structure depends on the type of coupling between dark sectors. By taking the
appropriate coupling to ensure the stable growth of structure, we observe that
the effect of the dark sectors' interaction overwhelms that of dark energy
perturbation on the growth function of dark matter perturbation. Due to the
influence of the interaction, the growth index can differ from the value
without interaction by an amount within the observational sensibility, which
provides a possibility to disclose the interaction between dark sectors through
future observations on the growth of large structure.Comment: 15 pages, 4 figures, revised version, to appear in JCA
Testing the interaction of dark energy to dark matter through the analysis of virial relaxation of clusters Abell Clusters A586 and A1689 using realistic density profiles
Interaction between dark energy and dark matter is probed through deviation
from the virial equilibrium for two relaxed clusters: A586 and A1689. The
evaluation of the virial equilibrium is performed using realistic density
profiles. The virial ratios found for the more realistic density profiles are
consistent with the absence of interaction.Comment: 16pp 1 fig; accepted by GeR
The phase-space structure of a dark-matter halo: Implications for dark-matter direct detection experiments
We study the phase-space structure of a dark-matter halo formed in a high
resolution simulation of a Lambda CDM cosmology. Our goal is to quantify how
much substructure is left over from the inhomogeneous growth of the halo, and
how it may affect the signal in experiments aimed at detecting the dark matter
particles directly. If we focus on the equivalent of ``Solar vicinity'', we
find that the dark-matter is smoothly distributed in space. The probability of
detecting particles bound within dense lumps of individual mass less than 10^7
M_\sun h^{-1} is small, less than 10^{-2}. The velocity ellipsoid in the Solar
neighbourhood deviates only slightly from a multivariate Gaussian, and can be
thought of as a superposition of thousands of kinematically cold streams. The
motions of the most energetic particles are, however, strongly clumped and
highly anisotropic. We conclude that experiments may safely assume a smooth
multivariate Gaussian distribution to represent the kinematics of dark-matter
particles in the Solar neighbourhood. Experiments sensitive to the direction of
motion of the incident particles could exploit the expected anisotropy to learn
about the recent merging history of our Galaxy.Comment: 13 pages, 13 figures, Phys. Rev. D in press. Postscript version with
high resolution figures available from
http://www.mpa-garching.mpg.de/~ahelmi/research/lcdm_dm.html; some changes in
the text; constraints on the effect of bound dark-matter lumps revised;
remaining conclusions unchange
Non-Markovian dynamics for an open two-level system without rotating wave approximation: Indivisibility versus backflow of information
By use of the two measures presented recently, the indivisibility and the
backflow of information, we study the non-Markovianity of the dynamics for a
two-level system interacting with a zero-temperature structured environment
without using rotating wave approximation (RWA). In the limit of weak coupling
between the system and the reservoir, and by expanding the time-convolutionless
(TCL) generator to the forth order with respect to the coupling strength, the
time-local non-Markovian master equation for the reduced state of the system is
derived. Under the secular approximation, the exact analytic solution is
obtained and the sufficient and necessary conditions for the indivisibility and
the backflow of information for the system dynamics are presented. In the more
general case, we investigate numerically the properties of the two measures for
the case of Lorentzian reservoir. Our results show the importance of the
counter-rotating terms to the short-time-scale non-Markovian behavior of the
system dynamics, further expose the relations between the two measures and
their rationality as non-Markovian measures. Finally, the complete positivity
of the dynamics of the considered system is discussed
An analytical model for the non-linear redshift-space power spectrum
We use N-body simulations to test the predictions of the redshift distortion
in the power spectrum given by the halo model in which the clustering of dark
matter particles is considered as a result both of the clustering of dark halos
in space and of the distribution of dark matter particles in individual dark
halo. The predicted redshift distortion depends sensitively on several model
parameters in a way different from the real-space power spectrum. An accurate
model of the redshift distortion can be constructed if the following properties
of the halo population are modelled accurately: the mass function of dark
halos, the velocity dispersion among dark halos, and the non-linear nature of
halo bias on small scales. The model can be readily applied to interpreting the
clustering properties and velocity dispersion of different populations of
galaxies once a cluster-weighted bias (or equivalently an halo occupation
number model) is specified for the galaxies. Some non-trivial bias features
observed from redshift surveys of optical galaxies and of IRAS galaxies
relative to the standard low-density cold dark matter model can be easily
explained in the cluster weighted bias model. The halo model further indicates
that a linear bias can be a good approximation only on for k <= 0.1 hMpc^{-1}.Comment: 10 pages, 10 figures, accepted for publication in MNRA
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