190 research outputs found
The abundance and clustering of dark haloes in the standard Lambda CDM cosmogony
Much evidence suggests that we live in a flat Cold Dark Matter universe with
a cosmological constant. Accurate analytic formulae are now available for many
properties of the dark halo population in such a Universe. Assuming current
``concordance'' values for the cosmological parameters, we plot halo abundance
against redshift as a function of halo mass, of halo temperature, of the
fraction of cosmic matter in haloes, of halo clustering strength, and of the
clustering strength of the z=0 descendants of high redshift haloes. These plots
are useful for understanding how nonlinear structure grows in the model. They
demonstrate a number of properties which may seem surprising, for example: 10^9
solar mass haloes are as abundant at z=20 as L_* galaxies are today; 10^6K
haloes are equally abundant at z=8 and at z=0; 10% of all matter is currently
in haloes hotter than 1 keV, while more than half is in haloes too cool to trap
photo-ionized gas; 1% of all matter at z=15 is in haloes hot enough to ionise
hydrogen; haloes of given mass or temperature are more clustered at higher
redshift; haloes with the abundance of present-day L_* galaxies are equally
clustered at all z10 are more
clustered at z=0 than are L_* galaxies.Comment: 10 pages, 2 ps figures, version to be published in MNRA
Virialization of Galaxy Clusters and Beyond
Using samples of structures identified by a multi-scale decomposition from
numerical simulation, we analyze the scale-dependence of the virialization of
clusters. We find that beyond the scale of full virialization there exists a
radius range over which clusters are quasi-virialized, i.e. while the internal
structure of an {\it individual} cluster is at substantial departure from
dynamical relaxation, some {\it statistical} properties of the multi-scale
identified clusters are approximately the same as those for the virialized
systems. The dynamical reason of the existence of quasi-virialization is that
some of the scaling properties of dynamically relaxed systems of cosmic
gravitational clustering approximately hold beyond the full virialization
regime. The "individual-statistical" duality of the quasi-virialization
provides an explanation of the observed puzzle that the total masses of
clusters derived from virial theorem are statistically the same as the
gravitational lensing determined masses, in spite of the presence of irregular
configuration and substructures in individual clusters. It also explains the
tight correlation between the velocity dispersion of optical galaxies and the
temperature of X-ray emitting gas. Consequently, the virial mass estimators
based on the assumptions of isothermal and hydrostatic model are statistically
applicable to scales on which the clusters are quasi-virialized. In the
quasi-virialization regime, the temperature functions of clusters also show
scaling. This feature is a useful discriminator among cosmological models.Comment: AAS Latex file, 22 pages+ 14 figures, accepted for publication in Ap
Constraining Cosmological Models by the Cluster Mass Function
We present a comparison between two observational and three theoretical mass
functions for eight cosmological models suggested by the data from the recently
completed BOOMERANG-98 and MAXIMA-1 cosmic microwave background (CMB)
anisotropy experiments as well as peculiar velocities (PVs) and type Ia
supernovae (SN) observations. The cosmological models have been proposed as the
best fit models by several groups. We show that no model is in agreement with
the abundances of X-ray clusters at .On the
other hand, we find that the BOOM+MAX+{\sl COBE}:I, Refined Concordance and
MDM are in a good agreement with the abundances of optical clusters.
The P11 and especially Concordance models predict a slightly lower abundances
than observed at . The BOOM+MAX+{\sl COBE}:II
and PV+CMB+SN models predict a slightly higher abundances than observed at
. The nonflat MAXIMA-1 is in a fatal conflict
with the observational cluster abundances and can be safely ruled out.Comment: 17 pages, 2 figures, reference added, figures changes, substantial
revision mad
Predicting the Clustering of X-Ray Selected Galaxy Clusters in Flux-Limited Surveys
(abridged) We present a model to predict the clustering properties of X-ray
clusters in flux-limited surveys. Our technique correctly accounts for past
light-cone effects on the observed clustering and follows the non-linear
evolution in redshift of the underlying DM correlation function and cluster
bias factor. The conversion of the limiting flux of a survey into the
corresponding minimum mass of the hosting DM haloes is obtained by using
theoretical and empirical relations between mass, temperature and X-ray
luminosity of clusters. Finally, our model is calibrated to reproduce the
observed cluster counts adopting a temperature-luminosity relation moderately
evolving with redshift. We apply our technique to three existing catalogues:
BCS, XBACs and REFLEX samples. Moreover, we consider an example of possible
future space missions with fainter limiting flux. In general, we find that the
amplitude of the spatial correlation function is a decreasing function of the
limiting flux and that the EdS models always give smaller correlation
amplitudes than open or flat models with low matter density parameter. In the
case of XBACs, the comparison with previous estimates of the observational
spatial correlation shows that only the predictions of models with Omega_0m=0.3
are in good agreement with the data, while the EdS models have too low a
correlation strength. Finally, we use our technique to discuss the best
strategy for future surveys. Our results show that the choice of a wide area
catalogue, even with a brighter limiting flux, is preferable to a deeper, but
with smaller area, survey.Comment: 20 pages, Latex using MN style, 11 figures enclosed. Version accepted
for publication in MNRA
The ROSAT-ESO Flux-Limited X-Ray (REFLEX) Galaxy Cluster Survey III: The Power Spectrum
We present a measure of the power spectrum on scales from 15 to 800 Mpc/h
using the ROSAT-ESO Flux-Limited X-Ray(REFLEX) galaxy cluster catalogue. The
REFLEX survey provides a sample of the 452 X-ray brightest southern clusters of
galaxies with the nominal flux limit S=3.0 10^{-12}erg/s/cm2 for the ROSAT
energy band (0.1-2.4)keV. Several tests are performed showing no significant
incompletenesses of the REFLEX clusters with X-ray luminosities brighter than
10^{43}erg/s up to scales of about 800 Mpc/h. They also indicate that cosmic
variance might be more important than previous studies suggest. We regard this
as a warning not to draw general cosmological conclusions from cluster samples
with a size smaller than REFLEX. Power spectra, P(k), of comoving cluster
number densities are estimated for flux- and volume-limited subsamples. The
most important result is the detection of a broad maximum within the comoving
wavenumber range 0.022<k<0.030 h/Mpc. The data suggest an increase of the power
spectral amplitude with X-ray luminosity. Compared to optically selected
cluster samples the REFLEX P(k)is flatter for wavenumbers k<0.05 h/Mpc thus
shifting the maximum of P(k) to larger scales. The smooth maximum is not
consistent with the narrow peak detected at k=0.05 h/Mpc using the Abell/ACO
richness data. In the range 0.02<k<0.4 h/Mpc general agreement is found
between the slope of the REFLEX P(k) and those obtained with optically selected
galaxies. A semi-analytic description of the biased nonlinear power spectrum in
redshift space gives the best agreement for low-density Cold Dark Matter models
with or without a cosmological constant.Comment: 22 pages, 20 figures, (A&A accepted), also available at
http://www.xray.mpe.mpg.de/theorie/REFLEX
The Galaxy Pairwise Velocity Dispersion as a Function of Local Density
The standard method of measuring the galaxy pairwise velocity dispersion on
small scales is heavily weighted by the densest regions in a way that is
difficult to calibrate. We propose a new statistic which measures the
small-scale velocity dispersion as an explicit function of density. Computing
this statistic for a volume-limited subsample of the Optical Redshift Survey,
we find that the small-scale velocity dispersion rises from 220 to 760 km/s as
density increases. We calculate this statistic for a series of mock catalogs
drawn from a hydrodynamic simulation of an \Omega h = 0.5 Cold Dark Matter
universe (standard CDM), and find that the observed velocity distribution lies
~1 \sigma below the simulations in each of eight density bins, formally ruling
out this model at the 7.4 \sigma level, quantifying the well-known problem that
this model produces too high a velocity dispersion. This comparison is
insensitive to the normalization of the power spectrum, although it is quite
sensitive to the density and velocity bias of galaxies relative to dark matter
on small scales. (abridged)Comment: 21 pages Latex with 6 embedded figures, submitted to Ap
Evolution of the Cluster Mass and Correlation Functions in LCDM Cosmology
The evolution of the cluster mass function and the cluster correlation
function from z = 0 to z = 3 are determined using 10^6 clusters obtained from
high-resolution simulations of the current best-fit LCDM cosmology (\Omega_m =
0.27, \sigma_8 = 0.84, h = 0.7). The results provide predictions for
comparisons with future observations of high redshift clusters. A comparison of
the predicted mass function of low redshift clusters with observations from
early Sloan Digital Sky Survey data, and the predicted abundance of massive
distant clusters with observational results, favor a slightly larger amplitude
of mass fluctuations (\sigma_8 = 0.9) and lower density parameter (\Omega_m =
0.2); these values are consistent within 1-\sigma with the current
observational and model uncertainties. The cluster correlation function
strength increases with redshift for a given mass limit; the clusters were more
strongly correlated in the past, due to their increasing bias with redshift -
the bias reaches b = 100 at z = 2 for M > 5 x 10^13 h^-1 M_sun. The
richness-dependent cluster correlation function, represented by the correlation
scale versus cluster mean separation relation, R0-d, is generally consistent
with observations. This relation can be approximated as R_0 = 1.7 d^0.6 h^-1
Mpc for d = 20 - 60 h^-1 Mpc. The R0-d relation exhibits surprisingly little
evolution with redshift for z < 2; this can provide a new test of the current
LCDM model when compared with future observations of high redshift clusters.Comment: 20 pages, 9 figures, accepted for publication in Ap
Constraints on a non-gaussian () CDM model
We consider constraints on the structure formation model based on
non-Gaussian fluctuations generated during inflation, which have
distributions. Using three data sets, the abundance of the clusters at ,
moderate and the correlation length, we show that constraints on the
non-Gaussianity and the amplitude of fluctuations and the density parameter can
be obtained. We obtain an upper bound for and a lower bound for the
non-Gaussianity and the amplitude of the fluctuations. Using the abundance of
clusters at , for the spectrum parameterized by cold dark matter
(CDM) shape parameter , we obtain an upper bound for the density
parameter and lower bounds for the amplitude and for the non-Gaussianity of fluctuations ,
where G=1 for Gaussian.Comment: 7 pages, 3 figures, MNRAS in pres
Cosmological constraints from clustering properties of galaxy clusters
In this paper, we discuss improvements of the Suto et al. (2000) model, in
the light of recent theoretical developments (new theoretical mass functions, a
more accurate mass-temperature relation and an improved bias model) to predict
the clustering properties of galaxy clusters and to obtain constraints on
cosmological parameters. We re-derive the two-point correlation function of
clusters of galaxies for OCDM and LambdaCDM cosmological models, and we compare
these results with the observed spatial correlation function for clusters in
RASS1 (ROSAT All-Sky Survey 1), and in XBACs (X-RAY Brighest Abell-Type)
samples. The comparison shows that the best agreement is obtained for the
LambdaCDM model with Omega=0.3. The values of the correlation length obtained,
(r_\simeq 28.2 \pm 5.2 \rm h^{-1}} Mpc for LambdaCDM), are larger than those
found in the literature and comparable with the results found in Borgani,
Plionis & Kolokotronis (1999). (REST IN THE PAPER ABSTRACT)Comment: printed in A&
The mass and temperature functions in a moving barrier model
In this paper, I use the extension of the excursion set model of Sheth &
Tormen (2002) and the barrier shape obtained in Del Popolo & Gambera (1998) to
calculate the unconditional halo mass function, and the conditional mass
function in several cosmological models. I show that the barrier obtained in
Del Popolo & Gambera (1998), which takes account of tidal interaction between
proto-haloes, is a better description of the mass functions than the spherical
collapse and is in good agreement with numerical simulations (Tozzi & Governato
1998, and Governato et al. 1999). The results are also in good agreement with
those obtained by Sheth & Tormen (2002), only slight differences are observed
expecially at the low mass end. I moreover calculate, and compare with
simulations, the temperature function obtained by means of the mass functions
previously calculated and also using an improved version of the M-T relation,
which accounts for the fact that massive clusters accrete matter
quasi-continuously, and finally taking account of the tidal interaction with
neighboring clusters. Even in this case the discrepancy between the
Press-Schecter predictions and simulations is considerably reduced.Comment: 23 pages; 11 encapsulated figures. Accepted for publication in MNRA
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