86 research outputs found
Cluster mass estimation from lens magnification
The mass of a cluster of galaxies can be estimated from its lens
magnification, which can be determined from the variation in number counts of
background galaxies. In order to derive the mass one needs to make assumptions
for the lens shear, which is unknown from the variation in number counts alone.
Furthermore, one needs to go beyond the weak lensing (linear) approximation as
most of the observational data is concentrated in the central parts of
clusters, where the lensing is strong. By studying the lensing properties of a
complete catalogue of galaxy cluster models, one can find reasonable
approximations about the lens shear as a function of the lens convergence. We
show that using these approximations one can fairly well reconstruct the
surface mass distribution from the magnification alone.Comment: 4 pages including 1 figure, LaTex, using sprocl.sty (included), To
appear in proceedings "Large Scale Structure: tracks and traces", Potsdam
1997, World Scientifi
Distribution functions for clusters of galaxies from N-body simulations
We present the results of an attempt to adapt the distribution function
formalism to characterize large-scale structures like clusters of galaxies that
form in a cosmological N-body simulation. While galaxy clusters are systems
that are not strictly in equilibrium, we show that their evolution can
nevertheless be studied using a physically motivated extension of the language
of equilibrium stellar dynamics. Restricting our analysis to the virialized
region, a prescription to limit the accessible phase-space is presented, which
permits the construction of both the isotropic and the anisotropic distribution
functions and . The method is applied to models extracted from a
catalogue of simulated clusters. Clusters evolved in open and flat background
cosmologies are followed during the course of their evolution, and are found to
transit through a sequence of what we define as `quasi-equilibrium' states. An
interesting feature is that the computed is well fit by an exponential
form. We conclude that the dynamical evolution of a cluster, undergoing
relaxation punctuated by interactions and violent mergers with consequent
energy-exchange, can be studied both in a qualitative and quantitative fashion
by following the time evolution of .Comment: 16 pages, LaTeX file, all figures included, revised version, accepted
for publication in MNRA
Dust and gas in distant cluster galaxies with ALMA
Going wider with ALMA: mapping a 'whole' (proto-)cluster with ALMA - is that feasible in a realistic amount of time
Overmerging and M/L ratios in phenomenological galaxy formation models
We show that the discrepancy between the Tully-Fisher relation and the
luminosity function predicted by most phenomenological galaxy formation models
is mainly due to overmerging of galaxy haloes. We have circumvented this
overmerging problem, which is inherent in both the Press-Schechter formalism
and dissipationless N-body simulations, by including a specific galaxy halo
formation recipe into an otherwise standard N-body code. This numerical
technique provides the merger trees which, together with simplified gas
dynamics and star formation physics, constitute our implementation of a
phenomenological galaxy formation model. Resolving the overmerging problem
provides us with the means to match both the I-band Tully-Fisher relation and
the B and K band luminosity functions within an EdS sCDM structure formation
scenario. It also allows us to include models for chemical evolution and
starbursts, which improves the match to observational data and renders the
modelling more realistic. We show that the inclusion of chemical evolution into
the modelling requires a significant fraction of stars to be formed in short
bursts triggered by merging events.Comment: 15 pages, 7 figures, to be published in MNRA
Properties of galaxy clusters in N-body simulations: Intrinsic properties
We study the influence of the various parameters of scenarios of large-scale
structure formation on properties of galaxy clusters, and investigate which
cluster properties are most sensitive to these parameters. We present a set of
large N-body simulations and derive the intrinsic properties of galaxy clusters
in these simulations, which represent a volume of Mpc^3. The
cosmological scenarios studied differ in either the shape of the power spectrum
of initial fluctuations, its normalization, the density parameter ,
or the Hubble parameter . Between each of the simulations, only one
parameter is set differently, so that we can study the influence of that
parameter on the cluster properties. The cluster properties that are studied
are the mass, line-of-sight velocity dispersion, peculiar velocity, intrinsic
shape, and orientation with respect to its surroundings. has a
large impact on the cluster properties. The latter, viz. the cluster number
density, mass, line-of-sight velocity dispersion and peculiar velocity, are
also determined by , though somewhat less. Other parameters, such
as , the tilt of the initial fluctuation spectrum, and the exact shape
of this spectrum, are generally less important. Unlike the other cluster
properties studied, the peculiar velocity is found to depend on all parameters
of the formation scenario. Using scaling relations between the average
properties of the cluster sample and the parameters of the formation scenario,
one may try and interpolate between the scenarios studied here in order to find
the parameters of the scenario that is most consistent with the data.Comment: 21 pages, Latex (mn.sty), 16 figure
A catalogue of galaxy cluster models
We present a technique to construct a fair sample of simulated galaxy
clusters, and build such a sample for a specific cosmological structure
formation scenario. Conventionally one extracts such a sample from a single
low-resolution large-scale simulation. Here we simulate the clusters
individually at high resolution. We construct a model catalogue selected on
expected final cluster mass. Such a catalogue will not have a well-defined
richness limit, because the relation between richness and mass is fairly broad.
However, by applying the appropriate completeness corrections, the results for
the mass-selected catalogue can be compared with observations for
richness-selected cluster catalogues. We build a model cluster catalogue for
the Omega_0=1 CDM scenario that is designed to mimic the ENACS sample of rich
Abell clusters. We use the distribution of richness, corrected for
incompleteness, to fix the present epoch. We find sigma_8=0.4-0.5, which is
consistent with other determinations. The catalogue is 70 per cent complete for
a richness larger than 50, but we do have a complete subsample for a richness
limit of 75. As a first test we compare the cumulative distribution of
line-of-sight velocity dispersions to those found for several observational
samples, and find that they match best for a value of around 0.4 for sigma_8.
This means that we find consistent values for sigma_8 for the CDM Omega_0=1
scenario on cluster scales.Comment: 24 pages, TeX with special macros included, postscript version also
available at http://www.tac.dk/~eelco/papers/cat.ps.gz or
http://www.roe.ac.uk/research/evk2.ps.gz; MNRAS, in pres
Hubble flow variations as a test for inhomogeneous cosmology
Context. Backreactions from large-scale inhomogeneities may provide an
elegant explanation for the observed accelerated expansion of the universe
without the need to introduce dark energy. Aims. We propose a cosmological test
for a specific model of inhomogeneous cosmology, called timescape cosmology.
Using large-scale galaxy surveys such as SDSS and 2MRS, we test the variation
of expansion expected in the -CDM model versus a more generic
differential expansion using our own calibrations of bounds suggested by
timescape cosmology. Method. Our test measures the systematic variations of the
Hubble flow towards distant galaxies groups as a function of the matter
distribution in the lines of sight to those galaxy groups. We compare the
observed systematic variation of the Hubble flow to mock catalogues from the
Millennium Simulation in the case of the -CDM model, and a deformed
version of the same simulation that exhibits more pronounced differential
expansion. Results. We perform a series of statistical tests, ranging from
linear regressions to Kolmogorov-Smirnov tests, on the obtained data. They
consistently yield results preferring -CDM cosmology over our
approximated model of timescape cosmology. Conclusions. Our analysis of
observational data shows no evidence that the variation of expansion differs
from that of the standard -CDM model.Comment: 20 pages, 21 figures, accepted for publication in A&
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