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 $f(E)$ and $f(E,L)$. 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 $f(E)$ 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 $f(E)$.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 $256^3 h^{-3}$ Mpc^3. The cosmological scenarios studied differ in either the shape of the power spectrum of initial fluctuations, its normalization, the density parameter $\Omega_{0}$, or the Hubble parameter $H_{0}$. 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. $\sigma_{8}$ 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 $\Omega_{0}$, though somewhat less. Other parameters, such as $H_{0}$, 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 $\Lambda$-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 $\Lambda$-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 $\Lambda$-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 $\Lambda$-CDM model.Comment: 20 pages, 21 figures, accepted for publication in A&