We have reanalyzed a data set of 99 low redshift (z < 0:1) Abell clusters studied previously by Rhee, van Haarlem & Katgert (1989), and determined their shapes. For this, three di erent measures are used: two of which were originally used by Rhee et al., and one of which was used by Plionis, Barrow & Frenk (1991) in their investigation of clusters in the Lick catalogue. We use Monte-Carlo simulations of clusters to investigate the errors in the methods. For low ellipticity, all methods overestimate the cluster elongation, whereas the opposite is true for a highly attened system. Also background galaxies and shot noise have a rather large in uence on the measured quantities. The corrected distribution of cluster ellipticities shows a peak at 0:4 and extends to 0:8, consistent with results of some previous studies. However, the present study uses more than twice the number of clusters as the earlier studies, and is self-consistent. That is, with the corrected distribution over projected cluster shapes we can reconstruct the observed distribution over projected cluster shapes and the observed relation between the number of galaxies in a cluster and its ellipticity. To achieve this, we have to assume that there is an anti-correlation between the true (projected) ellipticity of a cluster and its number of galaxies. It is not necessary to assume that the ellipticity of a cluster increases when one only includes the brighter galaxies (as suggested by Binney 1977). Using a redshift-independent richness criterion of Vink & Katgert (1994), it is shown that the richer clusters are intrinsically more nearly spherical than the poorer ones. Furthermore, the corrected distribution of cluster shapes is found to be more consistent with a population that consists of purely prolate clusters than with a purely oblate population. We compare the corrected true distribution of (projected) ellipticities with pre-2 P.A.M. de Theije, P. Katgert & E. van Kampen dictions from N-body simulations. For this, we use a catalogue of 75 N-body simulated clusters (van Kampen 1994) which assume a CDM spectrum with = 1:0. The simulations include a recipe for galaxy formation and merging. The model clusters are expected to be a representative sample of all real clusters. They show a good resemblance with the data both in radial pro le and number of galaxies. 'Observing' these simulated clusters in exactly the same way as the real clusters produces an ellipticity distribution that extends to much higher and that has too few nearly spherical clusters. Preliminary results of simulations of the formation of clusters in an = 0:2 universe suggest that, on average, clusters are more nearly spherical in this case, as is expected on theoretical grounds. This shows that the elongations of clusters can provide a useful constraint on the value of .

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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 2563h−1 Mpc3. The cosmological scenarios studied differ in either the shape of the power spectrum of the initial fluctuations, its normalization, the density parameter Ω0 or the Hubble parameter H0. 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 the surroundings of the cluster. The present‐day rms mass fluctuation on scales of 8 h−1 Mpc, σ8, which is largely determined by the normalization of the initial power spectrum, has a large impact on the cluster properties. The latter, namely the cluster number density, mass, line‐of‐sight velocity dispersion and peculiar velocity, are also determined by Ω0, though to a somewhat lesser extent. Other parameters, such as H0, 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. In a companion paper, the properties of the model clusters are compared with observations to try to discriminate between different cosmological scenarios. Using scaling relations between the average properties of the cluster sample and the parameters of the formation scenario, one may try to interpolate between the scenarios studied here in order to find the parameters of the scenario that is most consistent with the data.

This paper describes a simulated data set that can be used for tracker evaluation. The data set is generated in the framework of the Multistatic Tracking Working Group (MSTWG). The simulations are carried out using the TNO SIMONA simulator, which can generate raw hydrophone data for an arbitrary number of sources, receivers, and targets. All of these can be moving or stationary. The simulations can include e.g. reverberation, ambient noise and intra-ping displacement. The simulated hydrophone data are beamformed and matched-filtered, and a contactformation algorithm is applied. All contacts of one tsource, receiver, ping] triplet are stored in a so-called contact file. These files are used by the MSTWG to evaluate and compare different tracking algorithms, the results of which are described in other papers in this special session.

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