Peculiar velocities of galaxy clusters

We investigate the peculiar velocities predicted for galaxy clusters by theories in the cold dark matter family. A widely used hypothesis identifies rich clusters with high peaks of a suitably smoothed version of the linear density fluctuation field. Their peculiar velocities are then obtained by extrapolating the similarly smoothed linear peculiar velocities at the positions of these peaks. We test these ideas using large high resolution N-body simulations carried out within the Virgo supercomputing consortium. We find that at early times the barycentre of the material which ends up in a rich cluster is generally very close to a high peak of the initial density field. Furthermore the mean peculiar velocity of this material agrees well with the linear value at the peak. The late-time growth of peculiar velocities is, however, systematically underestimated by linear theory. At the time clusters are identified we find their rms peculiar velocity to be about 40% larger than predicted. Nonlinear effects are particularly important in superclusters. These systematics must be borne in mind when using cluster peculiar velocities to estimate the parameter combination #sigma#_8#OMEGA#"0"."6. (orig.)27 refs.Available from TIB Hannover: RR 4697(1096) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Evolution of structure in cold dark matter universes

We present an analysis of the clustering evolution of dark matter in four cold dark matter (CDM) cosmologies. We use a suite of high resolution, 17-million particle, N-body simulations which sample volumes large enough to give clustering statistics with unprecedented accuracy. We investigate a flat model with #OMEGA#_0 = 0.3, an open model also with #OMEGA#_0 = 0.3, and two models with #OMEGA# = 1, one with the standard CDM power spectrum and the other with the same power spectrum as the #OMEGA#_0 = 0.3 models. In all cases, the amplitude of primordial fluctuations is set so that the models reproduce the observed abundance of rich galaxy clusters by the present day. The mass two-point correlation function and power spectrum of all the simulations differ significantly from those of the observed galaxy distribution, in both shape and amplitude. Thus, for any of these models to provide an acceptable representation of reality, the distribution of galaxies must be biased relative to the mass in a non-trivial, scale-dependent, fashion. In the #OMEGA# = 1 models the required bias is always greater than unity, but in the #OMEGA#_0 = 0.3 models an ''antibias'' is required on scales smaller than #propor to#5h"-"1 Mpc. The mass correlation functions in the simulations are well fit by recently published analytic models. The velocity fields are remarkably similar in all the models, whether they be characterised as bulk flows, single-particle or pairwise velocity dispersions. This similarity is a direct consequence of our adopted normalisation and contradicts the common belief that the amplitude of the observed galaxy velocity fields can be used to constrain the value of #OMEGA#_0. The small-scale pairwise velocity dispersion of the dark matter is somewhat larger than recent determinations from galaxy redshift surveys, but (orig.)120 refs.SIGLEAvailable from TIB Hannover: RR 4697(1048) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Genus statistics of the Virgo N-body simulations and the 1.2-Jy redshift survey

We study the topology of the Virgo N-body simulations and compare it to the 1.2-Jy redshift survey of IRAS galaxies by means of the genus statistic. Four high-resolution simulations of variants of the CDM cosmology are considered: a flat standard model (SCDM), a variant of it with more large-scale power (#tau#CDM), and two low density universes, one open (OCDM, #OMEGA#_0 = 0.3) and one flat (#LAMBDA#CDM, #OMEGA#_0 = 0.3, #LAMBDA# = 0.7). In all cases, the initial fluctuation amplitudes are chosen so that the simulations approximately reproduce the observed abundance of rich clusters of galaxies at the present day. The fully sampled N-body simulations are examined down to strongly nonlinear scales, both with spatially fixed smoothing, and with an adaptive smoothing technique. While the #tau#CDM, #LAMBDA#CDM, and OCDM simulations have very similar genus statistics in the regime accessible to fixed smoothing, they can be separated with adaptive smoothing at small mass scales. In order to compare the N-body models with the 1.2-Jy survey, we extract large ensembles of mock catalogues from the simulations. These mock surveys are used to test for various systematic effects in the genus analysis and to establish the distribution of errors of the genus curve. We find that a simple multivariate analysis of the genus measurements is compromised both by non-Gaussian distributed errors and by noise that dominates the covariance matrix. We therefore introduce a principal components analysis of the genus curve38 refs.SIGLEAvailable from TIB Hannover: RR 4697(1046) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman