A B S T R A C TWe introduce a new technique for following the formation and evolution of galaxies in cosmological N-body simulations. Dissipationless simulations are used to track the formation and merging of dark matter haloes as a function of redshift. Simple prescriptions, taken directly from semi-analytic models of galaxy formation, are adopted for gas cooling, star formation, supernova feedback and the merging of galaxies within the haloes. This scheme enables us to explore the clustering properties of galaxies, and to investigate how selection by luminosity, colour or type influences the results. In this paper we study the properties of the galaxy distribution at z ¼ 0. These include B-and K-band luminosity functions, two-point correlation functions, pairwise peculiar velocities, cluster mass-to-light ratios, B ¹ V colours, and star formation rates. We focus on two variants of a cold dark matter (CDM) cosmology: a high-density (Q ¼ 1) model with shape-parameter G ¼ 0:21 (tCDM), and a low-density model with Q ¼ 0:3 and L ¼ 0:7 (LCDM). Both models are normalized to reproduce the Iband Tully-Fisher relation of Giovanelli et al. near a circular velocity of 220 km s ¹1 . Our results depend strongly both on this normalization and on the adopted prescriptions for star formation and feedback. Very different assumptions are required to obtain an acceptable model in the two cases. For tCDM, efficient feedback is required to suppress the growth of galaxies, particularly in low-mass field haloes. Without it, there are too many galaxies and the correlation function exhibits a strong turnover on scales below 1 Mpc. For LCDM, feedback must be weaker, otherwise too few L ¬ galaxies are produced and the correlation function is too steep. Although neither model is perfect, both come close to reproducing most of the data. Given the uncertainties in modelling some of the critical physical processes, we conclude that it is not yet possible to draw firm conclusions about the values of cosmological parameters from studies of this kind. Further observational work on global star formation and feedback effects is required to narrow the range of possibilities. Figure 3. Top panel: A comparison of the abundance of haloes as a function of circular velocity predicted by the Press-Schechter theory (solid) and the abundance derived from the simulation (dotted). F is in units of Number/ Mpc 3 / 0.1 interval of logðV c Þ. Middle panel: Magnitude of the central galaxy as a function of halo circular velocity. Error bars show the rms scatter between haloes. The Press-Schechter theory haloes are solid and the simulation is dotted. Bottom panel: The V-band field luminosity functions derived using the Press-Schechter approach (solid) and the simulations (dotted). F is in units of Number/ Mpc 3 / magnitude interval.

We present results on the X-ray properties of clusters and groups of galaxies, extracted from a large cosmological hydrodynamical simulation. We used the TREE+SPH code GADGET to simulate a concordance Λ cold dark matter cosmological model within a box of 192 h-1 Mpc on a side, 4803 dark matter particles and as many gas particles. The simulation includes radiative cooling assuming zero metallicity, star formation and supernova feedback. The very high dynamic range of the simulation allows us to cover a fairly large interval of cluster temperatures. We compute X-ray observables of the intracluster medium (ICM) for simulated groups and clusters and analyse their statistical properties. The simulated mass-temperature relation is consistent with observations once we mimic the procedure for mass estimates applied to real clusters. Also, with the adopted choices of Ωm= 0.3 and σ8= 0.8 for matter density and power spectrum normalization, respectively, the resulting X-ray temperature function agrees with the most recent observational determinations. The luminosity-temperature relation also agrees with observations for clusters with T>~ 2 keV. At the scale of groups, T<~ 1 keV, we find no change of slope in this relation. The entropy in central cluster regions is higher than predicted by gravitational heating alone, the excess being almost the same for clusters and groups. We also find that the simulated clusters appear to have suffered some overcooling. We find f*~= 0.2 for the fraction of baryons in stars within clusters, thus approximately twice as large as the value observed. Interestingly, temperature profiles of simulated clusters are found to increase steadily toward cluster centres. They decrease in the outer regions, much like observational data do at r>~ 0.2rvir, while not showing an isothermal regime followed by a smooth temperature decline in the innermost regions. Our results thus demonstrate the need for yet more efficient sources of energy feedback and/or the need to consider additional physical process which may be able to further suppress the gas density at the scale of poor clusters and groups, and, at the same time, to regulate the cooling of the ICM in central regions

We present a technique for estimating the mass in the outskirts of galaxy clusters where the usual assumption of dynamical equilibrium is not valid. The method assumes that clusters form through hierarchical clustering and requires only galaxy redshifts and positions on the sky. We apply the method to dissipationless cosmological N-body simulations where galaxies form and evolve according to semi-analytic modelling. The method recovers the actual cluster mass profile within a factor of two to several megaparsecs from the cluster centre. This error originates from projection effects, sparse sampling, and contamination by foreground and background galaxies. In the absence of velocity biases, this method can provide an estimate of the mass-to-light ratio on scales ~1-10 Mpc/h where this quantity is still poorly known.Comment: 14 pages, 7 figures, MN LaTeX style, MNRAS, in pres

We use the Fourth Data Release of the Sloan Digital Sky Survey (SDSS) to test the ubiquity of infall patterns around galaxy clusters and measure cluster mass profiles to large radii. The Cluster and Infall Region Nearby Survey (CAIRNS) found infall patterns in nine clusters, but the cluster sample was incomplete. Here we match X-ray cluster catalogs with SDSS, search for infall patterns, and compute mass profiles for a complete sample of X-ray-selected clusters. Very clean infall patterns are apparent in most of the clusters, with the fraction decreasing with increasing redshift due to shallower sampling. All 72 clusters in a well-defined sample limited by redshift (ensuring good sampling) and X-ray flux (excluding superpositions) show infall patterns sufficient to apply the caustic technique. This sample is by far the largest sample of cluster mass profiles extending to large radii to date. Similar to CAIRNS, cluster infall patterns are better defined in observations than in simulations. Further work is needed to determine the source of this difference. We use the infall patterns to compute mass profiles for 72 clusters and compare them to model profiles. Cluster scaling relations using caustic masses agree well with those using X-ray or virial mass estimates, confirming the reliability of the caustic technique. We confirm the conclusion of CAIRNS that cluster infall regions are well fitted by Navarro-Frenk-White (NFW ) and Hernquist profiles and poorly fitted by singular isothermal spheres. This much larger sample enables new comparisons of cluster properties with those in simulations. The shapes (specifically NFW concentrations) of the mass profiles agree well with the predictions of simulations. The mass in the infall region is typically comparable to or larger than that in the virial region. Specifically, the mass inside the turnaround radius is on average 2:19 AE 0:18 times that within the virial radius. This ratio agrees well with recent predictions from simulations of the final masses of dark matter halos.

In hierarchical clustering, galaxy clusters accrete mass through the aggregation of smaller systems. Thus, the velocity Ðeld of the infall regions of clusters contains signiÐcant random motion superposed on radial infall. Because the purely spherical infall model does not predict the amplitude of the velocity Ðeld correctly, methods estimating the cosmological density parameter based on this model yield unreli-) 0 able biased results. In fact, the amplitude of the velocity Ðeld depends on local dynamics and only very weakly on the global properties of the universe.We use N-body simulations of Ñat and open universes to show that the amplitude of the velocity Ðeld of the infall regions of dark matter halos is a direct measure of the escape velocity within these regions. We can use this amplitude to estimate the mass of dark matter halos within a few megaparsecs from the halo center. In this region dynamical equilibrium assumptions do not hold. The method yields a mass estimate with better than 30% accuracy. If galaxies trace the velocity Ðeld of the infall regions of clusters reliably, this method provides a straightforward way to estimate the amount of mass surrounding rich galaxy clusters from redshift data alone.