This is the first paper of a series that describes the methods and basic results of the GALICS model (Galaxies In Cosmological Simulations). GALICS is a hybrid model for hierarchical galaxy formation studies, combining the outputs of large cosmological N-body simulations with simple, semi-analytic recipes to describe the fate of the baryons within dark matter haloes. The simulations produce a detailed merging tree for the dark matter haloes, including complete knowledge of the statistical properties arising from the gravitational forces. We intend to predict the overall statistical properties of galaxies, with special emphasis on the panchromatic spectral energy distribution emitted by galaxies in the ultraviolet/optical and infrared/submillimetre wavelength ranges.In this paper, we outline the physically motivated assumptions and key free parameters that go into the model, comparing and contrasting with other parallel efforts. We specifically illustrate the success of the model in comparison with several data sets, showing how it is able to predict the galaxy disc sizes, colours, luminosity functions from the ultraviolet to far infrared, the Tully-Fisher and Faber-Jackson relations, and the fundamental plane in the local Universe. We also identify certain areas where the model fails, or where the assumptions needed to succeed are at odds with observations, and pay special attention to understanding the effects of the finite resolution of the simulations on the predictions made. Other papers in this series will take advantage of different data sets available in the literature to extend the study of the limitations and predictive power of GALICS, with particular emphasis put on high-redshift galaxies.

We present the Mock Map Facility, a powerful tool for converting theoretical outputs of hierarchical galaxy formation models into catalogues of virtual observations. The general principle is straightforward: mock observing cones can be generated using semi‐analytically post‐processed snapshots of cosmological N‐body simulations. These cones can then be projected to synthesize mock sky images. To this end, the paper describes in detail an efficient technique for creating such mock cones and images from the galaxies in cosmological simulations (galics) semi‐analytic model, providing the reader with an accurate quantification of the artefacts it introduces at every step. We show that replication effects introduce a negative bias on the clustering signal – typically peaking at less than 10 per cent around the correlation length. We also thoroughly discuss how the clustering signal is affected by finite‐volume effects, and show that it vanishes at scales larger than approximately one‐tenth of the simulation box size. For the purpose of analysing our method, we show that number counts and redshift distributions obtained with galics/momaf compare well with K‐band observations and the two‐degree field galaxy redshift survey. Given finite‐volume effects, we also show that the model can reproduce the automatic plate measuring machine angular correlation function. The momaf results discussed here are made publicly available to the astronomical community through a public data base. Moreover, a user‐friendly Web interface (http://galics.iap.fr) allows any user to recover her/his own favourite galaxy samples through simple SQL queries. The flexibility of this tool should permit a variety of uses ranging from extensive comparisons between real observations and those predicted by hierarchical models of galaxy formation, to the preparation of observing strategies for deep surveys and tests of data processing pipelines.

We present a comprehensive set of mock 2dF and SDSS galaxy redshift surveys constructed from a set of large, high-resolution cosmological N-body simulations. The radial selection functions and geometrical limits of the catalogues mimic those of the genuine surveys. The catalogues span a wide range of cosmologies, including both open and flat universes. In all the models the galaxy distributions are biased so as to approximately reproduce the observed galaxy correlation function on scales of 1--10 Mpc/h In some cases models with a variety of different biasing prescriptions are included. All the mock catalogues are publically available at http://star-www.dur.ac.uk/~cole/mocks/main.html . We expect these catalogues to be a valuable aid in the development of the new algorithms and statistics that will be used to analyse the 2dF and SDSS surveys when they are completed in the next few years. Mock catalogues of the PSCZ survey of IRAS galaxies are also available at the same WWW location.Comment: Accepted by MNRAS after minor revisions. All the mock catalogues presented and related information and software are available at http://star-www.dur.ac.uk/~cole/mocks/main.html . This WWW site also contains versions of this paper with high resolution (and colour) figures and many other figures illustrating the 2dF and SDSS survey

High‐resolution (HR) N‐body resimulations of 15 massive (1014–1015 M⊙) dark matter (DM) haloes have been combined with the hybrid galaxy formation model galics to study the formation and evolution of galaxies in clusters, within the framework of the hierarchical merging scenario. This paper describes the HR resimulation technique used to build the DM halo sample and discusses its reliability. New features incorporated in galics include a better description of galaxy positioning after DM halo merger events, a more reliable computation of the temperature of the intergalactic medium as a function of redshift, which also takes into account the reionization history of the Universe, and a semi‐analytic description of the ram pressure stripping of cold gas from galactic discs, suffered by galaxies during their motion through the diffuse hot intracluster medium. Within the multitude of available model results, we choose to focus here on the luminosity functions (LFs), morphological fractions and colour distributions of galaxies in clusters and in cluster outskirts, at z= 0. No systematic dependence on cluster richness is found either for the galaxy LFs, morphological mixes, or colour distributions. Moving from higher density (cluster cores) to lower density environments (cluster outskirts), we detect a progressive flattening of the LFs, an increase of the fraction of spirals and a decrease of that of ellipticals and S0s, and the progressive emergence of a bluer tail in the distributions of galaxy colours, especially for spirals. As compared with cluster spirals, early‐type galaxies show a flatter LF, and more homogeneous and redder colours. An overall good agreement is found between our results and the observations, particularly in terms of the cluster LFs and morphological mixes. However, some discrepancies are also apparent, with too faint magnitudes of the brightest cluster members, especially in the B band, and galaxy colours tending to be too red (or not blue enough) in the model, with respect to the observations. Finally, ram pressure stripping appears to affect very little our results.

We use a set of large, high-resolution cosmological N -body simulations to examine the redshift-space distortions of galaxy clustering on scales of order 10 − 200h −1 Mpc. Galaxy redshift surveys currently in progress will, on completion, allow us to measure the quadrupole distortion in the 2-point correlation function, ξ(σ, π), or its Fourier transform, the power spectrum, P (k, µ), to a high degree of accuracy. On these scales we typically find a positive quadrupole, as expected for coherent infall onto overdense regions and outflow from underdense regions, but the distortion is substantially weaker than that predicted by pure linear theory. We assess two models that may be regarded as refinements to linear theory, the Zel'dovich approximation and a dispersion model in which the non-linear velocities generated by the formation of virialized groups and clusters are treated as random perturbations to the velocities predicted by linear theory. We find that neither provides an adequate physical description of the clustering pattern. If used to model redshift space distortions on scales for 10 < λ < 200h −1 Mpc the estimated value of β (β = f (Ω 0 )/b where f (Ω 0 ) ≈ Ω 0.6 0 and b is the galaxy bias parameter) is liable to systematic errors of order ten per cent or more. We discuss how such systematics can be avoided by i) development of a more complete model of redshift distortions and ii) the direct use of galaxy catalogues generated from nonlinear N -body simulations.

Given the failure of existing models for redshift‐space distortions to provide a highly accurate measure of the β‐parameter, and the ability of forthcoming surveys to obtain data with very low random errors, it becomes necessary to develop better models for these distortions. Here we review the failures of the commonly used velocity dispersion models and present an empirical method for extracting β from the quadrupole statistic that has little systematic offset over a wide range of β and cosmologies. This empirical model is then applied to an ensemble of mock 2dF southern strip surveys, to illustrate the technique and see how accurately we can recover the true value of β. We compare this treatment with the error we expect to find caused only by the finite volume of the survey. We find that non‐linear effects reduce the range of scales over which β can be fitted, and introduce covariances between nearby modes in addition to those introduced by the convolution with the survey window function. The result is that we are only able to constrain β to a 1σ accuracy of 25 per cent (β=0.55±0.14 for the cosmological model considered). We explore one possible means of reducing this error, that of cluster collapse, and show that accurate application of this method can greatly reduce the effect of non‐linearities, improving the determination of β. We conclude by demonstrating that, when the contaminating effects of clusters are dealt with, this simple analysis of the full 2dF survey yields β=0.55±0.04. For this model, this represents a determination of β to an accuracy of 8 per cent and hence an important constraint on the cosmological density parameter Ω0.

This paper illustrates how mock observational samples of high‐redshift galaxies with sophisticated selection criteria can be extracted from the predictions of galics, a hybrid model of hierarchical galaxy formation that couples the outputs of large cosmological simulations and semi‐analytic recipes, to describe dark matter collapse and the physics of baryons. As an example of this method, we focus on the properties of Lyman‐break galaxies at redshift z∼ 3 (hereafter LBGs) in a Λ cold dark matter (ΛCDM) cosmology. With the momaf software package described in a companion paper, we generate a mock observational sample with selection criteria as similar as possible to those implied in the actual observations of z∼ 3 LBGs by Steidel, Pettini & Hamilton. We need to introduce an additional ‘maturity’ criterion to circumvent subtle effects due to mass resolution in the simulation. We predict a number density of 1.15 arcmin−2 at R≤ 25.5, in good agreement with the observed number density 1.2 ± 0.18 arcmin−2. Our model allows us to study the efficiency of the selection criterion to capture z∼ 3 galaxies. We find that the colour contours designed from models of spectrophotometric evolution of stellar populations are able to select more ‘realistic’ galaxies issued from models of hierarchical galaxy formation. We quantify the fraction of interlopers (12 per cent) and the selection efficiency (85 per cent), and we give estimates of the cosmic variance. We then study the clustering properties of our model LBGs. They are hosted by haloes with masses ∼1.6 × 1012 M⊙, with a linear bias parameter that decreases with increasing scale from b= 5 to 3. The amplitude and slope of the two‐dimensional correlation function is in good agreement with the data. We investigate a series of physical properties: ultraviolet (UV) extinction (a typical factor 6.2 at 1600 Å), stellar masses, metallicities and star formation rates, and we find them to be in general agreement with observed values. The model also allows us to make predictions at other optical and infrared/submillimetre wavelengths, that are easily accessible though queries to a web‐interfaced relational data base. Looking into the future of these LBGs, we predict that 75 per cent of them end up as massive ellipticals and lenticulars today, even though only 35 per cent of all our local ellipticals and lenticulars are predicted to have a LBG progenitor. In spite of some shortcomings that come from our simplifying assumptions and the subtle propagation of mass resolution effects, this new ‘mock observation’ method clearly represents a first step toward a more accurate comparison between hierarchical models of galaxy formation and real observational surveys.

We present a comprehensive set of mock 2dF and SDSS galaxy redshift surveys constructed from a set of large, high-resolution cosmological N -body simulations. The radial selection functions and geometrical limits of the catalogues mimic those of the genuine surveys. The catalogues span a wide range of cosmologies, including both open and flat universes. In all the models the galaxy distributions are biased so as to approximately reproduce the observed galaxy correlation function on scales of 1-10h −1 Mpc. In some cases models with a variety of different biasing prescriptions are included. All the mock catalogues are publically available at http://starwww.dur.ac.uk/˜cole/mocks/main.html . We expect these catalogues to be a valuable aid in the development of the new algorithms and statistics that will be used to analyse the 2dF and SDSS surveys when they are completed in the next few years. Mock catalogues of the PSCZ survey of IRAS galaxies are also available at the same WWW location.