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.

Using high‐resolution cosmological N‐body simulations, we investigate the tendency of dark matter haloes to have their axes of rotation aligned similarly to those of their neighbours, or pointing towards local large‐scale structures. We quantify these effects as a function of scale using measures somewhat analogous to the velocity dispersion and infall velocity statistics. We find a weak mutual alignment between haloes of separation ∼1 h−1 Mpc. More interestingly, we measure a slight (one per cent), but highly significant trend for angular momentum vectors to point towards overdensities in the halo distribution field over the range 1–30 h−1 Mpc, i.e. up to the largest scales reliably probed by our simulations. We investigate the dependence of the observed correlations on cosmology, halo size, and redshift, and discuss the possibility of being able to observe such effects with real galaxy surveys.

Abstract. N-body studies have previously shown that the bottom-up hierarchical formation of dark matter haloes is not as monotonic as implicitly assumed in the Press-Schechter formalism. During and following halo mergers, matter can be ejected into tidal tails, shells or low density "atmospheres" outside of the successor haloes' virialisation radii (or group-finder outermost radii). The implications that the possible truncation of star formation in this tidal "debris" may have for observational galaxy statistics are examined here using the ArFus N-body plus semi-analytical galaxy modelling software for standard star formation hypotheses. In the N-body simulations studied, the debris typically remains close to the successor halo and falls back into the successor haloes given sufficient time. A maximum debris loss of around 16% is found for redshift intervals of around ∆z = 0.4 at z ∼ 1, with little dependence on the matter density parameter Ω0 and the cosmological constant λ0. Upper and lower bounds on stellar losses implied by a given set of N-body simulation output data can be investigated by choice of the merging/identity criterion of haloes between successive N-body simulation output times. A median merging/identity criterion is defined and used to deduce an upper estimate of possible star formation and stellar population losses. A largest successor merging/identity criterion is defined to deduce an estimate which minimises stellar losses. The losses for star formation and luminosity functions are strongest for low luminosity galaxies -a likely consequence of the fact that the debris fraction is highest for low mass haloes -and at intermediate redshiftsThe losses in both cases are mostly around 10%-30%, have some dependence on Ω0 and negligible dependence on λ0. This upper bound on likely losses in star formation rates and stellar populations is smaller than the uncertainties in estimates of corresponding observational parameters. Hence, it may not be urgent to include a correction for this in Press-Schechter based galaxy formation models, except when statistics regarding dwarf galaxies are under study.