We use a set of twelve high-resolution N-body/hydrodynamical simulations in the ΛCDM cosmology to investigate the origin and formation rate of fossil groups (FGs), which are X-ray bright galaxy groups dominated by a large elliptical galaxy, with the second brightest galaxy being at least two magnitudes fainter. The simulations invoke star formation, chemical evolution with non-instantaneous recycling, metal dependent radiative cooling, strong star burst driven galactic super winds, effects of a meta-galactic UV field and full stellar population synthesis. We find an interesting correlation between the magnitude gap between the first and second brightest galaxy and the formation time of the group. It is found that FGs have assembled half of their final dark matter mass already at z 1, and subsequently typically grow by minor merging only, wheras non-FGs on average form later. The early assembly of FGs leaves sufficient time for galaxies of L ∼ L * to merge into the central one by dynamical friction, resulting in the large magnitude gap at z = 0. A fraction of 33±16% of the groups simulated are found to be fossil, whereas the observational estimate is ∼10-20%. The FGs are found to be X-ray over-luminous relative to non-FGs of the same optical luminosity, in qualitative agreement with observations. Finally, from a dynamical friction analysis is found that only because infall of L ∼ L * galaxies happens along filaments with small impact parameters do FGs exist at all.

Cosmological (Λ‐cold dark matter) treesph simulations of the formation and evolution of galaxy groups and clusters have been performed. The simulations invoke star formation, chemical evolution with non‐instantaneous recycling, metallicity‐dependent radiative cooling, strong starburst, and (optionally) active galactic nuclei, driven galactic superwinds, effects of a metagalactic ultraviolet field and thermal conduction. Results for two clusters, one Virgo‐like (T≃ 3 keV) and one (sub‐) Coma‐like (T≃ 6 keV), are presented. At z= 0 the stellar contents of both clusters consist of a central dominant (cD) galaxy surrounded by cluster galaxies and intracluster (IC) stars. The IC stars are found to contribute 20–40 per cent of the total cluster B‐band luminosity at z= 0 and to form at a mean redshift , which is on average approximately 0.5 Gyr older than the stars in cluster galaxies. UBVRIJHK surface brightness profiles of the IC star populations are presented; the profile of the larger cluster matches the observed V‐band profile of the cD in Abell 1413 (T≃ 8 keV) quite well. The typical colour of the IC stellar population is B−R= 1.4–1.5, comparable to the colour of sub‐L* E and S0 galaxies. The mean iron abundance of the IC stars is approximately solar in the central part of the cluster (r∼ 100 kpc) decreasing to approximately half solar at the virial radius. The IC stars are α‐element enhanced with a weak trend of [O/Fe] increasing with r and an overall [O/Fe]∼ 0.4 dex, indicative of dominant enrichment from Type II supernovae. The IC stars are kinematically significantly colder than the cluster galaxies: the velocity dispersions of the IC stars are in the inner parts of the clusters (r∼ 100–500 kpc) only approximately half of those of the cluster galaxies, increasing slightly outward to approximately 70 per cent at r= 1–2 Mpc. The typical projected velocity dispersion in the Virgo‐like cluster at R≳ 50 kpc is 300–600 km s−1, depending on orientation and projected distance from the cluster centre. Rotation is found to be dynamically insignificant for the IC stars. The velocity distributions of IC stars and clusters galaxies are highly radially anisotropic in one cluster and in the other close to being isotropic.

N‐body/hydrodynamical simulations of the formation and evolution of galaxy groups and clusters in a Λ cold dark matter (ΛCDM) cosmology are used in order to follow the building‐up of the colour–magnitude relation in two clusters and in 12 groups. We have found that galaxies, starting from the more massive, move to the red sequence (RS) as they get aged over times and eventually set upon a ‘dead sequence’ (DS) once they have stopped their bulk star formation activity. Fainter galaxies keep having significant star formation out to very recent epochs and lie broader around the RS. Environment plays a role as galaxies in groups and cluster outskirts hold star formation activity longer than the central cluster regions. However, galaxies experiencing infall from the outskirts to the central parts keep star formation on until they settle on to the DS of the core galaxies. Merging contributes to mass assembly until z∼ 1, after which major events only involve the brightest cluster galaxies. The emerging scenario is that the evolution of the colour–magnitude properties of galaxies within the hierarchical framework is mainly driven by star formation activity during dark matter haloes assembly. Galaxies progressively quenching their star formation settle to a very sharp ‘red and dead’ sequence, which turns out to be universal, its slope and scatter being almost independent of the redshift (since at least z∼ 1.5) and environment. Differently from the DS, the operatively defined RS evolves more evidently with z, the epoch when it changes its slope being closely corresponding to that at which the passive galaxies population takes over the star‐forming one: this goes from z≃ 1 in clusters down to 0.4 in normal groups.

We have performed a series of N‐body/hydrodynamical (treesph) simulations of clusters and groups of galaxies, selected from a cosmological volume within a Lambda cold dark matter (ΛCDM) framework: these objects have been resimulated at higher resolution to z= 0, in order to follow also the dynamical, thermal and chemical input on to the intracluster medium (ICM) from stellar populations within galaxies. The simulations include metallicity‐dependent radiative cooling, star formation according to different initial mass functions (IMFs), energy feedback as strong starburst‐driven galactic superwinds, chemical evolution with non‐instantaneous recycling of gas and heavy elements, effects of a metagalactic ultraviolet (UV) field and thermal conduction in the ICM. In this paper, the first in a series of three, we derive results, mainly at z= 0, on the temperature and entropy profiles of the ICM, its X‐ray luminosity, the cluster cold components [cold fraction as well as mass‐to‐light ratio (MLR)] and the metal distribution between ICM and stars. In general, models with efficient superwinds (produced by the action of supernovae and, in some simulations, of active galactic nuclei (AGNs), along with a top‐heavy stellar IMF, are able to reproduce fairly well the observed LX–T relation, the entropy profiles and the cold fraction: both features are found to be needed in order to remove high‐density and low‐entropy cold gas at core scales, although additional alternative feedback mechanisms would still be required to prevent late‐time central cooling flows, and subsequent overproduction of stars and heavy elements at the centre. Observed radial ICM temperature profiles can be matched, except for the gradual decline in temperature inside r∼ 0.1Rvir. Metal enrichment of the ICM gives rise to somewhat steep inner iron gradients; yet, the global level of enrichment compares well to observational estimates when a top‐heavy IMF is adopted, and after correcting for the stars formed at late times at the base of the cooling flows, the metal partition between stars and ICM gets into good agreement with observations. The overall abundance and profile of iron in the ICM is found essentially unchanged from z= 1 to present time. Finally, the α/Fe of the gas is found to increase steadily with radius, decreasing over time.