We introduce the Virgo Consortium’s FLAMINGO suite of hydrodynamical simulations for cosmology and galaxy cluster physics. To ensure the simulations are sufficiently realistic for studies of large-scale structure, the subgrid prescriptions for stellar and AGN feedback are calibrated to the observed low-redshift galaxy stellar mass function and cluster gas fractions. The calibration is performed using machine learning, separately for each of FLAMINGO’s three resolutions. This approach enables specification of the model by the observables to which they are calibrated. The calibration accounts for a number of potential observational biases and for random errors in the observed stellar masses. The two most demanding simulations have box sizes of 1.0 and 2.8 Gpc on a side and baryonic particle masses of 1 × 10⁸ and 1 × 10⁹ M_⊙, respectively. For the latter resolution the suite includes 12 model variations in a 1 Gpc box. There are 8 variations at fixed cosmology, including shifts in the stellar mass function and/or the cluster gas fractions to which we calibrate, and two alternative implementations of AGN feedback (thermal or jets). The remaining 4 variations use the unmodified calibration data but different cosmologies, including different neutrino masses. The 2.8 Gpc simulation follows 3 × 10¹¹ particles, making it the largest ever hydrodynamical simulation run to z = 0. Lightcone output is produced on-the-fly for up to 8 different observers. We investigate numerical convergence, show that the simulations reproduce the calibration data, and compare with a number of galaxy, cluster, and large-scale structure observations, finding very good agreement with the data for converged predictions. Finally, by comparing hydrodynamical and ‘dark-matter-only’ simulations, we confirm that baryonic effects can suppress the halo mass function and the matter power spectrum by up to ≈20 per cent.

We introduce the Cluster-EAGLE (c-eagle) simulation project, a set of cosmological hydrodynamical zoom simulations of the formation of 30 galaxy clusters in the mass range of 10¹⁴ < M₂₀₀/M_⊙ < 10^15.4 that incorporates the Hydrangea sample of Bahé et al. ( 2017). The simulations adopt the state-of-the-art eagle galaxy formation model, with a gas particle mass of 1.8 × 10⁶ M_⊙ and physical softening length of 0.7 kpc. In this paper, we introduce the sample and present the low-redshift global properties of the clusters. We calculate the X-ray properties in a manner consistent with observational techniques, demonstrating the bias and scatter introduced by using estimated masses. We find the total stellar content and black hole masses of the clusters to be in good agreement with the observed relations. However, the clusters are too gas rich, suggesting that the active galactic nucleus (AGN) feedback model is not efficient enough at expelling gas from the high-redshift progenitors of the clusters. The X-ray properties, such as the spectroscopic temperature and the soft-band luminosity, and the Sunyaev–Zel'dovich properties are in reasonable agreement with the observed relations. However, the clusters have too high central temperatures and larger-than-observed entropy cores, which is likely driven by the AGN feedback after the cluster core has formed. The total metal content and its distribution throughout the intracluster medium are a good match to the observations.

We introduce the Hydrangea simulations, a suite of 24 cosmological hydrodynamic zoom-in simulations of massive galaxy clusters (M_200c = 10¹⁴–10^15.4 M_⊙) with baryon particle masses of ∼10⁶ M_⊙. Designed to study the impact of the cluster environment on galaxy formation, they are a key part of the ‘Cluster–EAGLE’ project. They use a galaxy formation model developed for the EAGLE project, which has been shown to yield both realistic field galaxies and hot gas fractions of galaxy groups consistent with observations. The total stellar mass content of the simulated clusters agrees with observations, but central cluster galaxies are too massive, by up to 0.6 dex. Passive satellite fractions are higher than in the field, and at stellar masses M_star > 10¹⁰ M_⊙, this environmental effect is quantitatively consistent with observations. The predicted satellite stellar mass function matches data from local cluster surveys. Normalized to total mass, there are fewer low-mass (M_star ≲ 10¹⁰ M_⊙) galaxies within the virial radius of clusters than in the field, primarily due to star formation quenching. Conversely, the simulations predict an overabundance of massive galaxies in clusters compared to the field that persists to their far outskirts (>5 r_200c). This is caused by a significantly increased stellar mass fraction of (sub-)haloes in the cluster environment, by up to ∼0.3 dex even well beyond r_200c. Haloes near clusters are also more concentrated than equally massive field haloes, but these two effects are largely uncorrelated.

In the local Universe, galaxies in groups and clusters contain less gas and are less likely to be forming stars than their field counterparts. This effect is not limited to the central group/cluster regions, but is shown by recent observations to persist out to several virial radii. To gain insight into the extent and cause of this large-scale environmental influence, we use a suite of high-resolution cosmological hydrodynamic simulations to analyse galaxies around simulated groups and clusters of a wide range of mass (log ₁₀M_host/M_⊙ = [13.0, 15.2]). In qualitative agreement with the observations, we find a systematic depletion of both hot and cold gas and a decline in the star-forming fraction of galaxies as far out as ∼5r₂₀₀ from the host centre. A substantial fraction of these galaxies are on highly elliptical orbits and are not infalling for the first time (∼50 per cent at 2r₂₀₀, independent of host mass) or are affected by ‘pre-processing’ (less than 10 per cent of galaxies around groups, increasing to ∼50 per cent near a massive cluster), even a combination of these indirect mechanisms does not fully account for the environmental influence, particularly in the case of the hot gas content. Direct ram pressure interaction with an extended gas ‘halo’ surrounding groups and clusters is shown to be sufficiently strong to strip the hot gas atmospheres of infalling galaxies out to ∼5r₂₀₀. We show that this influence is highly anisotropic, with ram pressure along filaments enhanced by up to a factor of 100 despite significant co-flow of gas and galaxies.

We examine the properties of atomic hydrogen (H i) associated with galaxies in the Evolution and Assembly of GaLaxies and their Environments (EAGLE) simulations of galaxy formation. EAGLE's feedback parameters were calibrated to reproduce the stellar mass function and galaxy sizes at z = 0.1, and we assess whether this calibration also yields realistic H i properties. We estimate the self-shielding density with a fitting function calibrated using radiation transport simulations, and correct for molecular hydrogen with empirical or theoretical relations. The ‘standard-resolution’ simulations systematically underestimate H i column densities, leading to an H i deficiency in low-mass (M_⋆ < 10¹⁰ M_⊙) galaxies and poor reproduction of the observed H i mass function. These shortcomings are largely absent from EAGLE simulations featuring a factor of 8 (2) better mass (spatial) resolution, within which the H i mass of galaxies evolves more mildly from z = 1 to 0 than in the standard-resolution simulations. The largest volume simulation reproduces the observed clustering of H i systems, and its dependence on H i richness. At fixed M_⋆, galaxies acquire more H i in simulations with stronger feedback, as they become associated with more massive haloes and higher infall rates. They acquire less H i in simulations with a greater star formation efficiency, since the star formation and feedback necessary to balance the infall rate is produced by smaller gas reservoirs. The simulations indicate that the H i of present-day galaxies was acquired primarily by the smooth accretion of ionized, intergalactic gas at z ≃ 1, which later self-shields, and that only a small fraction is contributed by the reincorporation of gas previously heated strongly by feedback. H i reservoirs are highly dynamic: over 40 per cent of H i associated with z = 0.1 galaxies is converted to stars or ejected by z = 0.