We introduce the Virgo Consortium's EAGLE project, a suite of hydrodynamical simulations that follow the formation of galaxies and supermassive black holes in cosmologically representative volumes of a standard ΛCDM universe. We discuss the limitations of such simulations in light of their finite resolution and poorly constrained subgrid physics, and how these affect their predictive power. One major improvement is our treatment of feedback from massive stars and AGN in which thermal energy is injected into the gas without the need to turn off cooling or decouple hydrodynamical forces, allowing winds to develop without predetermined speed or mass loading factors. Because the feedback efficiencies cannot be predicted from first principles, we calibrate them to the present-day galaxy stellar mass function and the amplitude of the galaxy-central black hole mass relation, also taking galaxy sizes into account. The observed galaxy stellar mass function is reproduced to < ∼ 0.2 dex over the full resolved mass range, 10 8 < M * /M < ∼ 10 11 , a level of agreement close to that attained by semi-analytic models, and unprecedented for hydrodynamical simulations. We compare our results to a representative set of low-redshift observables not considered in the calibration, and find good agreement with the observed galaxy specific star formation rates, passive fractions, Tully-Fisher relation, total stellar luminosities of galaxy clusters, and column density distributions of intergalactic C iv and O vi. While the mass-metallicity relations for gas and stars are consistent with observations for M * > ∼ 10 9 M (M * > ∼ 10 10 M at intermediate resolution), they are insufficiently steep at lower masses. For the reference model the gas fractions and temperatures are too high for clusters of galaxies, but for galaxy groups these discrepancies can be resolved by adopting a higher heating temperature in the subgrid prescription for AGN feedback. The EAGLE simulation suite, which also includes physics variations and higher-resolution zoomed-in volumes described elsewhere, constitutes a valuable new resource for studies of galaxy formation.

We present results from thirteen cosmological simulations that explore the parameter space of the "Evolution and Assembly of GaLaxies and their Environments" (EAGLE) simulation project. Four of the simulations follow the evolution of a periodic cube L = 50 cMpc on a side, and each employs a different subgrid model of the energetic feedback associated with star formation. The relevant parameters were adjusted so that the simulations each reproduce the observed galaxy stellar mass function at z = 0.1. Three of the simulations fail to form disc galaxies as extended as observed, and we show analytically that this is a consequence of numerical radiative losses that reduce the efficiency of stellar feedback in high-density gas. Such losses are greatly reduced in the fourth simulation -the EAGLE reference model -by injecting more energy in higher density gas. This model produces galaxies with the observed size distribution, and also reproduces many galaxy scaling relations. In the remaining nine simulations, a single parameter or process of the reference model was varied at a time. We find that the properties of galaxies with stellar mass M (the "knee" of the galaxy stellar mass function) are largely governed by feedback associated with star formation, while those of more massive galaxies are also controlled by feedback from accretion onto their central black holes. Both processes must be efficient in order to reproduce the observed galaxy population. In general, simulations that have been calibrated to reproduce the low-redshift galaxy stellar mass function will still not form realistic galaxies, but the additional requirement that galaxy sizes be acceptable leads to agreement with a large range of observables. the regime where those outcomes are determined primarily by gravitational forces, have been corroborated by a diverse range of observational tests. These include, but are not limited to, cosmic shear induced by large-scale structure (e.g. Fu et al. 2008), the abundance of brightest cluster galaxies (BCGs, e.g. Rozo et al. 2010), tests of the cosmic expansion rate (e.g. Blake et al. 2011a) and the distanceredshift relation (e.g. Blake et al. 2011b), redshift-space distortions of the 2-point correlation function (e.g. Beutler et al. 2012) and the luminosity-distance relation of type Ia supernovae (e.g. Suzuki et al. 2012).The formation and evolution of galaxies is governed ulti-arXiv:1501.01311v2 [astro-ph.GA] 3 Apr 2015 1 We refer to losses on these scales as "subgrid losses". Losses induced by processes acting on scales that are resolved by cosmological simulations can also be significant, and dependent upon the subgrid implementation; we term these "macroscopic losses". 2 See also http://eagle.strw.leidenuniv.nl and http://icc.dur.ac.uk/Eagle 3 S15 also introduced a third model that better reproduces the observed properties of intragroup gas at intermediate resolution by adopting a higher AGN heating temperature ("AGNdT9").

We use a combination of three large N‐body simulations to investigate the dependence of dark matter halo concentrations on halo mass and redshift in the Wilkinson Microwave Anisotropy Probe year 5 (WMAP5) cosmology. The median relation between concentration and mass is adequately described by a power law for halo masses in the range 1011–1015 h−1 M⊙ and redshifts z < 2, regardless of whether the halo density profiles are fitted using Navarro, Frenk & White or Einasto profiles. Compared with recent analyses of the Millennium Simulation, which uses a value of σ8 that is higher than allowed by WMAP5, z= 0 halo concentrations are reduced by factors ranging from 23 per cent at 1011 h−1 M⊙ to 16 per cent at 1014 h−1 M⊙. The predicted concentrations are much lower than inferred from X‐ray observations of groups and clusters.