We present a detailed comparison of fundamental dark matter halo properties retrieved by a substantial number of different halo finders. These codes span a wide range of techniques including friends‐of‐friends, spherical‐overdensity and phase‐space‐based algorithms. We further introduce a robust (and publicly available) suite of test scenarios that allow halo finder developers to compare the performance of their codes against those presented here. This set includes mock haloes containing various levels and distributions of substructure at a range of resolutions as well as a cosmological simulation of the large‐scale structure of the universe. All the halo‐finding codes tested could successfully recover the spatial location of our mock haloes. They further returned lists of particles (potentially) belonging to the object that led to coinciding values for the maximum of the circular velocity profile and the radius where it is reached. All the finders based in configuration space struggled to recover substructure that was located close to the centre of the host halo, and the radial dependence of the mass recovered varies from finder to finder. Those finders based in phase space could resolve central substructure although they found difficulties in accurately recovering its properties. Through a resolution study we found that most of the finders could not reliably recover substructure containing fewer than 30–40 particles. However, also here the phase‐space finders excelled by resolving substructure down to 10–20 particles. By comparing the halo finders using a high‐resolution cosmological volume, we found that they agree remarkably well on fundamental properties of astrophysical significance (e.g. mass, position, velocity and peak of the rotation curve). We further suggest to utilize the peak of the rotation curve, vmax, as a proxy for mass, given the arbitrariness in defining a proper halo edge.

The description of the abundance and clustering of haloes for non‐Gaussian initial conditions has recently received renewed interest, motivated by the forthcoming large galaxy and cluster surveys, which can potentially yield constraints of the order of unity on the non‐Gaussianity parameter fNL. We present tests on N‐body simulations of analytical formulae describing the halo abundance and clustering for non‐Gaussian initial conditions. We calibrate the analytic non‐Gaussian mass function of Matarrese, Verde & Jimenez and LoVerde et al. and the analytic description of clustering of haloes for non‐Gaussian initial conditions on N‐body simulations. We find an excellent agreement between the simulations and the analytic predictions if we make the corrections and , where q≃ 0.75, in the density threshold for gravitational collapse and in the non‐Gaussian fractional correction to the halo bias, respectively. We discuss the implications of this correction on present and forecasted primordial non‐Gaussianity constraints. We confirm that the non‐Gaussian halo bias offers a robust and highly competitive test of primordial non‐Gaussianity.

We introduce the AGORA project, a comprehensive numerical study of well-resolved galaxies within the ΛCDM cosmology. Cosmological hydrodynamic simulations with force resolutions of ∼ 100 proper pc or better will be run with a variety of code platforms to follow the hierarchical growth, star formation history, morphological transformation, and the cycle of baryons in and out of 8 galaxies with halo masses M vir ≃ 10 10 , 10 11 , 10 12 , and 10 13 M ⊙ at z = 0 and two different ("violent" and "quiescent") assembly histories. The numerical techniques and implementations used in this project include the smoothed particle hydrodynamics codes GADGET and GASOLINE, and the adaptive mesh refinement codes ART, ENZO, and RAMSES. The codes will share common initial conditions and common astrophysics packages including UV background, metaldependent radiative cooling, metal and energy yields of supernovae, and stellar initial mass function. These are described in detail in the present paper. Subgrid star formation and feedback prescriptions will be tuned to provide a realistic interstellar and circumgalactic medium using a non-cosmological disk galaxy simulation. Cosmological runs will be systematically compared with each other using a common analysis toolkit, and validated against observations to verify that the solutions are robust -i.e., that the astrophysical assumptions are responsible for any success, rather than artifacts of particular implementations. The goals of the AGORA project are, broadly speaking, to raise the realism and predictive power of galaxy simulations and the understanding of the feedback processes that regulate galaxy "metabolism." The initial conditions for the AGORA galaxies as well as simulation outputs at various epochs will be made publicly available to the community. The proof-ofconcept dark matter-only test of the formation of a galactic halo with a z = 0 mass of M vir ≃ 1.7 × 10 11 M ⊙ by 9 different versions of the participating codes is also presented to validate the infrastructure of the project. The project website is http://www.AGORAsimulations.org/. Need for High-Resolution Galaxy SimulationAs the above discussion should make clear, the success of cosmological galaxy formation simulations in producing real- 30 The force softening of the Aquila GASOLINE simulation was 460 proper pc from z = 9 to z = 0, and the mass per dark matter and gas particle was 2.1 and 0.5 × 10 6 M ⊙ , respectively. The Eris simulation had the force softening of 120 proper pc from z = 9 to z = 0, and better mass per dark matter and gas particle of 9.8 and 2.0 × 10 4 M ⊙ , respectively.