Cosmological simulations are the key tool for investigating the different processes involved in the formation of the universe from small initial density perturbations to galaxies and clusters of galaxies observed today. The identification and analysis of bound objects, halos, is one of the most important steps in drawing useful physical information from simulations. In the advent of larger and larger simulations, a reliable and parallel halo finder, able to cope with the ever-increasing data files, is a must. In this work we present the freely available MPI parallel halo finder Ahf. We provide a description of the algorithm and the strategy followed to handle large simulation data. We also describe the parameters a user may choose in order to influence the process of halo finding, as well as pointing out which parameters are crucial to ensure untainted results from the parallel approach. Furthermore, we demonstrate the ability of Ahf to scale to high resolution simulations.

We use a suite of 31 simulated galaxies drawn from the MaGICC project to investigate the effects of baryonic feedback on the density profiles of dark matter haloes. The sample covers a wide mass range: 9.4 × 10 9 < M halo /M ⊙ < 7.8 × 10 11 , hosting galaxies with stellar masses: 5.0 × 10 5 < M ⋆ /M ⊙ < 8.3 × 10 10 , i.e. from dwarf to L ⋆ . The galaxies are simulated with blastwave supernova feedback and, for some of them, an additional source of energy from massive stars is included. Within this feedback scheme we vary several parameters, such as the initial mass function, the density threshold for star formation and energy from supernovae and massive stars.The main result is a clear dependence of the inner slope of the dark matter density profile, α in ρ ∝ r α , on the ratio between stellar-to-halo mass, M ⋆ /M halo . This relation is independent of the particular choice of parameters within our stellar feedback scheme, allowing a prediction for cusp vs core formation. When M ⋆ /M halo is low, < ∼ 0.01 per cent, energy from stellar feedback is insufficient to significantly alter the inner dark matter density and the galaxy retains a cuspy profile. At higher ratios of stellar-to-halo mass feedback drives the expansion of the dark matter and generates cored profiles. The flattest profiles form where M ⋆ /M halo ∼ 0.5 per cent. Above this ratio, stars formed in the central regions deepen the gravitational potential enough to oppose the supernova-driven expansion process, resulting in cuspier profiles. Combining the dependence of α on M ⋆ /M halo with the empirical abundance matching relation between M ⋆ and M halo provides a prediction for how α varies as a function of stellar mass. Further, using the Tully-Fisher relation allows a prediction for the dependence of the dark matter inner slope on the observed rotation velocity of galaxies. The most cored galaxies are expected to have V rot ∼ 50 km s −1 , with α decreasing for more massive disc galaxies: spirals with V rot ∼ 150 km s −1 have central slopes α −0.8, approaching again the NFW profile. This novel prediction for the dependence of α on disc galaxy mass can be tested using observational data sets and can be applied to theoretical modeling of mass profiles and populations of disc galaxies.

We introduce a mass dependent density profile to describe the distribution of dark matter within galaxies, which takes into account the stellar-to-halo mass dependence of the response of dark matter to baryonic processes. The study is based on the analysis of hydrodynamically simulated galaxies from dwarf to Milky Way mass, drawn from the MaGICC project, which have been shown to match a wide range of disk scaling relationships. We find that the best fit parameters of a generic double power-law density profile vary in a systematic manner that depends on the stellar-to-halo mass ratio of each galaxy. Thus, the quantity M ⋆ /M halo constrains the inner (γ) and outer (β) slopes of dark matter density, and the sharpness of transition between the slopes (α), reducing the number of free parameters of the model to two. Due to the tight relation between stellar mass and halo mass, either of these quantities is sufficient to describe the dark matter halo profile including the effects of baryons. The concentration of the haloes in the hydrodynamical simulations is consistent with N-body expectations up to Milky Way mass galaxies, at which mass the haloes become twice as concentrated as compared with pure dark matter runs.This mass dependent density profile can be directly applied to rotation curve data of observed galaxies and to semi analytic galaxy formation models as a significant improvement over the commonly used NFW profile.

We describe our new 'MLAPM halo finder' (MHF), which is based on the adaptive grid structure of the N-body code MLAPM. We then extend the MHF code in order to track the orbital evolution of gravitationally bound objects through any given cosmological N-body simulationour so-called 'MLAPM halo tracker' (MHT). The mode of operation of MHT is demonstrated using a series of eight high-resolution N-body simulations of galaxy clusters. Each of these haloes hosts more than one million particles within their virial radii r vir . We use MHT as well as MHF to follow the temporal evolution of hundreds of individual satellites, and show that the radial distribution of these substructure satellites follows a 'universal' radial distribution irrespective of the environment and formation history of the host halo. This in fact might pose another problem for simulations of cold dark matter structure formation, as there are recent findings by Taylor, Silk & Babul that the Milky Way satellites are found preferentially closer to the Galactic Centre and simulations underestimate the amount of central substructure. Further, this universal substructure profile is anti-biased with respect to the underlying dark matter profile. The halo finder MHF will become part of the open source MLAPM distribution.

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.