Alexander Knebe

AHF: Amiga's Halo Finder

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.

The dependence of dark matter profiles on the stellar-to-halo mass ratio: a prediction for cusps versus cores

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: 9.4×109 <Mhalo/M� <7.8×1011, hosting galaxies with stellarmasses in the range 5.0×105 <M∗/M� < 8.3×1010, 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 ∝ rα, on the stellar-to-halo mass ratio, M∗/Mhalo. This relation is independent of the particular choice of parameters within our stellar feedback scheme, allowing a prediction for cusp versus core formation. When M∗/Mhalo 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 stellar-to-halo mass ratios, feedback drives the expansion of the dark matter and generates cored profiles. The flattest profiles form where M∗/Mhalo ∼ 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∗/Mhalo with the empirical abundance matching relation between M∗ and Mhalo 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 Vrot ∼ 50 km s−1, with α decreasing for more massive disc galaxies: spirals with Vrot ∼ 150 km s−1 have central slopes α ≤−0.8, approaching again the Navarro–Frenk–White 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 modelling of mass profiles and populations of disc galaxies.

Haloes gone MAD: The Halo-Finder Comparison Project

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

The evolution of substructure – III. The outskirts of clusters

We present an investigation of satellite galaxies in the outskirts of galaxy clusters taken from a series of high-resolution N-body simulations. We focus on the so-called backsplash population, i.e. satellite galaxies that once were inside the virial radius of the host but now reside beyond it. We find that this population is significant in number and needs to be appreciated when interpreting the various galaxy morphology environmental relationships and decoupling the degeneracy between nature and nurture. Specifically, we find that approximately half of the galaxies with current cluster-centric distance in the interval 1-2 virial radii of the host are backsplash galaxies that once penetrated deep into the cluster potential, with 90 per cent of these entering to within 50 per cent of the virial radius. These galaxies have undergone significant tidal disruption, losing on average 40 per cent of their mass. This results in a mass function for the backsplash population different from those galaxies infalling for the first time. We further show that these two populations are kinematically distinct and should be observable within existent spectroscopic surveys.

Internal Alignment of the Halos of Disk Galaxies in Cosmological Hydrodynamic Simulations

Seven cosmological hydrodynamic simulations of disk galaxy formation are analyzed to determine the alignment of the disk within the dark matter halo and the internal structure of the halo. We find that the orientation of the outer halo, beyond ~0.1rvir, is unaffected by the presence of the disk. In contrast, the inner halo is aligned such that the halo minor axis aligns with the disk axis. The relative orientations of these two regions of the halo are uncorrelated. The alignment of the disk and inner halo appears to take place simultaneously through their joint evolution. The lack of connection between these two regions of the halo should be taken into account when modeling tidal streams in the halos of disk galaxies and when calculating intrinsic alignments of disk galaxies based on the properties of dark matter halos.

The halo mass function through the cosmic ages

In this paper we investigate how the halo mass function evolves with redshift, based on a suite of very large (with N_p = 3072^3 - 6000^3 particles) cosmological N-body simulations. Our halo catalogue data spans a redshift range of z = 0-30, allowing us to probe the mass function from the dark ages to the present. We utilise both the Friends-of-Friends (FOF) and Spherical Overdensity (SO) halofinding methods to directly compare the mass function derived using these commonly used halo definitions. The mass function from SO haloes exhibits a clear evolution with redshift, especially during the recent era of dark energy dominance (z 20, the SO algorithm finds a ~50-80% lower collapsed fraction in high mass haloes than does the FOF algorithm, due in part to the significant over-linking effects known to affect the FOF method.

A kinematic classification of the cosmic web

A new approach for the classification of the cosmic web is presented. In extension of the previous work of Hahn et al. and Forero-Romero et al., the new algorithm is based on the analysis of the velocity shear tensor rather than the gravitational tidal tensor. The procedure consists of the construction of the shear tensor at each (grid) point in space and the evaluation of its three eigenvectors. A given point is classified to be either a void, sheet, filament or a knot according to the number of eigenvalues above a certain threshold, 0, 1, 2 or 3, respectively. The threshold is treated as a free parameter that defines the web. The algorithm has been applied to a dark matter only simulation of a box of side length 64 h−1 Mpc and N = 10243 particles within the framework of the 5-year Wilkinson and Microwave Anisotropy Probe/Λ cold dark matter (ΛCDM) model. The resulting velocity-based cosmic web resolves structures down to ≲0.1 h−1 Mpc scales, as opposed to the ≈1 h−1 Mpc scale of the tidal-based web. The underdense regions are made of extended voids bisected by planar sheets, whose density is also below the mean. The overdense regions are vastly dominated by the linear filaments and knots. The resolution achieved by the velocity-based cosmic web provides a platform for studying the formation of haloes and galaxies within the framework of the cosmic web.

ANISOTROPY IN THE DISTRIBUTION OF SATELLITE GALAXY ORBITS

Nearby clusters such as Virgo and Coma possess galaxy distributions that tend to be aligned with the principal axis of the cluster itself. This has also been confirmed by a recent statistical analysis of some 300 Abell clusters, where the effect has been linked to the dynamical state of the cluster. Moreover, the orbits of satellite galaxies in galactic systems like our own Milky Way also demonstrate a high degree of anisotropy—the so-called Holmberg effect, the origin of which has been the subject of debate for more than 30 years. This study presents the analysis of cosmological simulations focusing on the orbits of satellite galaxies within dark matter halos. The apocenters of the orbits of these satellites are preferentially found within a cone of opening angle � 40 � around the major axis of the host halo, in accordance with the observed anisotropy found in galaxy clusters. We do, however, note that a link to the dynamical age of the cluster is not well established, as both of our oldest dark matter halos do show a clear anisotropy signal. Further analysis connects this distribution to the infall pattern of satellites along the filaments: the orbits are determined rather by the environment of the host halo than some ‘‘dynamical selection’’ during their life within the host’s virial radius. Subject headings: cosmology: theory — galaxies: formation — methods: numerical

A mass-dependent density profile for dark matter haloes including the influence of galaxy formation

ABSTRACT We introduce a mass dependent density profile to describe the distribution of dark matterwithin galaxies, which takes into account the stellar-to-halo mass dependence of the responseof dark matter to baryonic processes. The study is based on the analysis of hydrodynamicallysimulated galaxies from dwarf to Milky Way mass, drawn from the MaGICC project, whichhave been shown to match a wide range of disk scaling relationships. We find that the bestfit parameters of a generic double power-law density profile v ary in a systematic manner thatdepends on the stellar-to-halo mass ratio of each galaxy. Thus, the quantity M ⋆ /M halo con-strains the inner (γ) and outer (β) slopes of dark matter density, and the sharpness of transitionbetween the slopes (α), reducing the number of free parameters of the model to two. Due tothe tight relation between stellar mass and halo mass, either of these quantities is sufficient todescribe the dark matter halo profile including the effects o f baryons. The concentrationof thehaloes in the hydrodynamicalsimulations is consistent with N-body expectations up to MilkyWay mass galaxies, at which mass the haloes become twice as concentrated as compared withpure dark matter runs.This mass dependent density profile can be directly applied t o rotation curve data ofobserved galaxies and to semi analytic galaxy formation models as a significant improvementover the commonly used NFW profile.Key words:cosmology: dark matter galaxies: evolution - formation - hydrodynamics methods:N-bodysimulation

The evolution of substructure – II. Linking dynamics to environment

We present results from a series of high-resolution N-body simulations that focus on the formation and evolution of eight dark matter haloes, each of the order of a million particles within the virial radius. We follow the time evolution of hundreds of satellite galaxies with unprecedented time resolution, relating their physical properties to the differing halo environmental conditions. The self-consistent cosmological framework in which our analysis was undertaken allows us to explore satellite disruption within live host potentials, a natural complement to earlier work conducted within static potentials. Our host haloes were chosen to sample a variety of formation histories, ages and triaxialities; despite their obvious differences, we find striking similarities within the associated substructure populations. Namely, all satellite orbits follow nearly the same eccentricity distribution with a correlation between eccentricity and pericentre. We also find that the destruction rate of the substructure population is nearly independent of the mass, age and triaxiality of the host halo. There are, however, subtle differences in the velocity anisotropy of the satellite distribution. We find that the local velocity bias at all radii is greater than unity for all haloes and this increases as we move closer to the halo centre, where it varies from 1.1 to 1.4. For the global velocity bias, we find a small but slightly positive bias, although when we restrict the global velocity bias calculation to satellites that have had at least one orbit, the bias is essentially removed.