1,721,102 research outputs found
The structure and dynamical evolution of dark matter haloes
No full textWe use N-body simulations to investigate the structure and dynamical evolution of dark matter haloes in clusters of galaxies. Our sample consists of nine massive haloes from an Einstein-De Sitter universe with scale-free power spectrum and spectral index n=-1. Haloes are resolved by 20 000 particles each, on average, and have a dynamical resolution of 20-25 kpc, as shown by extensive tests. Large-scale tidal fields are included up to a scale L=150 Mpc using background particles. We find that the halo formation process can be characterized by the alternation of two dynamical configurations: a merging phase and a relaxation phase, defined by their signature on the evolution of the total mass and root mean square (rms) velocity. Haloes spend on average one- third of their evolution in the merging phase and two-thirds in the relaxation phase. Using this definition, we study the density profiles and show how they change during the halo dynamical history. In particular, we find that the average density profiles of our haloes are fitted by the Navarro, Frenk & White analytical model with an rms residual of 17 per cent between the virial radius R_v and 0.01R_v. The Hernquist analytical density profile fits the same haloes with an rms residual of 26 per cent. The trend with mass of the scale radius of these fits is marginally consistent with that found by Cole & Lacey: compared with their results our haloes are more centrally concentrated, and the relation between scale radius and halo mass is slightly steeper. We find a moderately large scatter in this relation, due both to dynamical evolution within haloes and to fluctuations in the halo population. We analyse the dynamical equilibrium of our haloes using the Jeans equation, and find that on average they are approximately in equilibrium within their virial radius. Finally, we find that the projected mass profiles of our simulated haloes are in very good agreement with the profiles of three rich galaxy clusters derived from strong and weak gravitational lensing observations
Populating a cluster of galaxies - I. Results at z=0
No full textWe simulate the assembly of a massive rich cluster and the formation of its constituent galaxies in a flat, low-density universe. Our most accurate model follows the collapse, the star formation history and the orbital motion of all galaxies more luminous than the Fornax dwarf spheroidal, while dark halo structure is tracked consistently throughout the cluster for all galaxies more luminous than the SMC. Within its virial radius this model contains about 2×107 dark matter particles and almost 5000 distinct dynamically resolved galaxies. Simulations of this same cluster at a variety of resolutions allow us to check explicitly for numerical convergence both of the dark matter structures produced by our new parallel N-body and substructure identification codes, and of the galaxy populations produced by the phenomenological models we use to follow cooling, star formation, feedback and stellar aging. This baryonic modelling is tuned so that our simulations reproduce the observed properties of isolated spirals outside clusters. Without further parameter adjustment our simulations then produce a luminosity function, a mass-to-light ratio, luminosity, number and velocity dispersion profiles, and a morphology-radius relation which are similar to those observed in real clusters. In particular, since our simulations follow galaxy merging explicitly, we can demonstrate that it accounts quantitatively for the observed cluster population of bulges and elliptical galaxies
Collisional dark matter and the structure of dark halos
No full textWe study how the internal structure of dark halos is affected if cold dark matter particles are assumed to have a large cross section for elastic collisions. We identify a cluster halo in a large cosmological N-body simulation and resimulate its formation with progressively increasing resolution. We compare the structure found in the two cases in which dark matter is treated as collisionless or as a fluid. For the collisionless case, the overall ellipticity of the cluster, the central density cusp, and the amount of surviving substructure are all similar to those found in earlier high-resolution simulations. Collisional dark matter results in a cluster that is more nearly spherical at all radii, has a steeper central density cusp, and has less-but still substantial-surviving substructure. As in the collisionless case, these results for a ``fluid'' cluster halo are expected to carry over approximately to smaller mass systems. The observed rotation curves of dwarf galaxies then argue that self-interacting dark matter can only be viable if intermediate cross sections produce structure that does not lie between the extremes we have simulated
The satellite population of the Milky Way in a Lambda CDM universe
No full textWe compare the structure and kinematics of the 11 known satellites of the Milky Way with high-resolution simulations of the formation of its dark halo in a ΛCDM universe. In contrast to earlier work, we find excellent agreement. The observed kinematics are exactly those predicted for stellar populations with the observed spatial structure orbiting within the most massive `satellite' substructures in our simulations. Less massive substructures have weaker potential wells than those hosting the observed satellites. If there is a halo substructure `problem', it consists of understanding why halo substructures have been so inefficient in making stars. Suggested modifications of dark matter properties (for example, self-interacting or warm dark matter) may well spoil the good agreement found for standard cold dark matter
Weakly self-interacting dark matter and the structure of dark halos
No full textWe study the formation of dark halos in a ΛCDM universe under the assumption that cold dark matter (CDM) particles have a finite cross section for elastic collisions. We compare evolution when CDM mean free paths are comparable to halo sizes with the collisionless and fluid limits. We show that a few collisions per particle per Hubble time at halo center can substantially affect the central density profile. Cross sections an order of magnitude larger produce sufficient relaxation for rich clusters to develop core radii in the range 100-200 h-1 kpc. The structural evolution of halos is a competition between collisional relaxation caused by individual particle interactions and violent relaxation resulting from the infall and merging processes by which clusters grow. Although our simulations concentrate on systems of cluster size, we can scale our results to address the halo structure expected for dwarf galaxies. We find that collision cross sections sufficiently large to significantly modify the cores of such galaxies produce cluster cores that are too large and/or too round to be consistent with observation. Thus, the simplest model for self-interacting dark matter is unable to improve fits to published dwarf galaxy rotation curves without violating other observational constraints
Giant cluster arcs as a constraint on the scattering cross- section of dark matter
No full textWe carry out ray tracing through five high-resolution simulations of a galaxy cluster, to study how its ability to produce giant gravitationally lensed arcs is influenced by the collision cross-section of its dark matter. In three cases typical dark matter particles in the cluster core undergo between 1 and 100 collisions per Hubble time; two more explore the long (`collisionless') and short (`fluid') mean free path limits. We study the size and shape distributions of arcs and compute the cross-section for producing `extreme' arcs of various sizes. Even a few collisions per particle modifies the core structure enough to destroy the ability of the cluster to produce long, thin arcs. For larger collision frequencies the cluster must be scaled up to unrealistically large masses before it regains the ability to produce giant arcs. None of our models with self-interacting dark matter (except the `fluid' limit) is able to produce radial arcs; even the case with the smallest scattering cross-section must be scaled to the upper limit of observed cluster masses before it produces radial arcs. Apparently the elastic collision cross-section of dark matter in clusters must be very small, below 0.1cm2g-1, to be compatible with the observed ability of clusters to produce both radial arcs and giant arcs
Dark matter annihilation in the halo of the Milky Way
No full textIf the dark matter in the Universe is made of weakly self-interacting particles, they may self-annihilate and emit γ-rays. We use high-resolution numerical simulations to estimate directly the annihilation flux from the central regions of the Milky Way and from dark matter substructures in its halo. Although such estimates remain uncertain because of their strong dependence on the structure of the densest regions, our numerical experiments suggest that less direct calculations have overestimated the emission both from the centre and from the halo substructure. We estimate a maximal enhancement of at most a factor of a few with respect to a smooth spherical halo of standard Navarro-Frenk-White (NFW) structure. We discuss detection strategies for the next generation of γ-ray detectors and find that the annihilation flux may be detectable, regardless of uncertainties about the densest regions, for the annihilation cross-sections predicted by currently popular elementary particle models for the dark matter
Substructures in cold dark matter haloes
No full textWe analyse the properties of substructures within dark matter haloes (subhaloes) using a set of high-resolution numerical simulations of the formation of structure in a ΛCDM universe. Our simulation set includes 11 high-resolution simulations of massive clusters as well as a region of mean density, allowing us to study the spatial and mass distribution of substructures down to a mass resolution limit of 109h-1 Msolar. We also investigate how the properties of substructures vary as a function of the mass of the `parent' halo in which they are located. We find that the substructure mass function depends at most weakly on the mass of the parent halo and is well described by a power law. The radial number density profiles of substructures are steeper in low mass haloes than in high-mass haloes. More massive substructures tend to avoid the centres of haloes and are preferentially located in the external regions of their parent haloes. We also study the mass accretion and merging histories of substructures, which we find to be largely independent of environment. We find that a significant fraction of the substructures residing in clusters at the present day were accreted at redshifts z < 1. This implies that a significant fraction of present-day `passive' cluster galaxies were still outside the cluster progenitor and were more active at z~ 1
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
- …
