Cusps in cold dark matter haloes Diemand, Jürg; Zemp, Marcel; Moore, Ben ...
Monthly notices of the Royal Astronomical Society,
December 2005, Letnik:
364, Številka:
2
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We resolve the inner region of a massive cluster forming in a cosmological Λ cold dark matter (CDM) simulation with a mass resolution of 2 × 106 M⊙ and before z= 4.4 even 3 × 105 M⊙. This is a ...billion times less than the cluster's final virial mass and a substantial increase over current ΛCDM simulations. We achieve this resolution using a new multimass refinement procedure and are now able to probe a dark matter halo density profile down to 0.1 per cent of the virial radius. The inner density profile of this cluster halo is well fitted by a power law ρ∝r−γ down to the smallest resolved scale. An inner region with roughly constant logarithmic slope is now resolved, which suggests that cuspy profiles describe the inner profile better than recently proposed profiles with a core. The cluster studied here is one out of a sample of six high-resolution cluster simulations, and its inner slope of about γ= 1.2 lies close to the sample average.
Haloes gone MAD: The Halo-Finder Comparison Project Knebe, Alexander; Knollmann, Steffen R.; Muldrew, Stuart I. ...
Monthly notices of the Royal Astronomical Society,
08/2011, Letnik:
415, Številka:
3
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ABSTRACT
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.
Redefining the Missing Satellites Problem Strigari, Louis E; Bullock, James S; Kaplinghat, Manoj ...
The Astrophysical journal,
11/2007, Letnik:
669, Številka:
2
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Numerical simulations of Milky Way-size cold dark matter (CDM) halos predict a steeply rising mass function of small dark matter subhalos and a substructure count that greatly outnumbers the observed ...satellites of the Milky Way. Several proposed explanations exist, but detailed comparison between theory and observation in terms of the maximum circular velocity (V sub(max)) of the subhalos is hampered by the fact thatV sub(max)for satellite halos is poorly constrained. We present comprehensive mass models for the well-known Milky Way dwarf satellites and derive likelihood functions to show that their masses within 0.6 kpc ( unk) are strongly constrained by the present data. We show that the unk mass function of luminous satellite halos is flat between similar to 10 super(7) and 10 super(8) M unk. We use the "Via Lactea" N-body simulation to show that the unk mass function of CDM subhalos is steeply rising over this range. We rule out the hypothesis that the 11 well-known satellites of the Milky Way are hosted by the 11 most massive subhalos. We show that models where the brightest satellites correspond to the earliest forming subhalos or the most massive accreted objects both reproduce the observed mass function. A similar analysis with the newly discovered dwarf satellites will further test these scenarios and provide powerful constraints on the CDM small-scale power spectrum andwarm dark matter models.
We show that most particle and subhalo orbits in simulated cosmological cold dark matter halos are surprisingly regular and periodic: the phase-space structure of the outer halo regions shares some ...of the properties of the classical self-similar secondary infall model. Some of the outer branches are clearly visible in the radial velocity-radius plane at certain epochs. However, they are severely broadened in realistic, triaxial halos with nonradial, clumpy, mass accretion. This prevents the formation of high-density caustics: even in the best cases there are only broad, very small ( < 10%) enhancements in the spherical density profile. Larger fluctuations in r(r) caused by massive satellites are common. Infall caustics are therefore too weak to affect lensing or dark matter annihilation experiments. Their detection is extremely challenging, as it requires a large number of accurate tracer positions and radial velocities in the outer halo. The stellar halo of the Milky Way is probably the only target where this could become feasible in the future.
The graininess of dark matter haloes Zemp, Marcel; Diemand, Jürg; Kuhlen, Michael ...
Monthly notices of the Royal Astronomical Society,
April 2009, Letnik:
394, Številka:
2
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We use the recently completed one billion particle Via Lactea IIΛ cold dark matter simulation to investigate local properties like density, mean velocity, velocity dispersion, anisotropy, orientation ...and shape of the velocity dispersion ellipsoid, as well as the structure in velocity space of dark matter haloes. We show that at the same radial distance from the halo centre, these properties can deviate by orders of magnitude from the canonical, spherically averaged values, a variation that can only be partly explained by triaxiality and the presence of subhaloes. The mass density appears smooth in the central relaxed regions but spans four orders of magnitude in the outskirts, both because of the presence of subhaloes as well as of underdense regions and holes in the matter distribution. In the inner regions, the local velocity dispersion ellipsoid is aligned with the shape ellipsoid of the halo. This is not true in the outer parts where the orientation becomes more isotropic. The clumpy structure in local velocity space of the outer halo cannot be well described by a smooth multivariate normal distribution. Via Lactea II also shows the presence of cold streams made visible by their high 6D phase space density. Generally, the structure of dark matter haloes shows a high degree of graininess in phase space that cannot be described by a smooth distribution function.
The structural evolution of substructure in cold dark matter (CDM) models is investigated combining "low-resolution" satellites from cosmological N-body simulations of parent halos with N = 10 ...super(7) particles with high-resolution individual subhalos orbiting within a static host potential. We show that, as a result of mass loss, convergence in the central density profiles requires the initial satellites to be resolved with N = 10 super(7) particles and parsec- scale force resolution. We find that the density profiles of substructure halos can be well fitted with a power-law central slope that is unmodified by tidal forces even after the tidal stripping of over 99% of the initial mass and an exponential cutoff in the outer parts. The solution to the missing-satellites problem advocated by Stoehr et al. in 2002 relied on the flattening of the dark matter halo central density cusps by gravitational tides, enabling the observed satellites to be embedded within dark halos with maximum circular velocities as large as 60 km s super(-1). In contrast, our results suggest that tidal interactions do not provide the mechanism for associating the dwarf spheroidal (dSph) satellites of the Milky Way with the most massive substructure halos expected in a CDM universe. Motivated by the structure of our stripped satellites, we compare the predicted velocity dispersion profiles of Fornax and Draco to observations, assuming that they are embedded in CDM halos. We demonstrate that models with isotropic and tangentially anisotropic velocity distributions for the stellar component fit the data only if the surrounding dark matter halos have maximum circular velocities in the range 20-35 km s super(-1). If the dSphs are embedded within halos this large, then the overabundance of satellites within the concordance lambda CDM cosmological model is significantly alleviated, but this still does not provide the entire solution.
If the dark matter particle is a neutralino, then the first structures to form are cuspy cold dark matter (CDM) haloes collapsing after redshifts z≈ 100 in the mass range 10−6–10−3 M⊙. We carry out a ...detailed study of the survival of these microhaloes in the Galaxy as they experience tidal encounters with stars, molecular clouds, and other dark matter substructures. We test the validity of analytic impulsive heating calculations using high-resolution N-body simulations. A major limitation of analytic estimates is that mean energy inputs are compared to mean binding energies, instead of the actual mass lost from the system. This energy criterion leads to an overestimate of the stripped mass and an underestimate of the disruption time-scale, since CDM haloes are strongly bound in their inner parts. We show that a significant fraction of material from CDM microhaloes can be unbound by encounters with Galactic substructure and stars; however, the cuspy central regions remain relatively intact. Furthermore, the microhaloes near the solar radius are those which collapse significantly earlier than average and will suffer very little mass-loss. Thus, we expect a fraction of surviving bound microhaloes, a smooth component with narrow features in phase space, which may be uncovered by direct detection experiments, as well as numerous surviving cuspy cores with proper motions of arcminutes per year, which can be detected indirectly via their annihilation into gamma-rays.
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