Dark Matter Halo Models Research Articles

A New Smoothing Algorithm and its Application in Gravitational Lensing_

Based on the DWT (discrete wavelet transform) method, we propose a new smoothing algorithm for computing surface densities from 3D numerical simulation samples. To check its effectiveness, we have applied this algorithm to two Monte-Carlo samples of gravitational lens simulation with different mass resolutions, generated from the isothermal ellipsoid model of dark matter halos. The calculated results indicate that this algorithm can reconstruct accurately the surface density distribution of the gravitational lens simulation sample, and that the lens caustics and critical curves derived from the surface densities agree well with the theoretical curves. We have compared the results calculated by using 3 different wavelet bases (Daub4, Daub6 and B-spline 3th), and identified the best one. Without sacrificing its smoothing capability, this algorithm has a very fast computing speed, suitable for later N-body numerical simulations, which require even higher resolutions.

Central and satellite colours in galaxy groups: a comparison of the halo model and SDSS group catalogues

Current analytic and semi-analytic dark matter halo models distinguish between the central galaxy in a halo and the satellite galaxies in halo substructures. It is expected that galaxy properties are correlated with host halo mass, and that central galaxies tend to be the most luminous, massive and reddest galaxies in haloes while the satellites around them are fainter and bluer. Using a recent halo-model description of the colour dependence of galaxy clustering (Skibba & Sheth), we investigate the colours of central and satellite galaxies predicted by the model and compare them to those of two galaxy group catalogues constructed from the Sloan Digital Sky Survey (SDSS; Yang et al.; Berlind et al.). In the model, the environmental dependence of galaxy colour is determined by that of halo mass, and the predicted colour mark correlations were shown to be consistent with SDSS measurements. The model assumes that satellites tend to follow a colour–magnitude sequence that approaches the red sequence at bright luminosities; the model's success suggests that bright satellites tend to be ‘red and dead’ while the star formation in fainter ones is in the process of being quenched. In both the model and the SDSS group catalogues, we find that at fixed luminosity or stellar mass, central galaxies tend to be bluer than satellites. In contrast, at fixed group richness or halo mass, central galaxies tend to be redder than satellites, and galaxy colours become redder with increasing mass. We also compare the central and satellite galaxy colour distributions, as a function of luminosity and as a function of richness, in the model and in the two group catalogues. Except for faint galaxies and small groups, the model and both group catalogues are in very good agreement.

Axion condensate as a model for dark matter halos

Localized solutions of an axion-like scalar model with a periodic self-interaction are analyzed as a model of dark matter halos. It is shown that such a cold Bose–Einstein type condensate can provide a substantial contribution to the observed rotations curves of galaxies, as well provide a soliton type interpretation of the dark matter ‘bullets’ observed via gravitational lensing in merging clusters.

A Cosmologically Motivated Description of the Dark Matter Halo Profile for the Low Surface Brightness Galaxy, Malin 1

ABSTRACTIn this paper we derive a possible mass profile for the low surface brightness galaxy, Malin 1, based upon previously published space-based and ground-based photometric properties and kinematics. We use properties of the bulge, normal disk, outer extended disk, and H i mass as inputs into mass profile models. We find that the dark matter halo model of Malin 1 is best described by a halo profile that has undergone adiabatic contraction, inconsistent with the findings for most disk galaxies to date, yet consistent with rotation curve studies of M31. More importantly, we find that Malin 1 is baryon dominated in its central regions out to a radius of (in the bulge region). Low surface brightness galaxies are often referred to as being dark matter dominated at all radii. If this is the case, then Malin 1 would seem to have characteristics similar to those of normal barred disk galaxies, as suggested by other recent work. We also find that Malin 1 falls on the rotation curve shear versus spiral arm pitch angle relation for normal galaxies, although more LSB galaxies need to be studied to determine if this is typical.

DISTRIBUTION FUNCTION OF DARK MATTER WITH CONSTANT ANISOTROPY

N-body simulations of dark matter halos show that the density is cusped near the center of the halo. The density profile behaves as r–γ in the inner parts, where γ ≃ 1 for the NFW model and γ ≃ 1.5 for the Moore model, but in the outer parts the two models agree with each other in the asymptotic behavior of the density profile. The simulations also show information about the anisotropy parameter β(r) of the velocity distribution: β ≈ 0 in the inner part and β ≈ 0.5 (radially anisotropic) in the outer part of the halo. We provide some distribution functions F(E, L) with the constant anisotropy parameter β for the two spherical models of dark matter halos: a new generalized NFW model and a generalized Moore model. There are two parameters α and ∊ for those two generalized models to determine the asymptotic behavior of the density profile. In this paper, we concentrate on the situation of β(r) = 1/2 from the viewpoint of the simulation.

Modeling the Gas Flow in the Bar of NGC 1365

We present new observations of the strongly-barred galaxy NGC 1365, including new photometric images and Fabry-Perot spectroscopy, as well as a detailed re-analysis of the neutral hydrogen observations from the VLA archive. We find the galaxy to be at once remarkably bi-symmetric in its I-band light distribution and strongly asymmetric in the distribution of dust and in the kinematics of the gas in the bar region. The velocity field mapped in the H-alpha line reveals bright HII regions with velocities that differ by 60 to 80 km/s from that of the surrounding gas, which may be due to remnants of infalling material. We have attempted hydrodynamic simulations of the bar flow to estimate the separate disk and halo masses, using two different dark matter halo models and covering a wide range of mass-to-light ratios (Upsilon) and bar pattern speeds (Omega_p). None of our models provides a compelling fit to the data, but they seem most nearly consistent with a fast bar, corotation at sim 1.2r_B, and Upsilon_I simeq 2.0 +- 1.0, implying a massive, but not fully maximal, disk. The fitted dark halos are unusually concentrated, a requirement driven by the declining outer rotation curve.

Dark Matter Halos: Velocity Anisotropy–Density Slope Relation

Dark matter (DM) halos formed in CDM cosmologies seem to be characterized by a power law phase-space density profile. The density of the DM halos is often fitted by the NFW profile but a better fit is provided by the Sersic fitting formula. These relations are empirically derived from cosmological simulations of structure formation but have not yet been explained on a first principle basis. Here we solve the Jeans equation under the assumption of a spherical DM halo in dynamical equilibrium, that obeys a power law phase space density and either the NFW-like or the Sersic density profile. We then calculate the velocity anisotropy, beta(r), analytically. Our main result is that for the NFW-like profile the beta - gamma relation is not a linear one (where gamma is the logarithmic derivative of the density rho[r]). The shape of beta(r) depends mostly on the ratio of the gravitational to kinetic energy within the NFW scale radius R_s. For the Sersic profile a linear beta - gamma relation is recovered, and in particular for the Sersic index of n = 6.0 case the linear fit of Hansen & Moore is reproduced. Our main result is that the phase-space density power law, the Sersic density form and the linear beta - gamma dependence constitute a consistent set of relations which obey the spherical Jeans equation and as such provide the framework for the dynamical modeling of DM halos.

Outflow or galactic wind: the fate of ionized gas in the halos of dwarf galaxies

Context. Hα images of star bursting irregular galaxies reveal a large amount of extended ionized gas structures, in some cases at kpc-distance away from any place of current star forming activity. A kinematic analysis of especially the faint structures in the halo of dwarf galaxies allows insights into the properties and the origin of this gas component. This is important for the chemical evolution of galaxies, the enrichment of the intergalactic medium, and for the understanding of the formation of galaxies in the early universe. Aims. We want to investigate whether the ionized gas detected in two irregular dwarf galaxies (NGC 2366 and NGC 4861) stays gravitationally bound to the host galaxy or can escape from it by becoming a freely flowing wind. Methods. Very deep Hα images of NGC 2366 and NGC 4861 were obtained to detect and catalog both small and large scale ionized gas structures down to very low surface brightnesses. Subsequently, high-resolution long-slit echelle spectroscopy of the Hα line was performed for a detailed kinematic analysis of the most prominent filaments and shells. To calculate the escape velocity of both galaxies and to compare it with the derived expansion velocities of the detected filaments and shells, we used dark matter halo models. Results. We detected a huge amount of both small scale (up to a few hundred pc) and large scale (about 1–2 kpc of diameter or length) ionized gas structures on our Hα images. Many of the fainter ones are new detections. The echelle spectra reveal outflows and expanding bubbles/shells with velocities between 20 and 110 km s −1 . Several of these structures are in accordance with filaments in the Hα images. A comparison with the escape velocities of the galaxies derived from the NFW dark matter halo model shows that all gas features stay gravitationally bound.

The dark matter haloes of massive, relaxed galaxy clusters observed with Chandra

We use the Chandra X-ray Observatory to study the dark matter halos of 34 massive, dynamically relaxed galaxy clusters, spanning the redshift range 0.06 < z < 0.7. The observed dark matter and total mass (dark-plus-luminous matter) profiles can be approximated by the Navarro Frenk & White (hereafter NFW) model for cold dark matter (CDM) halos; for {approx} 80 percent of the clusters, the NFW model provides a statistically acceptable fit. In contrast, the singular isothermal sphere model can, in almost every case, be completely ruled out. We observe a well-defined mass-concentration relation for the clusters with a normalization and intrinsic scatter in good agreement with the predictions from simulations. The slope of the mass-concentration relation, c {infinity} M{sub vir}{sup a}/(1 + z){sup b} with a = -0.41 {+-} 0.11 at 95 percent confidence, is steeper than the value a {approx} -0.1 predicted by CDM simulations for lower mass halos. With the slope a included as a free fit parameter, the redshift evolution of the concentration parameter, b = 0.54 {+-} 0.47 at 95 percent confidence, is also slower than, but marginally consistent with, the same simulations (b {approx} 1). Fixing a {approx} -0.1 leads to an apparent evolution that is significantly slower, b = 0.20 {+-} 0.45, although the goodness of fit in this case is significantly worse. Using a generalized NFW model, we find the inner dark matter density slope, a, to be consistent with unity at 95 percent confidence for the majority of clusters. Combining the results for all clusters for which the generalized NFW model provides a good description of the data, we measure ? = 0.88 {+-} 0.29 at 95 percent confidence, in agreement with CDM model predictions.

Dynamics of dwarf-spheroidals and the dark matter halo of the galaxy

The dynamics of the dwarf-spheroidal (dSph) galaxies in the gravitational field of the Galaxy is investigated with particular reference to their susceptibility to tidal break-up. Based on the observed paucity of the dSphs at small Galactocentric distances, we put forward the hypothesis that subsequent to the formation of the Milky Way and its satellites, those dSphs that had orbits with small perigalacticons were tidally disrupted, leaving behind a population that now has a relatively larger value of its average perigalacticon to apogalacticon ratio and consequently a larger value of its r.m.s. transverse to radial velocities ratio compared to their values at the time of formation of the dSphs. We analyze the implications of this hypothesis for the phase space distribution of the dSphs and that of the dark matter (DM) halo of the Galaxy within the context of a self-consistent model in which the functional form of the phase space distribution of DM particles follows the King model, i.e. the ‘lowered isothermal’ distribution and the potential of the Galaxy is determined self-consistently by including the gravitational cross-coupling between visible matter and DM particles. This analysis, coupled with virial arguments, yields an estimate of ≳270 km s −1 for the circular velocity of any test object at galactocentric distances of ∼100 kpc, the typical distances of the dSphs. The corresponding self-consistent values of the relevant DM halo model parameters, namely, the local (i.e., the solar neighbourhood) values of the DM density and velocity dispersion in the King model and its truncation radius, are estimated to be ∼0.3 GeV cm −3, >350 km s −1 and ≳150 kpc, respectively. Similar self-consistent studies with other possible forms of the DM distribution function will be useful in assessing the robustness of our estimates of the Galaxy’s DM halo parameters.

Milgrom Relation Models for Spiral Galaxies from Two-dimensional Velocity Maps

Using two-dimensional velocity maps and I-band photometry, we have created mass models of 40 spiral galaxies using the Milgrom relation (the basis of modified Newtonian dynamics) to complement previous work. A Bayesian technique is employed to compare several different dark matter halo models to Milgrom and Newtonian models. Pseudo-isothermal dark matter halos provide the best statistical fits to the data in a majority of cases, while the Milgrom relation generally provides good fits as well. We also find that Milgrom models give mass-to-light ratios that roughly correlate with galaxy color, as predicted by stellar population models. A subsample of galaxies in the Hydra cluster follows a tight relation between mass-to-light ratio and color, but one that is significantly different from relations found in previous studies. Ruling out the Milgrom relation with rotational kinematics is difficult due to systematic uncertainties in the observations, as well as underlying model assumptions. We discuss in detail two galaxies for which the Milgrom relation appears to fail and find that relaxing the assumption of constant stellar mass-to-light ratio can maintain Milgrom models' viability.

Toroidal halos in a nontopological soliton model of dark matter

Soliton type solutions of an axionlike scalar model with self-interaction are analyzed further as a toy model of dark matter halos. For a 'nonlinear superposition' of round and flattened configurations we found ringlike substructures in the density profile similarly as has been inferred for our Galaxy from the observed excess of the diffuse component of cosmic gamma rays.

Systematic Errors in the Determination of Hubble Constant due to the Asphericity and Nonisothermality of Clusters of Galaxies

We present statistical studies on the systematic errors in the determination of the Hubble constant H0 from joint analyses of X-ray and the Sunyaev-Zel'dovich (SZ) effect of clusters of galaxies. We focus on the effects of their triaxiality and nonisothermality. From the triaxial model of dark matter halos obtained from numerical simulations, we derive the distribution of the intracluster gas under the assumption of the hydrodynamic equilibrium. Both the isothermal and the polytropic gases are investigated. We run Monte Carlo simulations to generate samples of clusters according to the distributions of their masses, axial ratios, concentration parameters, and line-of-sight directions. The estimation of H0 is done by fitting X-ray and SZ profiles of a triaxial cluster with the isothermal and spherical β model. We find that for a sample of clusters with M = 1014 h-1 M☉ and z = 0.1, the estimated H0 is positively biased with H(estimated) ≈ 1.05H0(true) and H(estimated) ≈ 1.05H0(true) for the isothermal case. For the polytropic case with γ = 1.15, the bias is large with H(estimated) ≈ 1.35H0(true) and H(estimated) ≈ 3H0(true). For a mass-limited sample of clusters with Mlim = 1013 h-1 M☉, the results are similar. On the other hand, such a large overestimation has not been seen in real observations. It is noticed that the β-value for observed clusters is within the range of 0.5-0.8. Considering only the subsample in Monte Carlo simulations with β in the range of 0.5-0.8, we have H(estimated) = 1.002H0(true) and H(estimated) = 0.994H0(true) for the isothermal and polytropic cases, respectively. We further find that the value of β is more sensitive to the intrinsic asphericity of clusters than the axial ratio of two-dimensional X-ray images η is. Limiting to clusters with β ≥ 0.5 essentially excludes highly aspherical clusters from the sample. From this subsample of clusters, we can get a fair estimate on H0.

The Robustness of Dark Matter Density Profiles in Dissipationless Mergers

We present a comprehensive series of dissipationless N-body simulations to investigate the evolution of density distribution in equal-mass mergers between dark matter (DM) halos and multicomponent galaxies. The DM halo models are constructed with various asymptotic power-law indices ranging from steep cusps to core-like profiles and the structural properties of the galaxy models are motivated by the LCDM paradigm of structure formation. The adopted force resolution allows robust density profile estimates in the inner ~1% of the virial radii of the simulated systems. We demonstrate that the central slopes and overall shapes of the remnant density profiles are virtually identical to those of the initial systems suggesting that the remnants retain a remarkable memory of the density structure of their progenitors, despite the relaxation that accompanies merger activity. We also find that halo concentrations remain approximately constant through hierarchical merging involving identical systems and show that remnants contain significant fractions of their bound mass well beyond their formal virial radii. These conclusions hold for a wide variety of initial asymptotic density slopes, orbital energies, and encounter configurations, including sequences of consecutive merger events, simultaneous mergers of severals ystems, and mergers of halos with embedded cold baryonic components in the form of disks, spheroids, or both. As an immediate consequence, the net effect of gas cooling, which contracts and steepens the inner density profiles of DM halos, should be preserved through a period of dissipationless major merging. Our results imply that the characteristic universal shape of DM density profiles may be set early in the evolution of halos.

Globular Clusters with Dark Matter Halos. II. Evolution in a Tidal Field

In this second paper in our series, we continue to test primordial scenarios of globular cluster formation, which predict that globular clusters formed in the early universe in the potential of dark matter minihalos. In this paper we use high-resolution N-body simulations to model tidal stripping experienced by primordial dark matter-dominated globular clusters in the static gravitational potential of the host dwarf galaxy. We test both cuspy Navarro-Frenk-White (NFW) and flat-core Burkert models of dark matter halos. Our primordial globular cluster with an NFW dark matter halo survives severe tidal stripping and after 10 orbits is still dominated by dark matter in its outskirts. Our cluster with a Burkert dark matter halo loses almost all its dark matter to tidal stripping and starts losing stars at the end of our simulations. The results of this paper reinforce our conclusion in Paper I that current observations of globular clusters are consistent with the primordial picture of globular cluster formation.

Flattened halos in a nontopological soliton model of dark matter

Soliton type solutions of a scalar model with a ${\ensuremath{\Phi}}^{6}$ self-interaction are analyzed for their density profiles as toy model of dark matter halos. We construct exact solutions with nontrivial ellipticity due to angular momentum and propose a ``nonlinear superposition'' of round and flattened halos in order to improve the scaling relations and the correspondence of the predicted rotation curves to the empirical Burkert fit.

The Lyman-alpha glow of gas falling into the dark matter halo of a z = 3 galaxy.

Quasars are the visible signatures of gas falling into the deep potential well of super-massive black holes in the centres of distant galaxies. It has been suggested that quasars are formed when two massive galaxies collide and merge, leading to the prediction that quasars should be found in the centres of regions of largest overdensity in the early Universe. In dark matter (DM)-dominated models of the early Universe, massive DM halos are predicted to attract the surrounding gas, which falls towards their centres. The neutral gas is not detectable in emission by itself, but gas falling into the ionizing cone of such a quasar will glow in the Lyman-alpha line of hydrogen, effectively imaging the DM halo. Here we present a Lyalpha image of a DM halo at redshift z = 3, along with a two-dimensional spectrum of the gaseous halo. Our observations are best understood in the context of the standard model for DM haloes; we infer a mass of (2 - 7) x 10(12) solar masses (M(\circ)) for the halo.

Tests for Substructure in Gravitational Lenses

The flux anomalies in four-image gravitational lenses can be interpreted as evidence for the dark matter substructure predicted by cold dark matter (CDM) halo models. In principle, these flux anomalies could arise from alternate sources, such as absorption, scattering, or scintillation by the interstellar medium (ISM) of the lens galaxy, problems in the ellipsoidal macro models used to fit lens systems, or stellar microlensing. We apply several tests to the data that appear to rule out these alternate explanations. First, the radio flux anomalies show no significant dependence on wavelength, as would be expected for almost any propagation effect in the ISM or microlensing by the stars. Second, the flux anomaly distributions show the characteristic demagnifications of the brightest saddle point relative to the other images expected for low optical depth substructure, which cannot be mimicked by either the ISM or problems in the macro models. Microlensing by stars also cannot reproduce the suppression of the bright saddle points if the radio source sizes are consistent with the Compton limit for their angular sizes. Third, while it is possible to change the smooth lens models to fit the flux anomalies in some systems, we can rule out the necessary changes in all systems where we have additional lens constraints to check the models. Moreover, the parameters of these models are inconsistent with our present observations and expectations for the structure of galaxies. We conclude that low-mass halos remain the best explanation of the phenomenon.

Dark matter annihilation in the local group

Under the hypothesis of a dark matter composed by supersymmetric particles such as neutralinos, we investigate the possibility that their annihilation in the halos of nearby galaxies could produce detectable fluxes of $\ensuremath{\gamma}$ photons. Expected fluxes depend on several, poorly known quantities such as the density profiles of dark matter halos, the existence and prominence of central density cusps and the presence of a population of subhalos. We find that, for all reasonable choices of dark matter halo models, the intensity of the \ensuremath{\gamma}-ray flux from some of the nearest extragalactic objects, such as M31, is comparable to or higher than the diffuse galactic foreground. We show that next generation ground-based experiments could have the sensitivity to reveal such fluxes which could help us to unveil the nature of dark matter particles.

Searching for massive clusters in weak lensing surveys

We explore the ability of weak lensing surveys to locate massive clusters. We use both analytic models of dark matter halos and mock weak lensing surveys generated from a large cosmological N-body simulation. The analytic models describe average properties of weak lensing halos and predict the number counts, enabling us to compute an effective survey selection function. We test the model prediction for the peak number counts in weak lensing mass maps against the mock numerical data, and find that the noise due to intrinsic galaxy ellipticities causes a systematic effect which increases the peak counts. We develop a correction scheme for the systematic effect in an empirical manner, and show that, after the correction, the model prediction agrees well with the mock data. The mock data is also used to examine the completeness and efficiency of the weak lensing halo search with fully taking into account the noise and the projection effect by large-scale structures. We show that the detection threshold of S/N=4-5 gives an optimal balance between completeness and efficiency. Our results suggest that, for a weak lensing survey with a galaxy number density of ng=30/arcmin^2 with a mean redshift z=1, the mean number of peaks in the 10sq deg area is N_peak=62 for a detection threshold S/N=4. The contamination rate is 42%, and thus, on average, 36 out of 62 peaks (at least) are signals from real halos. Weak lensing surveys thus provide a reasonably efficient way to searching for massive clusters.