共查询到20条相似文献，搜索用时 15 毫秒
1.
Ravi K. Sheth 《Monthly notices of the Royal Astronomical Society》1998,300(4):1057-1070
A model of the gravitationally evolved dark matter distribution, in the Eulerian space, is developed. It is a simple extension of the excursion set model that is commonly used to estimate the mass function of collapsed dark matter haloes. In addition to describing the evolution of the Eulerian space distribution of the haloes, the model allows one to describe the evolution of the dark matter itself. It can also be used to describe density profiles, on scales larger than the virial radius of these haloes, and to quantify the way in which matter flows in and out of Eulerian cells. When the initial Lagrangian space distribution is white noise Gaussian, the model suggests that the Inverse Gaussian distribution should provide a reasonably good approximation to the evolved Eulerian density field, in agreement with numerical simulations. Application of this model to clustering from more general Gaussian initial conditions is discussed at the end. 相似文献
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We investigate the figure rotation of dark matter haloes identified in Λ cold dark matter (CDM) simulations. We find that when strict criteria are used to select suitable haloes for study, five of the 222 haloes identified in our z = 0 simulation output undergo coherent figure rotation over a 5 h −1 Gyr period. We discuss the effects of varying the selection criteria and find that pattern speeds for a much larger fraction of the haloes can be measured when the criteria are relaxed. Pattern speeds measured over a 1 h −1 Gyr period follow a lognormal distribution, centred at Ωp = 0.2 h rad Gyr−1 with a maximum value of 0.94 h rad Gyr−1 . Over a 5 h −1 Gyr period, the average pattern speed of a halo is about 0.1 h rad Gyr−1 and the largest pattern speed found is 0.24 h rad Gyr−1 . Less than half of the selected haloes showed alignment between their figure rotation axis and minor axis, the exact fraction being somewhat dependent on how one defines a halo. While the pattern speeds observed are lower than those generally thought capable of causing spiral structure, we note that coherent figure rotation is found over very long periods and argue that further simulations would be required before strong conclusions about spiral structure in all galaxies could be drawn. We find no correlation between halo properties such as total mass and the pattern speed. 相似文献
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K. Holley-Bockelmann M. Weinberg N. Katz 《Monthly notices of the Royal Astronomical Society》2005,363(3):991-1007
The evolution of a stellar bar transforms not only the galactic disc, but also the host dark matter halo. We present high-resolution, fully self-consistent N -body simulations that clearly demonstrate that dark matter halo central density cusps flatten as the bar torques the halo. This effect is independent of the bar formation mode and occurs even for rather short bars. The halo and bar evolution is mediated by resonant interactions between orbits in the halo and the bar pattern speed, as predicted by linear Hamiltonian perturbation theory. The bar lengthens and slows as it loses angular momentum, a process that occurs even in rather warm discs. We demonstrate that the bar and halo response can be critically underestimated for experiments that are unable to resolve the relevant resonant dynamics; this occurs when the phase space in the resonant region is undersampled or plagued by noise. 相似文献
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Marcel Zemp Jürg Diemand † Michael Kuhlen Piero Madau Ben Moore Doug Potter Joachim Stadel Lawrence Widrow 《Monthly notices of the Royal Astronomical Society》2009,394(2):641-659
We use the recently completed one billion particle Via Lactea II Λ 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. 相似文献
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R. H. Sanders 《Monthly notices of the Royal Astronomical Society》2007,380(1):331-338
The dynamical mass of clusters of galaxies, calculated in terms of MOdified Newtonian Dynamics (MOND), is a factor of 2 or 3 times smaller than the Newtonian dynamical mass but remains significantly larger than the observed baryonic mass in the form of hot gas and stars in galaxies. Here I consider further the suggestion that the undetected matter might be in the form of cosmological neutrinos with mass of the order of 2 eV. If the neutrinos and baryons have comparable velocity dispersions and if the two components maintain their cosmological density ratio, then the electron density in the cores of clusters should be proportional to T 3/2 , as appears to be true in non-cooling flow clusters. This is equivalent to the 'entropy floor' proposed to explain the steepness of the observed luminosity–temperature relation, but here preheating of the medium is not required. Two-fluid (neutrino–baryon) hydrostatic models of clusters, in the context of MOND, reproduce the observed luminosity–temperature relation of clusters. If the β law is imposed on the gas density distribution, then the self-consistent models predict the general form of the observed temperature profile in both cooling and non-cooling flow clusters. 相似文献
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Mark A. Walker 《Monthly notices of the Royal Astronomical Society》1999,308(2):551-556
If dark haloes are composed of dense gas clouds, as has recently been inferred, then collisions between clouds lead to galaxy evolution. Collisions introduce a core in an initially singular dark matter distribution, and can thus help to reconcile scale-free initial conditions – such as are found in simulations – with observed haloes, which have cores. A pseudo-Tully–Fisher relation, between halo circular speed and visible mass (not luminosity), emerges naturally from the model: M vis ∝ V 7/2 .
Published data conform astonishingly well to this theoretical prediction. For our sample of galaxies, the mass–velocity relationship has much less scatter than the Tully–Fisher relation, and holds as well for dwarf galaxies (where diffuse gas makes a sizeable contribution to the total visible mass) as it does for giants. It seems very likely that this visible-mass/velocity relationship is the underlying physical basis for the Tully–Fisher relation, and this discovery in turn suggests that the dark matter is both baryonic and collisional. 相似文献
Published data conform astonishingly well to this theoretical prediction. For our sample of galaxies, the mass–velocity relationship has much less scatter than the Tully–Fisher relation, and holds as well for dwarf galaxies (where diffuse gas makes a sizeable contribution to the total visible mass) as it does for giants. It seems very likely that this visible-mass/velocity relationship is the underlying physical basis for the Tully–Fisher relation, and this discovery in turn suggests that the dark matter is both baryonic and collisional. 相似文献
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Eyal Neistein Avishai Dekel 《Monthly notices of the Royal Astronomical Society》2008,388(4):1792-1802
We derive analytic merger rates for dark matter haloes within the framework of the extended Press–Schechter (EPS) formalism. These rates become self-consistent within EPS once we realize that the typical merger in the limit of a small time-step involves more than two progenitors, contrary to the assumption of binary mergers adopted in earlier studies. We present a general method for computing merger rates that span the range of solutions permitted by the EPS conditional mass function, and focus on a specific solution that attempts to match the merger rates in N -body simulations. The corrected EPS merger rates are more accurate than the earlier estimates of Lacey & Cole by ∼20 per cent for major mergers and by up to a factor of ∼3 for minor mergers of mass ratio 1:104 . Based on the revised merger rates, we provide a new algorithm for constructing Monte Carlo EPS merger trees, which could be useful in semi-analytic modelling. We provide analytic expressions and plot numerical results for several quantities that are very useful in studies of galaxy formation. This includes (i) the rate of mergers of a given mass ratio per given final halo, (ii) the fraction of mass added by mergers to a halo and (iii) the rate of mergers per given main progenitor. The creation and destruction rates of haloes serve for a self-consistency check. Our method for computing merger rates can be applied to conditional mass functions beyond EPS, such as those obtained by the ellipsoidal collapse model or extracted from N -body simulations. 相似文献
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Giuseppe Tormen Antonaldo Diaferio & D. Syer 《Monthly notices of the Royal Astronomical Society》1998,299(3):728-742
Using high-resolution cosmological N -body simulations, we investigate the survival of dark matter satellites falling into larger haloes. Satellites preserve their identity for some time after merging. We compute their loss of mass, energy and angular momentum as they are dissolved by dynamical friction, tidal forces and collisions with other satellites. We also analyse the evolution of their internal structure. Satellites with less than a few per cent of the mass of the main halo may survive for several billion years, whereas larger satellites rapidly sink into the centre of the main halo potential well and lose their identity. Penetrating encounters between satellites are frequent and may lead to significant mass loss and disruption. Only a minor fraction of cluster mass (10–15 per cent on average) is bound to substructure at most redshifts of interest. We discuss the application of these results to the survival and extent of dark matter haloes associated with galaxies in clusters, and to their interactions. We find that a minor fraction of galaxy-size dark matter haloes are disrupted by redshift z = 0. The fraction of satellites undergoing close encounters is similar to the observed fraction of interacting or merging galaxies in clusters at moderate redshift. 相似文献
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Paolo Catelan Francesco Lucchin Sabino Matarrese & Cristiano Porciani 《Monthly notices of the Royal Astronomical Society》1998,297(3):692-712
Accepted 1998 January 26. Received 1998 January 26; in original form 1997 August 13This paper presents a stochastic approach to the clustering evolution of dark matter haloes in the Universe. Haloes, identified by a Press–Schechter-type algorithm in Lagrangian space, are described in terms of 'counting fields', acting as non-linear operators on the underlying Gaussian density fluctuations. By ensemble-averaging these counting fields, the standard Press–Schechter mass function as well as analytic expressions for the halo correlation function and corresponding bias factors of linear theory are obtained, extending the recent results by Mo & White. The non-linear evolution of our halo population is then followed by solving the continuity equation, under the sole hypothesis that haloes move by the action of gravity. This leads to an exact and general formula for the bias field of dark matter haloes, defined as the local ratio between their number density contrast and the mass density fluctuation. Besides being a function of position and 'observation' redshift, this random field depends upon the mass and formation epoch of the objects and is both non-linear and non-local. The latter features are expected to leave a detectable imprint on the spatial clustering of galaxies, as described, for instance, by statistics like the bispectrum and the skewness. Our algorithm may have several interesting applications, among which is the possibility of generating mock halo catalogues from low-resolution N -body simulations. 相似文献
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Here Rydberg matter is proposed as a candidate for the missing dark matter or dark baryonic matter in the Universe. Spectroscopic and other experimental studies give valuable information on the properties of Rydberg matter, especially its very weak interaction with light caused by the very small overlap with low states, and because of the necessary two-electron transitions even for disturbed matter. Recently, the unidentified infrared (UIR) bands have been shown to agree well with calculations and experiments on Rydberg matter. This is the reason for the present, somewhat speculative, proposal that dark matter has, at least partially, the form of Rydberg matter. The UIR bands have also been observed directly in emission from Rydberg matter in the laboratory. The unique space-filling properties of Rydberg matter are described: a hydrogen atom in this matter occupies a volume 5×1012 times larger than in its ground state or in a hydrogen molecule. 相似文献
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Janne Holopainen Chris Flynn Alexander Knebe Stuart P. Gill Brad K. Gibson 《Monthly notices of the Royal Astronomical Society》2006,368(3):1209-1222
Using eight dark matter haloes extracted from fully self-consistent cosmological N -body simulations, we perform microlensing experiments. A hypothetical observer is placed at a distance of 8.5 kpc from the centre of the halo measuring optical depths, event durations and event rates towards the direction of the Large Magellanic Cloud. We simulate 1600 microlensing experiments for each halo. Assuming that the whole halo consists of massive astronomical compact halo objects (MACHOs), f = 1.0 , and a single MACHO mass is m M = 1.0 M⊙ , the simulations yield mean values of τ= 4.7+5.0 −2.2 × 10−7 and Γ= 1.6+1.3 −0.6 × 10−6 events star−1 yr−1 . We find that triaxiality and substructure can have major effects on the measured values so that τ and Γ values of up to three times the mean can be found. If we fit our values of τ and Γ to the MACHO collaboration observations, we find f = 0.23+0.15 −0.13 and m M = 0.44+0.24 −0.16 . Five out of the eight haloes under investigation produce f and m M values mainly concentrated within these bounds. 相似文献
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In the three years following the discovery of PSR J2051−0827, we have observed a large number of eclipse traverses over a wide frequency range. These data show that the pulsar usually undergoes complete eclipse at frequencies below 1 GHz. At higher frequencies the pulsar is often detected throughout this low-frequency eclipse region with pulse times of arrival being significantly delayed relative to the best-fitting timing model. Variability in the magnitude of the delay is clearly seen and occurs on time-scales shorter than the orbital period. Simultaneous dual frequency observations highlight the difference in the eclipse behaviour for two widely separated frequencies. The low-frequency eclipses are accompanied by a significant decrease in pulsed flux density, while the flux density variations during higher frequency eclipses are not well defined. We consider a number of eclipse mechanisms and find that scattering and cyclotron absorption in the magnetosphere of the companion are consistent with the phenomena presented here. 相似文献
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Alexey Boyarsky Denys Malyshev † rey Neronov Oleg Ruchayskiy 《Monthly notices of the Royal Astronomical Society》2008,387(4):1345-1360
Using the high-resolution spectrometer SPI on board the International Gamma-Ray Astrophysics Laboratory ( INTEGRAL ), we search for a spectral line produced by a dark matter (DM) particle with a mass in the range 40 keV < M DM < 14 MeV , decaying in the DM halo of the Milky Way. To distinguish the DM decay line from numerous instrumental lines found in the SPI background spectrum, we study the dependence of the intensity of the line signal on the offset of the SPI pointing from the direction toward the Galactic Centre. After a critical analysis of the uncertainties of the DM density profile in the inner Galaxy, we find that the intensity of the DM decay line should decrease by at least a factor of 3 when the offset from the Galactic Centre increases from 0° to 180°. We find that such a pronounced variation of the line flux across the sky is not observed for any line, detected with a significance higher than 3σ in the SPI background spectrum. Possible DM decay origin is not ruled out only for the unidentified spectral lines, having low (∼3σ) significance or coinciding in position with the instrumental ones. In the energy interval from 20 keV to 7 MeV, we derive restrictions on the DM decay line flux, implied by the (non-)detection of the DM decay line. For a particular DM candidate, the sterile neutrino of mass M DM , we derive a bound on the mixing angle. 相似文献