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1.
We investigate the cross-correlation between galaxy clusters and QSOs using Sloan Digital Sky Survey (SDSS) DR4 - 5000 deg2 data. With photometric redshifts of galaxies, we select galaxy clusters based on the local projected densities of LRGs brighter than Mr' = -22. The QSOs are from the main sample of SDSS QSO spectroscopic survey to i' - 19. A significant positive correlation is found between the clusters and QSOs. Under the assumption that the signal is caused by gravitational leasing, we fit the signal with singular isothermal sphere (SIS) model and NFW profile halo model. The velocity dispersionσv = 766 km s-1 is derived for the best-fit of SIS model. Best-fit for the NFW model requires the dark matter halo mass within 1.5 h-1 Mpc to be 4.6×1014 h-1 M⊙. The mass parameterΩcl of the cluster sample is deduced as 0.077 with the SIS model and 0.083 with the NFW model. Our results ofΩcl are smaller than those given by Croom & Shanks and by Myers et al.  相似文献   

2.
We explore degeneracies in strong lensing model so to make time delay data consistent with the WMAP (Wilkinson Microwave Anisotropy Probe) cosmology. Previous models using a singular isothermal lens often yield a time delay between the observed multiple images too small than the observed value if we "hardwire" the now widely quoted post-WMAP "high" value of the Hubble constant (H0 - 71 ± 4km s-1 Mpc-1). Alternatively, the lens density profile (star plus dark matter) is required to be locally steeper than r-2 (isothermal) profile near the Einstein radius (of the order 3 kpc) to fit the time delays; a naive extrapolation of a very steep profile to large radius would imply a lens halo with a scale length of the order only 3 kpc, too compact to be consistent with CDM. We explore more sophisticated, mathematically smooth, positive lens mass density profiles which are consistent with a large halo and the post-WMAP H0. Thanks to the spherical monopole degeneracy, the "reshuffling" of the mass in a lens model does not affect the quality of the fit to the image positions, amplifications, and image time delays. Even better, unlike the better-known mass sheet degeneracy, the stellar mass-to-light and the HO value are not affected either. We apply this monopole degeneracy to the quadruple imaged time-delay system PG1115 080. Finally we discuss the implications of the time delay data on the newly proposed relativistic MOND theory.  相似文献   

3.
We have accurately evaluated the halo pairwise velocity dispersion and the halo mean streaming velocity in the LCDM model (the fiat ω0 = 0.3 model) using a set of high-resolution N-body simulations. Based on the simulation results, we have developed a model for the pairwise velocity dispersion of halos. Our model agrees with the simulation results over all scales we studied. We have also tested the model of Sheth et al. for the mean streaming motion of halos derived from the pair-conservation equation. We found that their model reproduces the simulation data very well on large scale, but under-predicts the streaming motion on scales r < 10 h-1 Mpc. We have introduced an empirical relation to improve their model. These improved models are useful for predicting the redshift correlation functions and the redshift power spectrum of galaxies if the halo occupation number model, e.g. the cluster weighted  相似文献   

4.
We investigate the mass-temperature relation of clusters for both the spherical NFW halo model and a concentric triaxial halo model. We study the temperature and density distributions of both an isothermal and a polytropic intra-cluster gas in hydrostatic equilibrium. We find that both the uncertainties in the concentration parameter and in the eccentricities (in case of the triaxial halo) lead to a greater scatter in the emission-weighted temperature at a given halo mass for less massive clusters. This may be helpful when determining the intrinsic statistical error of the σ8 normalization of the linear power spectrum from cluster abundance.  相似文献   

5.
The motion of baryonic components of the Milky Way is governed by both luminous and dark matter content of the Galaxy. Thus, the dynamics of Milky Way globular clusters(GCs) can be used as tracers to infer the mass model of the Galaxy up to a large radius. In this work, we apply the directly observable line-of-sight velocities to test if the dynamics of the GC population are consistent with an assumed axisymmetric gravitational potential of the Milky Way. For this, we numerically compute the phase space distribution of the GC population where the orbits are either oriented randomly or co-/counter-rotating with respect to the stellar disk. Then we compare the observed position and line-of-sight velocity distribution of～150 GCs with those of the models. We found that, for the adopted mass model, the co-rotating scenario is the favored model based on various statistical tests. We do the analysis with and without the GCs associated with the progenitors of early merger events. This analysis can be extended in the near future to include precise and copious data to better constrain the Galactic potential up to a large radius.  相似文献   

6.
By employing an improved simulation of the evolution of black holes(BHs)based on the merger tree of dark matter halos,we explore the relationship between the central BH mass Mbh and velocity dispersion σflat high redshift z≥6 and quantify the mini-QSO's(with BH mass M=200-105M)contribution to cosmic reionization.The simulation demonstrates how seed BHs migrate onto the MBH-σ* relation by merging with each other and accreting gas at z≥6:1.The correlation between BHs and their host halos increases as the BHs ...  相似文献   

7.
We introduce a new halo/subhalo finder, HIKER(a Halo fInder based on KERnel-shift algorithm), which takes advantage of a machine learning method – the mean-shift algorithm combined with the Plummer kernel function, to effectively locate density peaks corresponding to halos/subhalos in density field. Based on these density peaks, dark matter halos are identified as spherical overdensity structures, and subhalos are bound substructures with boundaries at their tidal radius. By testing HIKER code with mock halos, we show that HIKER performs excellently in recovering input halo properties. In particular, HIKER has higher accuracy in locating halo/subhalo centres than most halo finders. With cosmological simulations,we further show that HIKER reproduces the abundance of dark matter halos and subhalos quite accurately,and the HIKER halo/subhalo mass functions and Vmax functions are in good agreement with two widely used halo finders, SUBFIND and AHF.  相似文献   

8.
We present a statistical method to derive the stellar density profiles of the Milky Way from spectroscopic survey data,taking into account selection effects.We assume the selection function,which can be altered during observations and data reductions,of the spectroscopic survey is based on photometric colors and magnitude.Then the underlying selection function for a line-of-sight can be recovered well by comparing the distribution of the spectroscopic stars in a color-magnitude plane with that of the photometric dataset.Subsequently,the stellar density profile along a line-of-sight can be derived from the spectroscopically measured stellar density profile multiplied by the selection function.The method is validated using Galaxia mock data with two different selection functions.We demonstrate that the derived stellar density profiles reconstruct the true ones well not only for the full set of targets,but also for sub-populations selected from the full dataset.Finally,the method is applied to map the density profiles for the Galactic disk and halo,using the LAMOST RGB stars.The Galactic disk extends to about R=19 kpc,where the disk still contributes about 10% to the total stellar surface density.Beyond this radius,the disk smoothly transitions to the halo without any truncation,bending or breaking.Moreover,no over-density corresponding to the Monoceros ring is found in the Galactic anti-center direction.The disk shows moderate north–south asymmetry at radii larger than 12 kpc.On the other hand,the R–Z tomographic map directly shows that the stellar halo is substantially oblate within a Galactocentric radius of 20 kpc and gradually becomes nearly spherical beyond 30 kpc.  相似文献   

9.
We develop a theoretical formulation for the large-scale dynamics of galaxy clusters involving two spherical ‘isothermal fluids‘ coupled by their mutual gravity and derive asymptotic similarity solutions analytically. One of the fluids roughly approximates the massive dark matter halo, while the other describes the hot gas, the relatively small mass contribution from the galaxies being subsumed in the gas. By properly choosing the self-similar variables, it is possible to consistently transform the set of time-dependent two-fluid equations of spherical symmetry with serf-gravity into a set of coupled nonlinear ordinary differential equations (ODEs). We focus on the analytical analysis and discuss applications of the solutions to galaxy clusters.  相似文献   

10.
We present an estimate of the strong lensing probability by dark halos, with emphasis on the role of the baryonic matter arising purely from radiative cooling. We treat the contribution of the cooled baryons optimistically with all the cooled baryons confined within a central core, and including no feedback process from stellar evolution. Our two-component model provides a strong lensing probability that is in good agreement with the observed distribution of multiple images of quasars, provided that the cooled baryons are deposited within a spherical region of radius of 0.1 times the virial radius and follow an isothermal profile. It is pointed out that strong lensing may be used as an additional probe‘of baryon physics in dark halos though this may meanwhile complicate the test of the inner density profiles of dark matter in halos using the observed strong lensing probability.  相似文献   

11.
We investigate two classes of conditions for galaxy quenching at 0.5 z 2.5 based on the structural scaling relations of galaxies in the five Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey(CANDELS) fields:the formation of a compact core and the environment.We confirm that in the entire redshift range,massive quiescent galaxies(M_* 10~(10) M_⊙) have much higher stellar mass surface densities within the central 1 kpc(∑_1) and smaller sizes than star-forming galaxies in the same stellar mass range.In addition,the quiescent fractions significantly increase with the increase of ∑_1 regardless of whether galaxies are centrals or satellites.In contrast,we find that the overall lower-mass quiescent galaxies(M_*～10~(10)M_⊙) have slightly higher E_1 and comparable sizes compared to starforming galaxies of the same mass and at the same redshift.At z 1.5,satellites have higher halo masses and larger quiescent fractions than those of centrals at a given ∑_1(stellar mass).Our findings indicate that the significant growth of the galaxy cores is closely related to the quenching of massive galaxies since z ～ 2.5,while the environmental effect plays an important role in the quenching of low-mass galaxies at z≤1.5.  相似文献   

12.
We study ten galaxy groups and clusters suggested in the literature to be "fossil systems(FSs)"based on Chandra observations. According to the M_(500)- T and L_X- T relations, the gas properties of FSs are not physically distinct from ordinary galaxy groups or clusters. We also first study the f_(gas, 2500)- T relation and find that the FSs exhibit the same trend as ordinary systems. The gas densities of FSs within 0.1_(r200) are ~ 10~(-3)cm~(-3), which is the same order of magnitude as galaxy clusters. The entropies within 0.1_(r200)(S_(0.1r200)) of FSs are systematically lower than those in ordinary galaxy groups, which is consistent with previous reports, but we find their S_(0.1r200)- T relation is more similar to galaxy clusters. The derived mass profiles of FSs are consistent with the Navarro, Frenk and White model in(0.1- 1)_(r200), and the relation between scale radius r_s and characteristic mass density δ_c indicates self-similarity of dark matter halos of FSs. The ranges of r_s and δ_c for FSs are also close to those of galaxy clusters. Therefore, FSs share more common characteristics with galaxy clusters. The special birth place of the FS makes it a distinct type of galaxy system.  相似文献   

13.
The mass density distribution of Newtonian self-gravitating systems is studied analytically in the field theoretical method. Modeling the system as a fluid in hydrostatic equilibrium, we apply Schwinger's functional derivative on the average of the field equation of mass density, and obtain the field equation of 2-point correlation function ξ(r) of the mass density fluctuation, which includes the next order of nonlinearity beyond the Gaussian approximation. The 3-point correlation occurs hierarchically in the equation,and is cut off by the Groth-Peebles ansatz, making it closed. We perform renormalization and write the equation with three nonlinear coefficients. The equation tells us that ξ depends on the point mass m and the Jeans wavelength scale λ_0, which are different for galaxies and clusters. Applying this to large scale structures, it predicts that the profile of ξcc for clusters is similar to ξgg for galaxies but with a higher amplitude, and that the correlation length increases with the mean separation between clusters, i.e., a scaling behavior r_0■0.4 d. The solution yields the galaxy correlation ξgg(r)■(r_0/r)1.7 valid only in a range1 < r < 10 h-1 Mpc. At larger scales the solution ξgg deviates below the power law and goes to zero around ~50 h-1 Mpc, just as the observations show. We also derive the field equation of the 3-point correlation function in the Gaussian approximation and its analytical solution, for which the Groth-Peebles ansatz with Q = 1 holds.  相似文献   

14.
We analyze the radial and vertical metallicity and [α/Fe] gradients of the disk stars of a disk galaxy simulated in a fully cosmological setting with the chemodynamical galaxy evolution code GCD+. We study how the radial abundance gradients vary as a function of height above the plane and find that the metallicity([α/Fe]) gradient becomes more positive(negative) with increasing height, changing sign around1.5 kpc above the plane. At the largest vertical height(2 |z| 3 kpc), our simulated galaxy shows a positive radial metallicity gradient. We find that the positive metallicity gradient is caused by the age-metallicity and age-velocity dispersion relation, where younger stars have higher metallicity and lower velocity dispersion. Due to the agevelocity dispersion relation, a greater fraction of younger stars reaches |z| 2 kpc at the outer region, because of the lower gravitational restoring force of the disk, i.e.flaring. As a result, the fraction of younger stars with higher metallicity due to the age-metallicity relation becomes higher at the outer radii, which makes the median metallicity higher at the outer radii. Combining this result with the recently observed age-metallicity and age-velocity dispersion relation for the Milky Way thick disk stars suggested by Haywood et al., we argue that the observed(small) positive radial metallicity gradient at large heights of the Milky Way disk stars can be explained by flaring of the younger thick and/or thin disk stars.  相似文献   

15.
It has been firmly established that there exists a tight correlation between the mass of the central black hole and velocity dispersion (or luminosity) in elliptical galaxies, “pseudobulges” and bulges of galaxies, although the nature of this correlation still remains unclear. We explore the possibility of extrapolating such a correlation to less massive, spherical systems like globular clusters. In particular, motivated by the apparent success in the globular cluster M15, we present an estimate of the central black hole mass for a number of globular clusters with available velocity dispersion data.  相似文献   

16.
For a composite system of gravitationally coupled stellar and gaseous discs, we have carried out a linear stability analysis for axisymmetric coplanar perturbations using the two-fluid formalism. The background stellar and gaseous discs are taken to be scale-free with all physical variables varying as powers of the cylindrical radius r with compatible exponents. The unstable modes set in as neutral modes or stationary perturbation configurations with angular frequency ω=0. The axisymmetric stable range is bounded by two marginal stability curves derived from stationary perturbation configurations. Because of the gravitational coupling between the stellar and the gaseous discs, one only needs to consider the parameter regime of the stellar disc. There exist two unstable regimes in general: a collapse regime corresponding to large-scale perturbations and a ring-fragmentation regime corresponding to short-wavelength perturbations. The composite system will collapse if it rotates too slowly and will succumb to ring-fragmentation instabilities if it rotates sufficiently fast. The overall stable range against axisymmetric perturbations is determined by a necossary D-criterion involving the effective Mach number squared Ds^2 (the squared ratio of the stellar disc rotation speed to the stellar velocity dispersion up to a numerical factor). Different mass ratio δ and sound speed ratio of the gaseous and stellar disc components will alter the overall stability. For spiral galaxies or circumnuclear discs, we further include the dynamical effect of a massive dark matter halo. Astrophysical applications to disc galaxies, proto-stellar discs and circumnuclear discs are given as examples.  相似文献   

17.
We calculate the gravitational lensing probabilities by cold dark matter (CDM) halos with different density profiles, and compare them with current observations from the Cosmic Lens All-Sky Survey (CLASS) and the Jodrell-Bank VLA Astrometric Survey (JVAS). We find that the lensing probability is dramatically sensitive to the clumping of the dark matter, or quantitatively, the concentration parameter. We also find that our predicted lensing probabilities in most cases show inconsistency with the observations. It is argued that high lensing probability may not be an effective tool for probing the statistical properties of inner structures of dark matter halos.  相似文献   

18.
Recently,some studies showed that the GeV gamma-ray excess signal from the central Milky Way can be explained by the annihilation of ~ 40 GeV dark matter through the bb channel.Based on the morphology of the gamma-ray flux,the best-fit inner slope of the dark matter density profile is γ= 1.26.However,recent analyses of the Milky Way dark matter profile favor γ= 0.6-0.8.In this article,we show that the GeV gamma-ray excess can also be explained by the Sommerfeld-enhanced dark matter annihilation through the bb channel with γ= 0.85-1.05.We constrain the parameters of the Sommerfeld-enhanced annihilation by using data from Fermi-LAT.We also show that the predicted gamma-ray fluxes emitted from dwarf galaxies generally satisfy recent upper limits on gamma-ray fluxes detected by Fermi-LAT.  相似文献   

19.
We have investigated late time acceleration for a spatially flat dust filled Universe in BransDicke theory in the presence of a positive cosmological constant Λ. Expressions for Hubble's constant,luminosity distance and apparent magnitude have been obtained for our model. The theoretical results are compared with observed values of the latest 287 high redshift(0.3 ≤ z ≤ 1.4) Type Ia supernova data taken from the Union 2.1 compilation to estimate present values of matter and dark energy parameters,(?_m)_0 and(?_Λ)_0. We have also estimated the present value of Hubble's constant H0 in light of an updated sample of Hubble parameter measurements including 19 independent data points. The results are found to be in good agreement with recent astrophysical observations. We also calculated various physical parameters such as matter and dark energy densities, present age of the Universe and deceleration parameter. The value for Brans-Dicke-coupling constant ω is set to be 40 000 based on accuracy of solar system tests and recent experimental evidence.  相似文献   

20.
From photometric observations and star counts, the existence of a bar in the cen-tral few kpc of the Galaxy is suggested. It is generally thought that our Galaxy is surrounded by a massive invisible halo. The gravitational potential of the Galaxy is therefore made non-axisymmetric generated by the central tfiaxial bar, by the outer triaxial halo, and/or by the spiral structures. Selecting nearly 300 open clusters with complete spatial velocity measure-ments and ages, we were able to construct the rotation curve of the Milky Way within a range of 3 kpc of the Sun. Using a dynamic model for an assumed elliptical disk, a clear weak el-liptical potential of the disk with ellipticity of ε(R0) = 0.060 ± 0.012 is detected, the Sun is found to be near the minor axis, displaced by 30°± 3°. The motion of the clusters is suggested to be on an oval orbit rather than on a circular one.  相似文献   

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