Gravitational stability of dark energy in galaxies and clusters of galaxies

We analyze the behavior of the scalar field as dark energy of the Universe in a static world of galaxies and clusters of galaxies. We find the analytical solutions of evolution equations of the density and velocity perturbations of dark matter and dark energy, which interact only gravitationally, along with the perturbations of metric in a static world with background Minkowski metric. It was shown that quintessential and phantom dark energy in the static world of galaxies and clusters of galaxies is gravitationally stable and can only oscillate by the influence of self-gravity. In the gravitational field of dark matter perturbations, it is able to condense monotonically, but the amplitude of density and velocity perturbations on all scales remains small. It was also illustrated that the “accretion” of phantom dark energy in the region of dark matter overdensities causes formation of dark energy underdensities-the regions with negative amplitude of density perturbations of dark energy.     

Old Stellar Populations in Nearby Dwarf Galaxies

What can we learn from the somewhat arduous study of old stellar populations in nearby galaxies? Unless the nearby universe is subtly anomalous, it should contain a relatively normal selection of galaxies whose histories are representative of field galaxies in general throughout the Universe. We can therefore take advantage of our ability to resolve local galaxies into individual stars to directly, and accurately, measure star formation histories. The star formation histories are determined from numerical models, based on stellar evolution tracks, of colour-magnitude diagrams. The most accurate information on star formation rates extending back to the earliest epoches can be obtained from the structure of the main sequence. However, the oldest main sequence turnoffs are very faint, and it is often necessary to use the brighter, more evolved, populations to infer the star formation history at older times. A complete star formation history can be compared with the spectroscopic properties of galaxies seen over a large range of lookback times in redshift surveys. There is considerable evidence that the faint blue galaxies seen in large numbers in cosmological surveys are the progenitors of the late-type irregular galaxies seen in copious numbers in the Local Group, and beyond. We consider how the `Madau-diagram', the star formation history of the Universe, would look if the Local Group were to be considered representative of the Universe as a whole.     

Feedback and metal enrichment in cosmological SPH simulations – II. A multiphase model with supernova energy feedback

We have developed a new scheme to treat a multiphase interstellar medium in smoothed particle hydrodynamics simulations of galaxy formation. This scheme can represent a co-spatial mixture of cold and hot ISM components, and is formulated without scale-dependent parameters. It is thus particularly suited to studies of cosmological structure formation where galaxies with a wide range of masses form simultaneously. We also present new algorithms for energy and heavy element injection by supernovae, and show that together these schemes can reproduce several important observed effects in galaxy evolution. Both in collapsing systems and in quiescent galaxies our codes can reproduce the Kennicutt relation between the surface densities of gas and of star formation. Strongly metal-enhanced winds are generated in both cases with ratios of mass-loss to star formation which are similar to those observed. This leads to a self-regulated cycle for star formation activity. The overall impact of feedback depends on galaxy mass. Star formation is suppressed at most by a factor of a few in massive galaxies, but in low-mass systems the effects can be much larger, giving star formation an episodic, bursty character. The larger the energy fraction assumed available in feedback, the more massive the outflows and the lower the final stellar masses. Winds from forming discs are collimated perpendicular to the disc plane, reach velocities up to  ∼1000 km s⁻¹  , and efficiently transport metals out of the galaxies. The asymptotically unbound baryon fraction drops from >95 per cent to ∼30 per cent from the least to the most massive of our idealized galaxies, but the fraction of all metals ejected with this component exceeds 60 per cent regardless of mass. Such winds could plausibly enrich the intergalactic medium to observed levels.     

Large-scale structure of galaxies in the Ophiuchus region

The large-scale structure around the Ophiuchus cluster of galaxies in the vicinity of the Galactic Centre ( l =05, b =95, cz =8500 km s⁻¹) is investigated on the basis of a galaxy survey and spectroscopic observations made for a 12°×17° area. The galaxy survey was performed using six ESO/SERC Sky Survey Atlas films, and 4021 galaxies were detected in total. Recession velocities were newly obtained for 179 galaxies to make the total number of galaxies in the survey area with known velocities 219.
In the distribution of bright galaxies, we identified seven new clumps of galaxies. Comparing the surface number density of bright galaxies with the Galactic extinction, which is estimated from the 100‐μm flux density in the IRAS Sky Survey Atlas, we demonstrate that the seven clumps are not spurious as a result of the inhomogeneity of the Galactic extinction. Among the seven clumps, two are found to be clusters and four to be groups on the basis of the histogram of recession velocities and the number of member galaxies. The Ophiuchus cluster, two newly identified clusters, and four groups are all concentrated at 9000 km s⁻¹. Field galaxies are also distributed centred at 8500 km s⁻¹. Hence field galaxies occupy a common three-dimensional region with galaxies in the clusters and groups, and altogether they form a large-scale structure of supercluster size. As opposed to the overdensity in the supercluster region, the mean number density of galaxies in the velocity range 0–5000 km s⁻¹ is only 25 per cent of the mean number density of the Universe, comparable with the density of the well-known Böotes void. Hence this nearby three-dimensional region in Ophiuchus is a void of galaxies also.     

galev evolutionary synthesis models – I. Code, input physics and web interface

galev (GALaxy EVolution) evolutionary synthesis models describe the evolution of stellar populations in general, of star clusters as well as of galaxies, both in terms of resolved stellar populations and of integrated light properties over cosmological time-scales of ≥13 Gyr from the onset of star formation shortly after the big bang until today.
For galaxies, galev includes a simultaneous treatment of the chemical evolution of the gas and the spectral evolution of the stellar content, allowing for what we call a chemically consistent treatment: we use input physics (stellar evolutionary tracks, stellar yields and model atmospheres) for a large range of metallicities and consistently account for the increasing initial abundances of successive stellar generations.
Here we present the latest version of the galev evolutionary synthesis models that are now interactively available at http://www.galev.org . We review the currently used input physics, and also give details on how this physics is implemented in practice. We explain how to use the interactive web interface to generate models for user-defined parameters and also give a range of applications that can be studied using galev , ranging from star clusters, undisturbed galaxies of various types E–Sd to starburst and dwarf galaxies, both in the local and the high-redshift Universe.     

Supersonic motions of galaxies in clusters

We study motions of galaxies in galaxy clusters formed in the concordance Λ cold dark matter cosmology. We use high-resolution cosmological simulations that follow the dynamics of dark matter and gas and include various physical processes critical for galaxy formation: gas cooling, heating and star formation. Analysing the motions of galaxies and the properties of intracluster gas in a sample of eight simulated clusters at z = 0, we study the velocity dispersion profiles of the dark matter, gas and galaxies. We measure the mean velocity of galaxy motions and gas sound speed as a function of radius and calculate the average Mach number of galaxy motions. The simulations show that galaxies, on average, move supersonically with the average Mach number of ≈1.4, approximately independent of the cluster-centric radius. The supersonic motions of galaxies may potentially provide an important source of heating for the intracluster gas by driving weak shocks and via dynamical friction, although these heating processes appear to be inefficient in our simulations. We also find that galaxies move slightly faster than the dark matter particles. The magnitude of the velocity bias,   b _v ≈ 1.1  , is, however, smaller than the bias estimated for subhaloes in dissipationless simulations. Interestingly, we find velocity bias in the tangential component of the velocity dispersion, but not in the radial component. Finally, we find significant random bulk motions of gas. The typical gas velocities are of order ≈20–30 per cent of the gas sound speed. These random motions provide about 10 per cent of the total pressure support in our simulated clusters. The non-thermal pressure support, if neglected, will bias measurements of the total mass in the hydrostatic analyses of the X-ray cluster observations.     

Small scale entropy and adiabatic density perturbations — Antimatter in the Universe

Fluctuations of all scales are equally interesting from the point of view of the characteristics of the singular state and not only those which led to the formation of astronomical objects such as clusters of galaxies, separate galaxies, globular clusters and quasars. In this article estimates are given of the homogeneity of the overall density of hot plasma and the relation between the quantity of baryons and antibaryons at early stages of evolution of the Universe. These estimates are made for small scales, considerably smaller than the scale of the astronomical objects enumerated above. Considerations about the energy balance of hot plasma and distortions of the spectrum of relic radiation due to dissipation of density fluctuations of matter are used for these estimates. The corrected upper limit to early energy injection is given. In our preceding paper (Sunyaev and Zeldovich, 1970) this upper limit was underestimated. Difficulties with a model of the Universe which is symmetric in baryon charge are noted.     

Testing cosmological structure formation using redshift-space distortions

Observations of redshift-space distortions in spectroscopic galaxy surveys offer an attractive method for observing the build-up of cosmological structure. In this paper, we develop and test a new statistic based on anisotropies in the measured galaxy power spectrum, which is independent of galaxy bias and matches the matter power spectrum shape on large scales. The amplitude provides a constraint on the derivative of the linear growth rate through   f σ₈(mass)  . This demonstrates that spectroscopic galaxy surveys offer many of the same advantages as weak lensing surveys, in that they both use galaxies as test particles to probe all matter in the Universe. They are complementary as redshift-space distortions probe non-relativistic velocities and therefore the temporal metric perturbations, while weak lensing tests the sum of the temporal and spatial metric perturbations. The degree to which our estimator can be pushed into the non-linear regime is considered and we show that a simple Gaussian damping model, similar to that previously used to model the behaviour of the power spectrum on very small scales, can also model the quasi-linear behaviour of our estimator. This enhances the information that can be extracted from surveys for Λ cold dark matter (ΛCDM) models.     

Investigating Clusters of Galaxies with Planck and Herschel

With the Planck and Herschel satellite missions of the European Space Agency, the far-infrared and submillimeter window will offer new investigation tools toward clusters of galaxies in the distant Universe. These are the Sunyaev Zel'dovich (SZ) effect of the cosmic microwave background and the thermal emission of dust grains. The power of the SZ effect is such that Planckis expected to discover thousands of new clusters at redshifts larger than 0.2, where only a few tens are known today. The dust can be present at large scale in the intracluster medium, and we show that even at very low abundances it is able to be a major cooling agent for the whole cluster. However the dominating dust emission will be that of the background infrared star forming galaxies. In all cases, the data processing of space borne sensitive submillimeter observations of clusters of galaxies such as the one that Planck and Herschel will provide, will require a very carefull combined analysis of the SZ effect and dust thermal emission.     

The anatomy of the NGC 5044 group – II. Stellar populations and star formation histories

The distribution of galaxy properties in groups and clusters holds important information on galaxy evolution and growth of structure in the Universe. While clusters have received appreciable attention in this regard, the role of groups as fundamental to formation of the present-day galaxy population has remained relatively unaddressed. Here, we present stellar ages, metallicities and α-element abundances derived using Lick indices for 67 spectroscopically confirmed members of the NGC 5044 galaxy group with the aim of shedding light on galaxy evolution in the context of the group environment.
We find that galaxies in the NGC 5044 group show evidence for a strong relationship between stellar mass and metallicity, consistent with their counterparts in both higher and lower mass groups and clusters. Galaxies show no clear trend of age or α-element abundance with mass, but these data form a tight sequence when fitted simultaneously in age, metallicity and stellar mass. In the context of the group environment, our data support the tidal disruption of low-mass galaxies at small group-centric radii, as evident from an apparent lack of galaxies below  ∼10⁹ M_⊙  within ∼100 kpc of the brightest group galaxy. Using a joint analysis of absorption- and emission-line metallicities, we are able to show that the star-forming galaxy population in the NGC 5044 group appears to require gas removal to explain the ∼1.5 dex offset between absorption- and emission-line metallicities observed in some cases. A comparison with other stellar population properties suggests that this gas removal is dominated by galaxy interactions with the hot intragroup medium.     

Discs in early-type lensing galaxies: effects on magnification ratios and measurements of H0

Observations of early-type galaxies, both in the local Universe and in clusters at medium redshifts, suggest that these galaxies often contain discs or disc-like structures. Using the results of Kelson et al. for the incidence of disc-components among the galaxies in the redshift   z = 0.33  cluster CL 1358+62, we investigate the effect of disc structures on the lensing properties of early-type galaxies. Statistical properties, like magnification cross-sections and the expected number of quad (four-image) lens systems, are not affected greatly by the inclusion of discs that contain less than ∼10 per cent of the total stellar mass. However, the properties of individual lens systems are affected. We estimate that 10–30 per cent of all quad-lens systems, with early-type deflector galaxies, would be affected measurably by the presence of disc components. Intriguingly, the image magnification ratios are altered significantly. The amplitude of the predicted change is sufficient to explain the observed magnification ratios in systems like B1422+231 without requiring compact substructure. Furthermore, time-delays between images also change; fitting a bulge-only model to early-type lenses that in fact contain a disc would yield a value of the Hubble constant H ₀ that is systematically too low by ∼25 per cent.     

Properties of spherical galaxies and clusters with an NFW density profile

Using the standard dynamical theory of spherical systems, we calculate the properties of spherical galaxies and clusters whose density profiles obey the universal form first obtained in high-resolution cosmological N -body simulations by Navarro, Frenk & White (NFW). We adopt three models for the internal kinematics: isotropic velocities, constant anisotropy and increasingly radial OsipkovMerritt anisotropy. Analytical solutions are found for the radial dependence of the mass, gravitational potential, velocity dispersion, energy and virial ratio and we test their variability with the concentration parameter describing the density profile and amount of velocity anisotropy. We also compute structural parameters, such as half-mass radius, effective radius and various measures of concentration. Finally, we derive projected quantities, the surface mass density and line-of-sight as well as aperture-velocity dispersion, all of which can be directly applied in observational tests of current scenarios of structure formation. On the mass scales of galaxies, if constant mass-to-light is assumed, the NFW surface density profile is found to fit HubbleReynolds laws well. It is also well fitted by Sérsic R ^{1/ m} laws, for     but in a much narrower range of m and with much larger effective radii than are observed. Assuming in turn reasonable values of the effective radius, the mass density profiles imply a mass-to-light ratio that increases outwards at all radii.     

The effects of photoionization on galaxy formation – II. Satellite galaxies in the Local Group

We use a self-consistent model of galaxy formation and the evolution of the intergalactic medium to study the effects of the reionization of the Universe at high redshift on the properties of satellite galaxies like those seen around the Milky Way. Photoionization suppresses the formation of small galaxies, so that surviving satellites are preferentially those that formed before the Universe reionized. As a result, the number of satellites expected today is about an order of magnitude smaller than the number inferred by identifying satellites with subhaloes of the same circular velocity in high-resolution simulations of the dark matter. The resulting satellite population has an abundance similar to that observed in the Local Group, although the distribution of circular velocities differs somewhat from the available data. We explore many other properties of satellite galaxies, including their gas content, metallicity and star formation rate, and find generally good agreement with available data. Our model predicts the existence of many as yet undetected satellites in the Local Group. We quantify their observability in terms of their apparent magnitude and surface brightness, and also in terms of their constituent stars. A near-complete census of the Milky Way's satellites would require imaging to   V ≈20  and to a surface brightness fainter than 26 V -band magnitudes per square arcsecond. Satellites with integrated luminosity   V =15  should contain of order 100 stars brighter than   B =26  , with central stellar densities of a few tens per square arcminute. Discovery of a large population of faint satellites would provide a strong test of current models of galaxy formation.     

The sizes of minivoids in the local Universe: an argument in favour of a warm dark matter model?

Using high-resolution simulations within the cold dark matter (CDM) and warm dark matter (WDM) models, we study the evolution of small-scale structure in the local volume, a sphere of 8-Mpc radius around the Local Group. We compare the observed spectrum of minivoids in the local volume with the spectrum of minivoids determined from the simulations. We show that the ΛWDM model can easily explain both the observed spectrum of minivoids and the presence of low-mass galaxies observed in the local volume, provided that all haloes with circular velocities greater than 20 km s⁻¹ host galaxies. On the contrary, within the ΛCDM model the distribution of the simulated minivoids reflects the observed one if haloes with maximal circular velocities larger than  35 km s⁻¹  host galaxies. This assumption is in contradiction with observations of galaxies with circular velocities as low as 20 km s⁻¹ in our local Universe. A potential problem of the ΛWDM model could be the late formation of the haloes in which the gas can be efficiently photoevaporated. Thus, star formation is suppressed and low-mass haloes might not host any galaxy at all.     

Cosmological constraints on the neutrino mass from CMB anisotropy and large-scale structure of the Universe

Using cosmological data on the CMB anisotropy and large-scale structure of the Universe, we have obtained new constraints on the sum of the masses of three generations of active neutrinos: Σm _ν < 1.05 eV (95% confidence level). Data of the third year of the WMAP mission served as the source of CMB anisotropy data. The mass functions of X-ray clusters of galaxies were taken as the data on the large-scale structure of the Universe. The observational properties of the clusters were obtained during the ROSAT mission and the assumption that the baryon fraction is universal in the Universe was used to determine the total cluster mass.     

A primordial feature at the scale of superclusters of galaxies

We investigate a spatially flat cold dark matter model (with the matter density parameter     with a primordial feature in the initial power spectrum. We assume that there is a bump in the power spectrum of density fluctuations at wavelengths     , which corresponds to the scale of superclusters of galaxies . There are indications for such a feature in the power spectra derived from redshift surveys and also in the power spectra derived from peculiar velocities of galaxies. We study the mass function of clusters of galaxies, the power spectrum of the cosmic microwave background (CMB) temperature fluctuations, the rms bulk velocity and the rms peculiar velocity of clusters of galaxies. The baryon density is assumed to be consistent with the big bang nucleosynthesis value. We show that with an appropriately chosen feature in the power spectrum of density fluctuations at the scale of superclusters, the mass function of clusters, the CMB power spectrum, the rms bulk velocity and the rms peculiar velocity of clusters are in good agreement with the observed data.     

Apparent superluminal velocities due to the curvature of space

Instead of the difficult concept of a three-dimensional curved space one can use two partial models of two-dimensional curved subspaces. If a two-dimensional subspace is curved the arbitrary angle of view must have a distorted shape which has in consequence an optical illusion. The distorted shape of ? must be corrected to the real angle¹φ, valid in uncurved space according to the relation φ=¹φf(ρ) where ρ is the central angle in theoretical sphere. If a two-dimensional subspace is just a spheric surface, the correction factor isf _s (ρ)?ρ/sin ρ, appropriate to the angle distance ρ from observer to the observed object. In this way it is possible to explain why, in remote parts of the Universe, it was observed that their assumed components were receding at velocities larger than that of light, because the correction factorf(ρ) can change up to infinity for ρ=π. On the other hand there are many clustering of galaxies, because the correction factor for a hyperbolic spool space or internal part of annuloid is between zero and one: 0<f _A (ρ))<1. Conception considered explains the cause of polarizing effect and interference phenomenon of light without assuming of components, too, but it is in contradiction with the recent interpretation of the spectrum. If we consider the Universe closed into itself due to curvature the existence of superluminal velocities is a undirect evidence for it. If we consider local hyperbolic spool spaces or that of annuloid, the existence of galaxies clustering is a needful but not sufficient condition for it.     

基于伽马射线的类轴子粒子探测及暗物质子晕搜寻研究

目前已经有很多观测证据表明宇宙中存在着大量暗物质,其能量密度占据了目前宇宙总能量密度的1/4.根据高精度的数值模拟和引力透镜观测,我们已经对从矮星系到星系团中的暗物质空间分布有了较好的理解,但是对于暗物质究竟是什么我们还一无所知.由此,物理学家提出了很多假想的粒子模型.     

Influence of the non-luminous component on dynamical and structural properties of galaxy clusters

In this paper a family of two-component non-isothermal hydrostatic models are elaborated in order to simulate clusters of galaxies in which a dark background exists. The influence of an invisible component on the dynamical analysis is examined. We show that the mass-to-light ratio MLR is not linearly related to the actual amount of the unseen mass, leading to the conclusion that the MLR is not a reliable indicator of the amount of dark matter in clusters of galaxies. The determination of the mass depends strongly on the relative concentration of both the luminous and the dark component. This is the so-called Limber effect which is quantified on the ground of our two-component hydrostatic models. The asymptotic behaviour of the density profile of both components is investigated. We found that they allow useful information to be obtained on the distribution of the unseen component and on its thermodynamical state. We also show that intra-cluster medium (ICM) can not mimic the dark matter and consequently one has to invoke the dark matter to explain the high MLRs in galaxy clusters.