On Einstein clusters as galactic dark matter haloes

We consider global and gravitational lensing properties of the recently suggested Einstein clusters of weakly interacting massive particles (WIMPs) as galactic dark matter haloes. Being tangential pressure dominated, Einstein clusters are strongly anisotropic systems which can describe any galactic rotation curve by specifying the anisotropy. Due to this property, Einstein clusters may be considered as dark matter candidates. We analyse the stability of the Einstein clusters against both radial and non-radial pulsations, and we show that the Einstein clusters are dynamically stable. With the use of the Buchdahl type inequalities for anisotropic bodies, we derive upper limits on the velocity of the particles defining the cluster. These limits are consistent with those obtained from stability considerations. The study of light deflection shows that the gravitational lensing effect is slightly smaller for the Einstein clusters as compared to the singular isothermal density sphere model for dark matter. Therefore, lensing observations may discriminate, at least, in principle, between Einstein cluster and the other dark matter models.     

Studying hot gas using the ultraviolet: High ions in galactic halos

Among the many spectral lines falling in the ultraviolet are the resonance doublets of the highly ionized species O VI, N V, C IV, and Si IV. By using both space-based UV observations and ground-based optical observations, these lines can be used to trace diffuse astrophysical plasma over a large fraction of the age of the Universe. In this paper we discuss observations of these absorption lines in damped Lyman-α systems at z≈2–3, using the UVES spectrograph on the VLT in Chile.     

Neutral hydrogen in the Galaxy and the galactic shocks

The effect of the differential rotation on the statistical behaviour of a dynamical system has been studied in the presence of a scalar field. The existence of neutrally stable regimes in a disk-like system has been shown which formally correspond to the Landau levels of a charged particle whose motion is quantized in a magnetic field. The existence also of the so-called anomalous Doppler effect has been pointed out, and a modified form is given to include the differential rotation. Finally, quantitative results are given concerning the instability of the system.     

Neutral hydrogen in the vicinity of galactic radio sources

In the preceding paper by Vengeret al. (1984) the results of observations of neutral gas in the vicinity of some galacticHii regions were considered; and it was demonstrated that 17 of the regions observed are surrounded by expandingHi envelopes. This paper describes a model of interaction between theHii regions and the surrounding interstellar medium constructed on the basis of the said results. It is assumed that the main dynamic factor in the envelope formation mechanism is the total stellar wind from the stars which ionze theHii regions. The employment of the observations of the line at =21 cm, of the radio continuum and IR dust continuum made it possible to determine the differential mass spectra of the stars exciting theHii zones and calculate some characteristics of the stellar population. The mass spectrum index of the objects considered turned out to be much higher than that for the mass spectra of background stars and scattered clusters of stars.     

Neutral hydrogen in the vicinity of galactic radio sources

Observations of neutral gas in the vicinity of 24 SNRs (Supernova remnants) were performed on a RATAN-600 radio telescope with resolution of 2×130×6.3 km s^–1. The observations demonstrate that kinematics and spatial structure of the gas in the vicinity of 12 SNRs is typical of the expanding evelopes. This paper considers in detail only those enveloping the SNRs. The dimensions and expanding velocities of the envelopes are defined and the evolutionary parameters of the SNRs estimated.     

The role of diffuse matter in galactic coronas

An upper limit to the gas content in the coronas of giant galaxiesM _gas<0.04M _cor has been calculated on the basis of observational data on the emission of clusters of galaxies in different wavelength ranges. An analysis of companion galaxies has revealed a correlation of their morphological types with the distances to central giant galaxies. A gasodynamical interpretation of this regularity is suggested and a possible picture of the evolution of the galactic coronas is presented.     

The clustering of QSOs and the dark matter halos that host them

The spatial clustering of QSOs is an important measurable quantity which can be used to infer the properties of dark matter halos that host them.We construct a simple QSO model to explain the linear bias of QSOs measured by recent observations and explore the properties of dark matter halos that host a QSO.We assume that major mergers of dark matter halos can lead to the triggering of QSO phenomena,and the evolution of luminosity for a QSO generally shows two accretion phases,i.e.,initially having a constant Eddington ratio due to the self-regulation of the accretion process when supply is sufcient,and then declining in rate with time as a power law due to either diminished supply or long term disk evolution.Using a Markov Chain Monte Carlo method,the model parameters are constrained by fitting the observationally determined QSO luminosity functions（LFs）in the hard X-ray and in the optical band simultaneously.Adopting the model parameters that best fit the QSO LFs,the linear bias of QSOs can be predicted and then compared with the observational measurements by accounting for various selection efects in diferent QSO surveys.We find that the latest measurements of the linear bias of QSOs from both the SDSS and BOSS QSO surveys can be well reproduced.The typical mass of SDSS QSOs at redshift 1.53 6×1012h 1M⊙and the typical mass of BOSS QSOs at z².4 is²×1012h 1M⊙.For relatively faint QSOs,the mass distribution of their host dark matter halos is wider than that of bright QSOs because faint QSOs can be hosted in both big halos and smaller halos,but bright QSOs are only hosted in big halos,which is part of the reason for the predicted weak dependence of the linear biases on the QSO luminosity.     

Local galactic field of forces and the interstellar matter

The density distributions of the two main components in interstellar hydrogen are calculated using 21 cm line data from the Berkeley Survey and the Pulkovo Survey. The narrow, dense component (state I of neutral hydrogen) has a Gaussianz-distribution with a scale-height of 50 pc in the local zones (the galactic disk). For the wide, tenuous component (hydrogen in state II) we postulate a distribution valid in the zones where such a material predominates (70 pc?z? 350 pc the galactic stratum) i.e., $$n_H \left( z \right) = n_H \left( 0 \right)exp \left( { - \left( {z/300{\text{ }}pc} \right)^{3/2} } \right).$$ Similar components are found in the dust distribution and in the available stellar data reaching sufficiently highz-altitudes. The scale-heights depend on the stellar type: the stratum in M III stars is considerably wider than in A stars (500–700 pc against 300 pc). The gas to dust ratio is approximately the same in both components: 0.66 atom cm^?3 mag^?1 kpc in the galactic plane. A third state of the gas is postulated associating it the observed free electron stratum at a scale-height of 660 pc (hydrogen fully ionized at high temperatures). The ratio between the observed dispersions in neutral hydrogen (thermal width plus turbulence) and the total dispersions corresponding to the real inner energies in the medium is obtained by a comparison with the dispersion distribution σ(z) of the different stellar types associated with the disk and the stratum $$\sigma ^2 \left( {total} \right) = \sigma ^2 \left( {21{\text{ cm line}}} \right) \cdot {\text{ }}Q^2 ,$$ from which we graphically obtainedQ ²=2.9 ± 0.3, although that number could be lower in the densest parts of the spiral arms. Its dependence on magnetic field and cosmic rays is analysed, indicating equipartition of the different energy components in the interstellar medium and consistency with the observed values of the magnetic field: i.e., fluctuations with an average of ～ 3 μG (associated with the disk) in a homogeneous background of ～ 1 μG (associated with the stratum). A minimum and maximumK _z-force are obtained assuming extreme conditions for the total density distribution (gas plus stars). TheK _z-force obtained from the interstellar gas in its different states using approximations of the Boltzmann equation is a reasonable intermediate case between maximum and minimumK _z. The mass density obtained in the galactic plane is 0.20±0.05M _⊙ pc^?3, and the results indicate that the galactic disk is somewhat narrower and denser than has usually been believed. The effects of wave-like distributions of matter in thez-coordinate are analysed in relation with theK _z-force, and comparisons with theoretical results are performed. A qualitative model for the galactic field of force is postulated together with a classification of the different zones of the Galaxy according to their observed ranges in velocity dispersions and the behaviour of the potential well at differentz-altitudes. The disk containing at least two-thirds of the total mass atz<100 pc, the stratum containing one-third or less of the total mass atz≤600–800 pc, and the halo at higherz-altitudes with a small fraction of such a mass which is difficult to evaluate.     

An empirical model to form and evolve galaxies in dark matter halos

Based on the star formation histories of galaxies in halos with different masses, we develop an empirical model to grow galaxies in dark matter halos. This model has very few ingredients, any of which can be associated with observational data and thus be efficiently assessed. By applying this model to a very high resolution cosmological N-body simulation, we predict a number of galaxy properties that are a very good match to relevant observational data. Namely, for both centrals and satellites, the galaxy stellar mass functions up to redshift z ≈ 4 and the conditional stellar mass functions in the local universe are in good agreement with observations. In addition, the two point correlation function is well predicted in the different stellar mass ranges explored by our model. Furthermore, after applying stellar population synthesis models to our stellar composition as a function of redshift, we find that the luminosity functions in the 0.1_u,0.1_g,0.1_r,0.1_i and 0.1_z bands agree quite well with the SDSS observational results down to an absolute magnitude at about –17.0. The SDSS conditional luminosity function itself is predicted well. Finally, the cold gas is derived from the star formation rate to predict the HI gas mass within each mock galaxy. We find a remarkably good match to observed HI-to-stellar mass ratios. These features ensure that such galaxy/gas catalogs can be used to generate reliable mock redshift surveys.     

The mass to light ratio and the initial mass function in galactic discs

A low mass-to-light ratio for the baryonic component of spiral galaxies is advocated by a number of dynamical studies and by cosmological simulations of galaxy formation. We discuss the possibility of obtaining low mass-to-light ratios for the stellar component in discs, by changing the Initial Mass Function and the Star Formation History. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of primordial magnetic field on the formation rate of dark matter halos

We construct and demonstrate a method for computing the formation rate of the dark matter halo in the hierarchical model set up. This method uses the Press-Schecter distribution for the halos and hence applies only to the spherical halos. But this can be generalized to ellipsoidal structures also if one uses the Sheth-Torman distribution. After obtaining the formation rate, we study the effect of primordial magnetic field on the dynamics of these halos. We investigate the effect for different field strengths and conclude that a magnetic field stronger than 10 nG would impact the halos larger than \(10^{8}\) solar masses while a weaker field affects the formation rate of smaller halos.     

On the structure and mass of the galactic halo

On the basis of a globular cluster study a crude estimate of the total mass of the galactic halo within 20 kpc from the centre is done. It gives a minimal halo mass of the order of , yielding possibilities for a mass as large as . The content of the interstellar matter in the halo is estimated too. It is found that the gas content is a few percents the minimal mass, the gas temperature is very high — about 1×10⁶ K, the magnetic field weak — about 0.25 nT. A weak nonthermal radio emission might be expected from such a halo.     

Dark halos acting as chaos controllers in asymmetric triaxial galaxy models

We study the regular or chaotic character of orbits in a 3D dynamical model,describing a triaxial galaxy surrounded by a spherical dark halo component.Our numerical experiments suggest that the percentage of chaotic orbits decreases exponentially as the mass of the dark halo increases.A linear increase of the percentage of the chaotic orbits was observed as the scale length of the halo component increases. In order to distinguish between regular and chaotic motion,we chose to use the total angular momentum ...     

The entropy-corrected holographic dark energy in Brans-Dicke cosmology with varying mass fermions

We aim in this paper to study Brans-Dicke cosmology in the presence of varying mass fermions and a self-interaction potential. Furthermore, we also probe the entropy corrected holographic dark energy (ECHDE) in the model in two non-interacting and interacting scenarios. The model parameters are constrained by using the recent SNe Ia observational data and tested against observational data of Hubble parameter. For a comparison, we also constrained and tested the cosmological parameters in ΛCDM model with the same observational data. We show that in non of the scenarios the model prediction is better than ΛCDM model.     

Possibility of mass loss in the red-giant stage from H-R diagrams of galactic clusters

A statistical research on evolved stars beyond hydrogen exhaustion is performed by comparing the H-R diagrams of about 60 open clusters with a set of isochronous curves without mass loss derived from Iben's evolutionary tracks and time scales for Population I stars. Interpreting the difference in magnitude between the theoretical positions thus calculated and the observed ones as due to mass loss, when negative, the results indicate that this loss may be conspicuous only for very massive and red stars. However, a comparison with an analogous work of Lindoff reveals that the uncertainties connected with the bolometric and colour corrections may invalidate by a large amount the conclusions which might be drawn from such research.Presented at the Trieste Colloquium on Mass Loss from Stars, September 12–16, 1968.     

The missing mass problem and the cosmological constant problem in the Brans-Dicke theory

The missing mass problem and the cosmological constant problem, which arised in the theory of general relativity, are re-examined in the Brans-Dicke theory. It is shown that the two problems can be solved in simple ways in the Brans-Dicke theory.     

Lifetimes of stars in the main sequence and the maximum mass of stars in the galactic disk

Generalized and interconsistent approximation formulas are derived to describe the relationship between the hydrogen-burning time and the zero-age stellar mass for the mass and elemental composition ranges characteristic of stars that have been formed during the lifetime of the universe. The maximum masses of population I stars are estimated based on the known statistical relationships among stellar characteristics that agree with observational data.