Cosmic strings and the orientation of galaxies and clusters

The formation of galaxies, clusters, and superclusters is discussed in the cosmic string model and the constraints are given for the corresponding loops which form these structures by the accretion of the loops. The alignment of position angles of galaxies with that of their parent cluster and of clusters with that of the supercluster they lie on, appears naturally. Various different observations and statistical results of position angles are (at least qualitatively) explained in the cosmic string theory of formation of the large-scale structure.     

Cosmic strings and strings in gravitational waves

We consider strings with the Nambu action as extremal surfaces in a given space-time, thus, we ignore their back reaction. Especially, we look for strings sharing one symmetry with the underlying space-time. If this is a non-null symmetry the problem of determining ihc motion of the string can be dimensionally reduced. We get exact solutions for the following cases: straight and circle-like strings in a Friedmann background, straight strings in an anisotropic Kasner background, different types of strings in the metric of a gravitational wave. The solutions will be discussed. Wir betrachten Strings mit der Nambu-Wirkung als Extremalflächen in einer gegebenen Raum-Zeit, d. h., wir ignorieren ihre Rückwirkung. Wir interessieren uns dabei besonders für solche Strings, die eine Isometrie mit der unterliegenden Raum-Zeit gemeinsam haben. Handelt es sich dabei um eine nicht-lichtartige Symmetrie, so läß Bt sich das Problem der Bestimmung der Stringbewegung dimensionsreduzieren. Wir erhalten exakte Lösungen für die folgenden Fälle: gerade und kreisrunde Strings im Friedmann-Hintergrund, gerade Strings im anisotropen Kasner-Hintergrund sowie verschiedene Stringtypen in der Metrik einer Gravitationswelle. Die Lösungen werden diskutiert.     

Cosmic rays and cosmic strings

It has been suggested that the highest-energy cosmic rays might be protons resulting from collapsing cosmic strings in the Universe. We point out that this mechanism, although attractive, has important shortcomings, notably the fact that gamma rays produced along with the protons and those produced by the protons in their interactions with the cosmic background radiation generate cascades in the Universe and result in unacceptably high fluxes of cosmic gamma rays in the region of hundreds of MeV.     

Cosmic strings in Lyra geometry

Exact cylindrically-symmetric solutions of Sen equations both with and without the source term in Lyra geometry are presented. It is found that the exterior vacuum solution, when continuously joined to the interior solution describing the uniform density cylinder of finite radius, may represent a cosmic string. The rate of light deflection that is independent of the impact parameter suggests that the solution has the gravitational lensing property, and for typical values of the linear mass density it yields predictions similar to general relativity.     

The formation and evolution of clusters of galaxies in different cosmogonies

The formation of galaxy clusters in hierarchically clustering universes is investigated by means of high-resolution N -body simulations. The simulations are performed using a newly developed multimass scheme which combines a PM code with a high-resolution N -body code. Numerical effects resulting from time-stepping and gravitational softening are investigated, as well as the influence of the simulation box size and of the assumed boundary conditions. Special emphasis is laid on the formation process and the influence of various cosmological parameters. Cosmogonies with massive neutrinos are also considered. Differences between clusters in the same cosmological model seem to dominate over differences caused by differing background cosmogony. The cosmological model can alter the time evolution of cluster collapse, but the merging pattern remains fairly similar, e.g. the number of mergers and the mass ratio of mergers. The gross properties of a halo, such as its size and total angular momentum, also evolve in a similar manner for all cosmogonies, and can be described using analytical models. It is shown that the density distribution of a halo shows a characteristic radial dependence which follows a power law with a slope of =1 at small radii and =3 at large radii, independent of the background cosmogony or the considered redshift. The shape of the density profiles follows the generic form proposed by Navarro et al. for all hierarchically clustering scenarios, and retains very little information about the formation process or the cosmological model. Only the central matter concentration of a halo is correlated with the formation time and therefore the corresponding cosmogony. We emphasize the role of non-radial motions of the halo particles in the evolution of the density profile.     

Cosmic strings with self-interacting hot dark matter

We compute the linear power spectrum of cosmic-string-seeded fluctuations in the context of neutrinos with a strong self-interaction and show that it is very similar to that obtained in the context of 'normal' neutrinos. We compare our results with observational data and show that for any value of the cosmological parameters h and Ω₀ the interacting hot dark matter power spectrum requires a scale-dependent biasing parameter.     

Star formation in galaxies falling into clusters along supercluster-scale filaments

With the help of a statistical parameter derived from optical spectra, we show that the current star formation rate of a galaxy, falling into a cluster along a supercluster filament, is likely to undergo a sudden enhancement before the galaxy reaches the virial radius of the cluster. From a sample of 52 supercluster-scale filaments of galaxies joining a pair of rich clusters of galaxies within the two-degree Field Redshift Survey region, we find a significant enhancement of star formation, within a narrow range between ∼2 and  3  h ⁻¹₇₀ Mpc  of the centre of the cluster into which the galaxy is falling. This burst of star formation is almost exclusively seen in the fainter dwarf galaxies  ( M _B ≥−20)  . The relative position of the peak does not depend on whether the galaxy is a member of a group or not, but non-group galaxies have on average a higher rate of star formation immediately before falling into a cluster. From the various trends, we conclude that the predominant process responsible for this rapid burst is the close interaction with other galaxies falling into the cluster along the same filament, if the interaction occurs before the gas reservoir of the galaxy gets stripped off due to the interaction with the intracluster medium.     

Star formation rates and chemical abundances of emission-line galaxies in intermediate-redshift clusters

We examine the evolutionary status of luminous, star-forming galaxies in intermediate-redshift clusters by considering their star formation rates (SFRs) and the chemical and ionization properties of their interstellar emitting gas. Our sample consists of 17 massive, star-forming, mostly disc galaxies with   M_B ≲−20  , in clusters with redshifts in the range  0.31 ≲ z ≲ 0.59  , with a median of  〈 z 〉= 0.42  . We compare these galaxies with the identically selected and analysed intermediate-redshift field sample of Mouhcine et al., and with local galaxies from the Nearby Field Galaxy Survey of Jansen et al.
From our optical spectra, we measure the equivalent widths of  [O  ii ]λ3727, Hβ  and [O  iii ]λ5007 emission lines to determine diagnostic line ratios, oxygen abundances and extinction-corrected SFRs. The star-forming galaxies in intermediate-redshift clusters display emission-line equivalent widths which are, on average, significantly smaller than measured for field galaxies at comparable redshifts. However, a contrasting fraction of our cluster galaxies have equivalent widths similar to the highest observed in the field. This tentatively suggests a bimodality in the SFRs per unit luminosity for galaxies in distant clusters. We find no evidence for further bimodalities, or differences between our cluster and field samples, when examining additional diagnostics and the oxygen abundances of our galaxies. This maybe because no such differences exist, perhaps because the cluster galaxies which still display signs of star formation have recently arrived from the field. In order to examine this topic with more certainty, and to further investigate the way in which any disparity varies as a function of cluster properties, larger spectroscopic samples are needed.     

Cosmic strings and viscous matter in a Kantowski-Sachs universe

We study the dynamics of a universe where the matter content is represented by a radially directed distribution of cosmic strings plus matter admitting both shear and bulk viscosity. We assume specific forms for the equation of state of both the strings and matter and discuss the cosmological solutions obtained.     

Star formation, massive stars, and super star clusters in nearby galaxies

The Square Kilometer Array (SKA) will enable studies of star formation in nearby galaxies with a level of detail never before possible outside of the Milky Way. Because the earliest stages of stellar evolution are often inaccessible at optical and near-infrared wavelengths, high spatial resolution radio observations are necessary to explore extragalactic star formation. The SKA will have the sensitivity to detect individual ultracompact HII regions out to the distance of nearly 50 Mpc, allowing us to study their spatial distributions, morphologies, and populations statistics in a wide range of environments. Radio observations of Wolf-Rayet stars outside of the Milky Way will also be possible for the first time, greatly expanding the range of conditions in which their mass loss rates can be determined from free-free emission. On a vastly larger scale, natal of super star clusters will be accessible to the SKA out to redshifts of nearly z 0.1. The unprecedented sensitivity of radio observations with the SKA will also place tight constraints on the star formation rates as low as 1M yr⁻¹ in galaxies out to a redshift of z 1 by directly measuring the thermal radio flux density without assumptions about a galaxy’s magnetic field strength, cosmic ray production rate, or extinction.     

Peculiar velocities of galaxies and clusters

We present a simple model for the shape of the distribution function of galaxy peculiar velocities. We show how both non-linear and linear theory terms combine to produce a distribution which has an approximately Gaussian core with exponential wings. The model is easily extended to study how the statistic depends on the type of particle used to trace the velocity field (dark matter particles, dark matter haloes, galaxies), and on the density of the environment in which the test particles are located. Comparisons with simulations suggest that our model is accurate. We also show that the evolution of the peculiar velocities depends on the local, rather than the global, density. Since clusters populate denser regions on average, using cluster velocities with the linear theory scaling may lead to an overestimate of the global value of Ω₀. Conversely, using linear theory with the global value of Ω₀ to scale cluster velocities from the initial to the present time results in an underestimate of their true velocities. In general, however, the directions of motions of haloes are rather well described by linear theory. Our results help to simplify models of redshift-space distortions considerably.     

On the periodical structures in populations of galaxies and of clusters of galaxies

Analysing the distribution of the redshift of clusters of galaxies, obtained from the catalogue of SCHMIDT (1986), two already known periodical structures were confirmed and both periods were extended by a number of periods — approximately by two times (We elucidated 5 new periods in the first structure and 4 in the second.). The statistical significance of these structures was estimated to 0.01 in the first case and to 0.02 in the second one. Particularly, the second structure may be the largest known object in the Universe (about 900 Mpc at H = 75 km s^-1 Mpc^-1). These results agree with the predictions of the axion-dominated early Universe. In addition, we discovered periodical structures of an another type — in luminosity functions of galaxies, which are members of rich clusters. Bei der Analyse des Katalogs von SCHMIDT (1986) wurden zwei periodische Strukturen in den Verteilungen der Rotverschiebungen von Galaxienhaufen festgestellt. In den beiden Strukturen wurden 5 bzw. 4 neue Perioden beobachtet, weshalb sich die Periodenzahl in jeder der beiden Strukturen etwa verdoppelt hat. Die statistische Signifikanzen dieser Strukturen wurden zu 0.01 und 0.02 abgeschätzt. Möglicherweise entspricht die zweite Struktur dem größten bekannten Objekt des Weltalls (etwa 900 Mpc bei H = 75 km s^-1 Mpc^-1). Diese Resultate stützen die Theorie eines axiondominierten frühen Universums. Außerdem wurden weitere periodische Strukturen in der Leuchtkraftfunktion von Galaxien aus reichen Galaxien-haufen entdeckt.     

Cosmic rays in central regions of active galaxies

We consider the acceleration of charged particles by the Fermi mechanism on magnetic field irregularities in active galactic regions. The relativistic particles are shown to be accelerated most efficiently, while the acceleration of nonrelativistic particles by this mechanism is possible only in highly nonuniform galactic nuclei, that is, in nuclei with strong turbulization. The conditions for the acceleration of charged particles in active galactic nuclei at various stages of their evolution are investigated.     

Modelling rich clusters of galaxies

We develop a self-consistent dynamical model for spherically-symmetric clusters of galaxies. The total mass profile and velocity dispersion profiles of galaxies are derived by taking both, galaxies and intracluster gas, in hydrostatic equilibrium, and by assuming the latter to follow a polytropic distributionT^–1. We use the strongest and better established correlations among observed properties of clusters to fix the values of the resulting free parameters, and so, to reduce the general freedom of the model.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

The cosmological constant and the clusters of galaxies

The dependence of the formation redshift of clusters of galaxies on the cosmological constant is discussed on the basis of recent measurements of gravitational masses and radii from X-ray astronomy techniques. For a flat CDM model with several different values of the cosmological constant, the cluster number density as a function of the redshift is calculated. Comparing the calculated number density at z − 0 with the observed value limits the spectrum parameter Γ to the range 0.15–0.32. We also show that if Γ and/or Ω_Λ were larger, many more clusters of galaxies would have been formed at high redshifts.     

New polar ring galaxies in rich clusters of galaxies

Four polar ring galaxies discovered in rich clusters of galaxies are presented. Brief comments on their structural properties are given.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.