54807942
Self-similarity in a Kantowski-Sachs universe with a string cloud
We study a cosmological model with very simple solutions characterized by string clouds. Assuming self-similar symmetry for a Kantowski- Sachs spacetime we study the gravitational effects of the cosmic strings. It is also assumed that the primitive universe enters in a false vacuum-dominated era, accelerating the expansion in a time period of the order of 10 -35 sec (a phase transition), satisfying all the energy conditions. Finally, we examine the possibility that in the last stages of this evolution, the geometry of the universe could be flat.
117082338
Review of the Second School on Cosmic Rays and Astrophysics
118112489
Air Shower Simulations
Air shower simulations are a vital part of the design of air shower experiments and the analysis of their data. We describe the basic features of air showers and explain why numerical simulations are the appropriate approach to model the shower simulation. The CORSIKA program, the standard simulation program in this field, is introduced and its features, performance and limitations are discussed. The basic principles of hadronic interaction models and some gerneral simulation techniques are explained. Also a brief introduction to the installation and use of CORSIKA is given.
14875226
Galaxy groups in the 2dF Galaxy Redshift Survey: a compactness analysis of groups
A comprehensive study on compactness has been carried out on the 2dF Galaxy Group Catalogue constructed by Merchan and Zandivarez. The compactness indices defined in this work take into account different geometrical constraints in order to explore a wide range of possibilities. Our results show that there is no clear distinction between groups with high and low levels of compactness when considering particular properties such as the radial velocity dispersion, the relative fraction of galaxies per spectral type, and the luminosity functions of their galaxy members. Studying the trend of the fraction of galaxies per spectral type as a function of the dimensionless crossing time, some signs of dynamical evolution are observed. From the comparison with previous works on compactness, we realize that special care should be taken for some compactness criteria definitions in order to avoid possible biases in the identification.
15049813
Radiating Fluid Spheres in the Effective Variables Approximation
We study the evolution of spherically symmetric radiating fluid distributions using the effective variables method, implemented ab initio in Schwarzschild coordinates. To illustrate the procedure and to establish some comparison with the original method, we integrate numerically the set of equations at the surface for two different models. The first model is derived from the Schwarzschild interior solution. The second model is inspired in the Tolman VI solution.
14588387
Galaxy groups in the 2dF Galaxy Redshift Survey: galaxy spectral type segregation in groups
The behaviour of the relative fraction of galaxies with different spectral types in groups is analysed as a function of projected local galaxy density and the group-centric distance. The group sample was taken from the 2dF Group Galaxy Calatogue constructed by Merchan & Zandivarez. Our group sample was constrained to have a homogeneous virial mass distribution with redshift. Galaxies belonging to this group sample were selected in order to minimize possible biases, such as preferential selection of high-luminosity objects. We find a clear distinction between high virial mass groups (MV≳ 1013.5 M⊙) and the less massive ones. While the massive groups show a significant dependence of the relative fraction of low star formation galaxies on local galaxy density and group-centric radius, groups with lower masses show no significant trends. We also cross-correlate our group subsample with the previously identified clusters, finding that this sample shows a very similar behaviour to that observed in the high virial mass group subsample.
117908138
COMPARING GALAXY POPULATIONS IN COMPACT AND LOOSEGROUPS OF GALAXIES
Aims. We compare the properties of galaxies in compact groups, loose groups, and the field to deepen our understanding of the physical mechanisms acting upon galaxy evolution in different environments. Methods. We select samples of galaxies in compact groups, loose groups, and field galaxies from the Sloan Digital Sky Survey. We compare the properties of the galaxy populations in these different environments: absolute magnitude, colour, size, surface brightness, stellar mass, and concentration. We also study the fraction of red and early-type galaxies, the luminosity function, the colourluminosity, and luminosity-size relations. Results. The population of galaxies in compact groups differ from that of loose groups and the field. The fraction of red and early-type galaxies is larger in compact groups. Galaxies in compact groups are, on average, systematically smaller in size, more concentrated, and have higher surface brightnesses than galaxies in the field and in loose groups. At a fixed absolute magnitude, or fixed surface brightness, galaxies in compact groups are smaller in size. Conclusions. The physical mechanisms that transform galaxies into earlier types could be more effective within compact groups, owing to the typically high densities and small velocity dispersions of these environments, which could explain the large fraction of red and early-type galaxies we found in compact groups. Galaxies inhabiting compact groups have undergone a major transformation compared to galaxies that inhabit loose groups.
118549208
An analytic Pade-motivated QCD coupling
We consider a modification of the Minimal Analytic (MA) coupling of Shirkov and Solovtsov. This modified MA (mMA) coupling reflects the desired analytic properties of the space‐like observables. We show that an approximation by Dirac deltas of its discontinuity function ρ is equivalent to a Pade (rational) approximation of the mMA coupling that keeps its analytic structure. We propose a modification to mMA that, as preliminary results indicate, could be an improvement in the evaluation of low‐energy observables compared with other analytic couplings.