Can Conformal Weyl Gravity be Considered a Viable Cosmological Theory?

We present exact analytical solutions to the Conformal Weyl Gravity cosmological equations that are valid for both the matter and radiation dominated eras. The Primordial Nucleosynthesis process is also exhaustively studied. The main conclusion of our work is that cosmological models derived from this theory are not likely to reproduce the observational properties of our Universe. They fail to fulfill simultaneously the observational constraints on present cosmological parameters and on primordial light element abundances.Comment: 7 pages, (1 Figure included), uuencode compressed Postscript. (To be published in ApJ, June 1994.). FTUAM-93-2

Hydrodynamical simulations of galaxy properties: Environmental effects

Using N-body+hydro simulations we study relations between the local environments of galaxies on 0.5 Mpc scale and properties of the luminous components of galaxies. Our numerical simulations include effects of star formation and supernova feedback in different cosmological scenarios: the standard Cold Dark Matter model, the Broken Scale Invariance model (BSI), and a model with cosmological constant (LCDM). In this paper, we concentrate on the effects of environment on colors and morphologies of galaxies, on the star formation rate and on the relation between the total luminosity of a galaxy and its circular velocity. We demonstrate a statistically significant theoretical relationship between morphology and environment. In particular, there is a strong tendency for high-mass galaxies and for elliptical galaxies to form in denser environments, in agreement with observations. We find that in models with denser environments (CDM scenario) ~ 13 % of the galactic halos can be identified as field ellipticals, according to their colors. In simulations with less clustering (BSI and LCDM), the fraction of ellipticals is considerably lower (~ 2-3 %). The strong sensitivity of morphological type to environment is rather remarkable because our results are applicable to ``field'' galaxies and small groups. If all galaxies in our simulations are included, we find a statistically significant dependence of the galaxy luminosity - circular velocity relation on dark matter overdensity within spheres of radius 0.5 Mpc, for the CDM simulations. But if we remove ``elliptical'' galaxies from our analysis to mimic the Tully-Fisher relation for spirals, then no dependence is found in any model.Comment: 44 pages, 21 figures (17 included). Submitted to New Astronomy. GIFF color plots and the complete paper in Postscript (including color figures) can be found at http://astrosg.ft.uam.es/~gustavo/newas

Entropy of gas and dark matter in galaxy clusters

On the basis of a large scale 'adiabatic', namely non-radiative and non-dissipative, cosmological smooth particle hydrodynamic simulation we compare the entropy profiles of the gas and the dark matter (DM) in galaxy clusters. The quantity K_g = T_g \rho_g^{-2/3} provides a measure for the entropy of the intra-cluster gas. By analogy with the thermodynamic variables of the gas the velocity dispersion of the DM is associated with a formal temperature and thereby K_DM = \sigma_DM^2 \rho_DM^{-2/3} is defined. This DM entropy is related to the DM phase space density by K_DM \propto Q_DM^{-2/3}. In accord with other studies the DM phase space density follows a power law behaviour, Q_DM \propto r^{-1.82}, which corresponds to K_DM \propto r^{1.21}. The simulated intra-cluster gas has a flat entropy core within (0.8 \pm 0.4) R_s, where R_s is the NFW scale radius. The outer profile follows the DM behaviour, K_g \propto r^{1.21}, in close agreement with X-ray observations. Upon scaling the DM and gas densities by their mean cosmological values we find that outside the entropy core a constant ratio of K_g / K_{DM} = 0.71 \pm 0.18 prevails. By extending the definition of the gas temperature to include also the bulk kinetic energy the ratio of the DM and gas extended entropy is found to be unity for r > 0.8 R_s. The constant ratio of the gas thermal entropy to that of the DM implies that observations of the intra-cluster gas can provide an almost direct probe of the DM.Comment: 7 pages, 8 figures, accepted for publication in MNRAS, web page of the The Marenostrum Numerical Cosmology Project : http://astro.ft.uam.es/~marenostrum

Strong lensing in the MareNostrum Universe II: scaling relations and optical depths

The strong lensing events that are observed in compact clusters of galaxies can, both statistically and individually, return important clues about the structural properties of the most massive structures in the Universe. Substantial work is ongoing in order to understand the degree of similarity between the lensing cluster population and the population of clusters as a whole, with members of the former being likely more massive, compact, and substructured than members of the latter. In this work we exploit synthetic clusters extracted from the {\sc MareNostrum Universe} cosmological simulation in order to estimate the correlation between the strong lensing efficiency and other bulk properties of lensing clusters, such as the virial mass and the bolometric X-ray luminosity. We found that a positive correlation exist between all these quantities, with the substantial scatter being smaller for the luminosity-cross section relation. We additionally used the relation between the lensing efficiency and the virial mass in order to construct a synthetic optical depth that agrees well with the true one, while being extremely faster to be evaluated. We finally estimated what fraction of the total giant arc abundance is recovered when galaxy clusters are selected according to their dynamical activity or their X-ray luminosity. Our results show that there is a high probability for high-redshift strong lensing clusters to be substantially far away from dynamical equilibrium, and that $30-40\%$ of the total amount of giant arcs are lost if looking only at very X-ray luminous objects.Comment: 15 pages, 10 figures. Accepted by A&

The Shape-Alignment relation in Λ\LambdaCDM Cosmic Structures

In this paper we study the supercluster - cluster morphological properties using one of the largest ($2\times 512^{3}$ SPH+N-body simulations of large scale structure formation in a $\Lambda$CDM model, based on the publicly available code GADGET. We find that filamentary (prolate-like) shapes are the dominant supercluster and cluster dark matter halo morphological feature, in agreement with previous studies. However, the baryonic gas component of the clusters is predominantly spherical. We investigate the alignment between cluster halos (using either their DM or baryonic components) and their parent supercluster major-axis orientation, finding that clusters show such a preferential alignment. Combining the shape and the alignment statistics, we also find that the amplitude of supercluster - cluster alignment increases although weakly with supercluster filamentariness.Comment: Accepted for puplication in MNRAS, 10 pages, 15 figure