Clustering of galaxies at high redshifts

Recent observations show a large concentration of galaxies at high redshift. At first sight, strong clustering of galaxies at high redshifts seems to be in contradiction with the models of structure formation. In this paper we show that such structures are a manifestation of the strong clustering of rare peaks in the density field. We compute the frequency of occurrence of such large concentrations of galaxies in some models of structure formation.     

Comparison of the SFI peculiar velocities with the IRAS 1.2-Jy gravity field

We present a comparison between the peculiar velocity fields measured from a recently completed l -band Tully–Fisher survey of field spirals (SFI) and that derived from the IRAS 1.2-Jy redshift survey galaxy distribution. The analysis is based on the expansion of these data in redshift space using smooth orthonormal functions, and is performed using low- and high-resolution expansions, with an effective smoothing scale which increases almost linearly with redshift. The effective smoothing scales at 3000 km s⁻¹ are 1500 and 1000 km s⁻¹ for the low- and high-resolution filters. The agreement between the high- and low-resolution SFI velocity maps is excellent. The general features in the filtered SFI and IRAS velocity fields agree remarkably well within 6000 km s⁻¹. This good agreement between the fields allows us to determine the parameter β = Ω^0.6 / b , where Ω is the cosmological density parameter, and b is the linear biasing factor. From a likelihood analysis on the SFI and IRAS modes we find that β = 0.6 ± 0.1, independently of the resolution of the modal expansion. For this value of β, the residual fields for the two filters show no systematic variations within 6000 km s⁻¹. Most remarkable is the lack of any coherent, redshift-dependent dipole flow in the residual field.     

Influence of cluster environments on X-ray cluster morphology

We investigated the influence of environment on cluster morphology by examining the alignment of the cluster position angle with respect to the nearest neighbour cluster. The cluster position angle and ellipticity were measured using high spatial resolution X-ray data taken from the Chandra archive, while the nearest neighbour sample was extracted from the Abell cluster catalogue. We found high confidence for an alignment when neighbour distance (Dn) was less than  70  h ⁻¹  Mpc, while we found no significant variations of the confidence level at various values of Dn less than  70  h ⁻¹  Mpc.     

The effect of reionization on the cosmic microwave background–density correlation

In this paper, we show how the rescattering of cosmic microwave background photons after cosmic reionization can give a significant linear contribution to the temperature–matter cross-correlation measurements. These anisotropies, which arise via a late-time Doppler effect, are on scales much larger than the typical scale of non-linear effects at reionization; they can contribute to degree scale cross-correlations and could affect the interpretation of similar correlations resulting from the integrated Sachs–Wolfe effect. While expected to be small at low redshifts, these correlations can be large given a probe of the density at high redshift, and so could be a useful probe of the cosmic reionization history.     

Virialization of cosmological structures in models with time-varying equation of state

We study the virialization of the cosmic structures in the framework of flat cosmological models where the dark energy component plays an important role in the global dynamics of the Universe. In particular, our analysis focuses on the study of the spherical matter perturbations, as the latter decouple from the background expansion, start to 'turn around' and finally collapse. We generalize this procedure, taking into account models with an equation of state which vary with time, and provide a complete formulation of the cluster virialization attempting to address the non-linear regime of structure formation. In particular, assuming that clusters have collapsed prior to the epoch of z _f≃ 1.4, in which the most distant cluster has been found, we show that the behaviour of the spherical collapse model depends on the functional form of the equation of state.     

Statistical analysis of galaxy surveys – III. The non-linear clustering of red and blue galaxies in the 2dFGRS

We present measurements of the higher order clustering of red and blue galaxies as a function of scale and luminosity made from the two-degree field galaxy redshift survey (2dFGRS). We use a counts-in-cells analysis to estimate the volume-averaged correlation functions,     , as a function of scale up to the order of   p = 5  , and also the reduced void probability function. Hierarchical amplitudes are constructed using the estimates of the correlation functions:     . We find that (i) red galaxies display stronger clustering than blue galaxies at all orders measured; (ii) red galaxies show values of   S _p   that are strongly dependent on luminosity whereas blue galaxies show no segregation in   S _p   within the errors; this is remarkable given the segregation in the variance; (iii) the linear relative bias shows the opposite trend to the hierarchical amplitudes, with little segregation for the red sequence and some segregation for the blue; (iv) faint red galaxies deviate significantly from the 'universal' negative binomial reduced void probabilities followed by all other galaxy populations. Our results show that the characteristic colour of a galaxy population reveals a unique signature in its spatial distribution. Such signatures will hopefully further elucidate the physics responsible for shaping the cosmological evolution of galaxies.