Effects Of Cosmic Variance Research Articles

Constraints on Bars in the Local Universe from 5000 SDSS Galaxies

AbstractWe present the first study of bars in the local Universe, based on the Sloan Digitized Sky Survey (SDSS). The large sample of ~5000 local galaxies provides the largest study to date of local bars and minimizes the effect of cosmic variance. The sample galaxies have Mg ≤ −18.5 mag and cover the redshift range 0.01 ≤ z < 0.04. We use a color cut in the color-magnitude diagram and the Sérsic index n to identify disk galaxies. We characterize bars and disks using r-band images and the method of iterative ellipse fits and quantitative criteria developed in Jogee at al. (2004, ApJL, 615, L105). After excluding highly inclined (i>60°) systems our results are: (1) the optical (r-band) fraction of barred galaxies among local disk galaxies is 43% (Figure 1, left panel), which confirms the ubiquity of local bars, in agreement with other optical studies based on smaller samples (e.g. Eskridge et al 2000, AJ, 119, 536, Marinova & Jogee 2006, astro-ph/0608039); (2) the optical bar fraction rises for bluer galaxies, suggesting a relation between bars and star formation (Figure 1, middle panel); (3) preliminary analyzes suggest that the optical bar fraction increases steeply with the galaxy effective radius (reff, Figure 1, right panel); (4) the optical bar fraction at z ~ 0 is ~35% for bright disks (Mg ≤ −19.3 mag) and strong (bar ellipticity >0.4), large-scale (bar semi-major axis >1.5 kpc) bars, which is comparable to the value of ~30 ± 6% reported earlier (Jogee et al 2004) for similar disks and bars at z ~ 0.2 − 1.0.

Morphology of mock SDSS catalogues

We measure the geometry, topology and morphology of superclusters in mock Sloan Digital Sky Survey (SDSS) catalogues prepared and reported by Cole et al. (1998). The mock catalogues refer to rCDM and ACDM flat cosmological models and are populated by galaxies so that these act as biased tracers of mass, conforming with the observed two-point correlation function (TPCF) measured using the APM catalogue on scales between 1 and 10 h -1 Mpc. We compute the Minkowski functionals (hereafter, MFs) for the cosmic density fields using SURFGEN and use the available 10 realizations of rCDM to study the effect of cosmic variance in estimation of MFs and shapefinders; the statistics derived from MFs, and used to study the sizes and shapes of the superclusters. The MFs and shapefinders are found to be extremely well constrained statistics, useful in assessing the effect of higher-order correlation functions on the clustering of the galaxy distribution. We show that though all the mock catalogues of galaxies have the same TPCF and similar clustering amplitude, the global MFs due to rCDM show systematically lower amplitude compared to those due to ACDM. This is an indirect, but detectable effect due to non-zero, higher-order correlation functions, and enables us to distinguish successfully the two models of structure formation. Next we establish that scale-dependent biasing leads to different clustering properties for galaxies as against the dark matter background. Thus, we stress the importance of incorporating a realistic treatment of bias in preparing and analysing the mock catalogues before confronting our models with the observed Universe. We measure the characteristic thickness (T), breadth (B) and length (L) of the superclusters using the 10 rCDM realizations. While T ≤ B and T, B ∈ [1, 17] h -1 Mpc, we find the top 10 superclusters to be as long as 90 h -1 Mpc, with the longest superclusters identified at percolation to be rare objects with their length as large as 150 h -1 Mpc. The longest structures of rCDM are found to be almost twice as long as those in ACDM. The dominant morphology of the large superclusters at percolation threshold is found to be filamentary.

Cosmic opacity to CMB photons and polarization measurements

Anisotropy data analysis leaves a significant degeneracy between primeval spectral index ( n s ) and cosmic opacity to CMB photons ( τ). Low- l polarization measures, in principle, can remove it. We perform a likelihood analysis to see how cosmic variance possibly affects such a problem. We find that, for a sufficiently low noise level ( σ P pix) and if τ is not negligibly low, the degeneracy is greatly reduced, while the residual impact of cosmic variance on n s and τ determinations is under control. On the contrary, if σ P pix is too high, cosmic variance effects appear to be magnified. We apply general results to specific experiments and find that, if favorable conditions occur, it is possible that a 2- σ detection of a lower limit on τ is provided by the SPOrt experiment. Furthermore, if the PLANCK experiment will measure polarization with the expected precision, the error on low- l harmonics is adequate to determine τ, without significant magnification of the cosmic variance. This however indicates that high sensitivity might be more important than high resolution in τ determinations. We also outline that a determination of τ is critical to perform detailed analyses on the nature of dark energy and/or on the presence of primeval gravitational waves.

Constraining the power spectrum using clusters

We analyze an extended redshift sample of Abell/ACO clusters and compare the results with those coming from numerical simulations of the cluster distribution, based on the truncated Zel'dovich approximation (TZA), for a list of eleven dark matter (DM) models. For each model we run several realizations, so that we generate a set of 48 independent mock Abell/ACO cluster samples per model, on which we estimate cosmic variance effects. Other than the standard CDM model, we consider (a) Ω 0 = 1 CDM models based on lowering the Hubble parameter and/or on tilting the primordial spectrum; (b) Ω 0 = 1 Cold + Hot DM models with 0.1 ≤ Ω ν ≤0.5; (c) low-density flat ΛCDM models with 0.3 ≤ Ω 0 ≤0.5. We compare real and simulated cluster distributions by analysing correlation statistics, the probability density function, and supercluster properties from percolation analysis. We introduce a generalized definition of the spectrum shape parameter Γ in terms of σ 25/ σ 8, where σ r is the rms fluctuation amplitude within a sphere of radius r. As a general result, we find that the distribution of galaxy clusters provides a constraint only on the shape of the power spectrum, but not on its amplitude: a shape parameter 0.18 ≲ Γ ≲ 0.25 and an effective spectral index at the 20 h −1 Mpc scale −1.1 ≲ n eff ≲ −0.9 are required by the Abell/ACO data. In order to obtain complementary constraints on the spectrum amplitude, we consider the cluster abundance as estimated using the Press-Schechter approach, whose reliability is explicitly tested against N-body simulations. By combining results from the analysis of the distribution and the abundance of clusters we conclude that, of the cosmological models considered here, the only viable models are either Cold + Hot DM ones with 0.2 ≲ Ω ν ≲ 0.3, better if shared between two massive ν species, and ΛCDM ones with 0.3 ≲ Ω 0≲0.5.