Large-scale Distribution Of Galaxies Research Articles

Spatial and observational homogeneities of the galaxy distribution in standard cosmologies

Context. An important aim of standard relativistic cosmology is the empirical verification of its geometrical concept of homogeneity by considering various definitions of distance and astronomical observations occurring along the past light cone. Aims. We analyze the physical consequences of distinguishing between spatial homogeneity (SH), defined by the Cosmological Principle, and observational homogeneity (OH). We argue that OH is falsifiable by means of astronomical observations, whereas SH can be verified only indirectly. Methods. We simulate observational counts of cosmological sources, such as galaxies, by means of a generalized number-distance expression that can be specialized to produce either the counts of the Einstein-de Sitter (EdS) cosmology, which has SH by construction, or other types of counts, which do, or do not, have OH by construction. Expressions for observational volumes are derived using the various cosmological-distance definitions in the EdS cosmological model. The observational volumes and simulated counts are then used to derive differential densities. We present the behavior of these densities for increasing redshift values. Results. Simulated counts that have OH by construction do not always exhibit SH features. The reverse situation is also true. In addition, simulated counts with no OH features at low redshift begin to show OH characteristics at high redshift. The comoving distance appears to be the only distance definition for which both SH and OH are applicable simultaneously, even though with limitations. Conclusions. We demonstrate that observations indicative of a possible absence of OH do not necessarily falsify the standard Friedmannian cosmology, which implies that this cosmology does not always produce observable homogeneous densities. We conclude that using different cosmological distances in the characterization of the galaxy distribution can produce significant ambiguities in reaching conclusions about the large-scale galaxy distribution in the Universe.

Dimensionality and morphology of particle and bubble clusters in turbulent flow

We conduct numerical experiments to investigate the spatial clustering of particles and bubbles in simulations of homogeneous and isotropic turbulence. On varying the Stokes parameter and the densities, striking differences in the clustering of the particles can be observed. To quantify these visual findings we use the Kaplan–Yorke dimension. This local scaling analysis shows a dimension of approximately 1.4 for the light bubble distribution, whereas the distribution of very heavy particles shows a dimension of approximately 2.6. However, clearly different parameter combinations yield the same dimensions. To overcome this degeneracy and to further develop the understanding of clustering, we perform a morphological (geometrical and topological) analysis of the particle distribution. For such an analysis, Minkowski functionals have been successfully employed in cosmology, in order to quantify the global geometry and topology of the large-scale distribution of galaxies. In the context of dispersed multiphase flow, these Minkowski functionals – being morphological order parameters – allow us to discern the filamentary structure of the light particle distribution from the wall-like distribution of heavy particles around empty interconnected tunnels. Movies are available with the online version of the paper.

The Vimos VLT deep survey

The VVDS-Wide survey has been designed to trace the large-scale distribution of galaxies at z ~ 1 on comoving scales reaching ~100~h-1 Mpc, while providing a good control of cosmic variance over areas as large as a few square degrees. This is achieved by measuring redshifts with VIMOS at the ESO VLT to a limiting magnitude IAB = 22.5, targeting four independent fields with sizes of up to 4 deg2 each. We discuss the survey strategy which covers 8.6 deg2 and present the general properties of the current redshift sample. This includes 32 734 spectra in the four regions, covering a total area of 6.1 deg2 with a sampling rate of 22 to 24%. This paper accompanies the public release of the first 18 143 redshifts of the VVDS-Wide survey from the 4 deg2 contiguous area of the F22 field at RA = 22^h. We have devised and tested an objective method to assess the quality of each spectrum, providing a compact figure-of-merit. This is particularly effective in the case of long-lasting spectroscopic surveys with varying observing conditions. Our figure of merit is a measure of the robustness of the redshift measurement and, most importantly, can be used to select galaxies with uniform high-quality spectra to carry out reliable measurements of spectral features. We also use the data available over the four independent regions to directly measure the variance in galaxy counts. We compare it with general predictions from the observed galaxy two-point correlation function at different redshifts and with that measured in mock galaxy surveys built from the Millennium simulation. The purely magnitude-limited VVDS Wide sample includes 19 977 galaxies, 304 type I AGNs, and 9913 stars. The redshift success rate is above 90% independent of magnitude. A cone diagram of the galaxy spatial distribution provides us with the current largest overview of large-scale structure up to z ~ 1, showing a rich texture of over- and under-dense regions. We give the mean N(z) distribution averaged over 6.1 deg2 for a sample limited in magnitude to IAB = 22.5. Comparing galaxy densities from the four fields shows that in a redshift bin Δz = 0.1 at z ~ 1 one still has factor-of-two variations over areas as large as ~ 0.25 deg2. This level of cosmic variance agrees with that obtained by integrating the galaxy two-point correlation function estimated from the F22 field alone. It is also in fairly good statistical agreement with that predicted by the Millennium simulations. The VVDS WIDE survey currently provides the largest area coverage among redshift surveys reaching z ~ 1. The variance estimated over the survey fields shows explicitly how clustering results from deep surveys of even 1 deg2 size should be interpreted with caution. The survey data represent a rich data base to select complete sub-samples of high-quality spectra and to study galaxy ensemble properties and galaxy clustering over unprecedented scales at these redshifts. The redshift catalog of the 4 deg2 F22 field is publicly available at http://cencosw.oamp.fr.

Towards a self-organizing Universe

In this work we show that the large scale structure of the Universe is naturally linked to the E-infinity theory and to the concept of a self-structured Universe. The key idea is that the nonlinear evolution of the Universe towards a fixed point located at 4 + ϕ3(where ϕ is the golden mean) must be reflected by the evolution of matter structures. Using data from 2dFGRS we show that the fractal dimension of the large scale galaxy distribution seems to depend on the redshift. This dependence suggests that the Universe has started to form structures about thirteen billions years ago and is evolving towards a Cantorian structure.

Cosmological parameters from strong gravitational lensing and stellar dynamics in elliptical galaxies

Context. Observations of the cosmic microwave background, light element abundances, large-scale distribution of galaxies, and distant supernovae are the primary tools for determining the cosmological parameters that define the global structure of the Universe. Aims. Here we illustrate how the combination of observations related to strong gravitational lensing and stellar dynamics in elliptical galaxies offers a simple and promising way to measure the cosmological matter and dark-energy density parameters. Methods. A gravitational lensing estimate of the mass enclosed inside the Einstein circle can be obtained by measuring the Einstein angle, once the critical density of the system is known. A model-dependent dynamical estimate of this mass can also be obtained by measuring the central velocity dispersion of the stellar component. By assuming the well-tested homologous 1/r 2 (isothermal) profile for the total (luminous+dark) density distribution in elliptical galaxies acting as lenses, these two mass measurements can be properly compared. Thus, a relation between the Einstein angle and the central stellar velocity dispersion is derived, and the cosmological matter and the dark-energy density parameters can be estimated from this. Results. We determined the accuracy of the cosmological parameter estimates by means of simulations that include realistic measurement uncertainties on the relevant quantities. Interestingly, the expected constraints on the cosmological parameter plane are complementary to those coming from other observational techniques. Then, we applied the method to the recent data sets of the Sloan Lens ACS (SLACS) and the Lenses Structure and Dynamics (LSD) Surveys, and showed that the concordance value between 0.7 and 0.8 for the dark-energy density parameter is included in our 99% confidence regions. Conclusions. The small number of lenses available to date prevents us from precisely determining the cosmological parameters, but it still proves the feasibility of the method. When applied to samples made of hundreds of lenses that are expected to become available from forthcoming deep and wide surveys, this technique will be an important alternative tool for measuring the geometry of the Universe.

The problem of cosmological dark matter and statistical physics

Although dark matter is supposed to provide with more than 0.9 of the total fraction of the mass-energy in universe, its amount and properties can only be defined a posteriori. In this context, a crucial point concerns the identification of a possible clear feature of dark matter fields which is not arbitrary, i.e. a property which has to be satisfied by dark matter fluctuations under some very general theoretical conditions. We discuss the fact that this property, in standard cosmological models, is represented by super-homogeneity, i.e. a very fine tuned balance between negative and positive correlations of density fluctuations, which must be imprinted both in the anisotropies of the CMBR and in the large scale distribution of galaxies. We review the main aspects of this property, considering examples of super-homogeneous systems well-studied in statistical physics, and discuss its possible observational evidences.

The signature of large scale structures on the very high energy gamma ray sky

If the diffuse extragalactic gamma ray emission traces the large scale structures of theuniverse, peculiar anisotropy patterns are expected in the gamma ray sky. In particular,because of the cut-off distance introduced by the absorption of 0.1–10 TeV photons on theinfrared/optical background, prominent correlations with the local structures within arange of few hundreds of Mpc should be present. We provide detailed predictionsof the signal based on the PSCz map of the local universe. We also use mockN-body catalogues complemented with the halo model of structures to study some statisticalfeatures of the expected signatures. The results are largely independent of cosmologicaldetails, and depend mostly on the index of correlation (or bias) of the sources withrespect to the large scale distribution of galaxies. For instance, the predictedsignal in the case of a quadratic correlation (as may happen for a dark matterannihilation contribution to the diffuse gamma flux) differs substantially from a linearcorrelation case, providing a complementary tool for unveiling the nature of thesources of the diffuse gamma ray emission. The chances for the present and futurespace and ground based observatories to measure these features are discussed.

Three-dimensional mapping of dark matter reveals the expected filamentary scaffold

Model simulations of the large-scale distribution of galaxies have long suggested that galaxies form on a filamentary network of dark matter. Now gravitational lensing has yielded a look at that network.

Differential Density Statistics of the Galaxy Distribution and the Luminosity Function

This paper uses data obtained from the galaxy luminosity function (LF) to calculate two types of radial number density statistics of the galaxy distribution as discussed in Ribeiro, namely, the differential density γ and the integral differential density γ*. By applying the theory advanced by Ribeiro & Stoeger, which connects the relativistic cosmology number counts with the astronomically derived LF, the differential number counts dN/dz are extracted from the LF and used to calculate both γ and γ* with various cosmological distance definitions, namely, area distance, luminosity distance, galaxy area distance, and redshift distance. LF data are taken from the CNOC2 galaxy redshift survey, and γ and γ* are calculated for two cosmological models: Einstein-de Sitter and an Ω = 0.3, Ω = 0.7 standard cosmology. The results confirm the strong dependency of both statistics on the distance definition, as predicted in Ribeiro, as well as showing that plots of γ and γ* against the luminosity and redshift distances indicate that the CNOC2 galaxy distribution follows a power-law pattern for redshifts higher than 0.1. These findings support Ribeiro's theoretical proposition that using different cosmological distance measures in statistical analyses of galaxy surveys can lead to significant ambiguity in drawing conclusions about the behavior of the observed large-scale distribution of galaxies.

Spectroscopic studies of $z \sim5.7$ and $z\sim6.5$ galaxies: implications for reionisation

The recent development of large, complete samples which identify high-redshift galaxies at z~5.7 and z~6.5 from deep, wide-field surveys provides detailed information on the earliest galaxies, their numbers, spatial and kinematic distributions, and implications for early reionization of the IGM. In this contribution we present results of spectroscopic studies of z~5.7 and z~6.5 galaxies identified from our deep, Lyman alpha narrowband and multicolor surveys conducted with the SuprimeCam mosaic CCD camera on the 8.3-m Subaru telescope and observed with the DEIMOS multi-object spectrograph on Keck. The luminosity function of the z~6.5 galaxies is shown to be similar to the luminosity function of the z~5.7 galaxy samples, suggesting that a substantial star-forming population is already in place at z~6.5. Comparisons of both individual and stacked spectra of galaxies in these two samples show that the Lyman alpha emission profiles, equivalent widths, and continuum break strengths do not substantially change over this redshift interval. The wide-field nature of the surveys also permits mapping the large-scale distribution of the high-redshift galaxies in spatial structures extending across individual SuprimeCam fields (~60 Mpc). Field-to-field variations in the number of objects at z~6.5 may shortly be able to place constraints on the porosity of the reionization boundary.

Scaling laws in the distribution of galaxies

Past surveys have revealed that the large-scale distribution of galaxies in the universe is far from random: it is highly structured over a vast range of scales. Surveys being currently undertaken and being planned for the next decades will provide a wealth of information about this structure. The ultimate goal must be not only to describe galaxy clustering as it is now, but also to explain how this arose as a consequence of evolutionary processes acting on the initial conditions that we see in the cosmic microwave background anisotropy data. In order to achieve this we need to build mathematically quantifiable descriptions of cosmic structure. Identifying where scaling laws apply and the nature of those scaling laws is an important part of understanding which physical mechanisms have been responsible for the organization of clusters of galaxies, superclusters, and the voids between them. Finding where these scaling laws are broken is equally important since this indicates the transition to different underlying physics. In describing scaling laws it is helpful to make analogies with fractals, mathematical constructs that can possess a wide variety of scaling properties. We must beware, however, of saying that the universe is a fractal on some range of scales: it merely exhibits a specific kind of fractal-like behavior on those scales. The richness of fractal scaling behavior is an important supplement to the usual battery of statistical descriptors. This article reviews the history of how we have learned about the structure of the universe and presents the data and methodologies that are relevant to an understanding of any scaling properties that structure may have. The ultimate goal is to have a complete understanding of how that structure emerged. We are getting close!

Simulations of Large Scale Structure Formation: The Connection to Smaller Scales

Maps of the cosmos, in particular maps of the cosmic microwave background and of the large scale distribution of galaxies have been crucial ingredients in the development of the standard model of structure formation, sometimes also labeled “concordance model”. This model has proven to be remarkably successful in explaining an impressive array of observations on scales of hundreds of kpc to thousands of Mpc. In this contribution I will attempt to extend those studies to smaller, (sub)galactic scales and will confront detailed gas-dynamical simulations of the formation of individual galaxies with observational data on these scales, reporting some successes and failures of this endeavor. Ongoing surveys that are mapping the distribution of stars in the Milky Way should be able to clearly identify the imprints of the hierarchical galaxy formation process providing an independent check of the validity of the structure formation paradigm.

Shapley and Hubble: Different Views Brought Galaxies Into Focus

Shapley and Hubble: Different Views Brought Galaxies Into Focus

Mapping the cosmic web with the Sloan Digital Sky Survey

Wide-angle, moderately deep redshift surveys such as that conducted as part of the Sloan Digital Sky Survey (SDSS) allow study of the relationship between the structural elements of the large-scale distribution of galaxies – including groups, cluster, superclusters, and voids – and the dependence of galaxy formation and evolution on these enviroments. We present a progress report on mapping efforts with the SDSS and discuss recently constructed catalogs of clusters, voids, and void galaxies, and evidence for a Mpc supercluster or “Great Wall.” Analysis of multi-band photometry and moderate-resolution spectroscopy from the SDSS reveals environmental dependence of the star formation history of galaxies that extends over more than a factor of 100 in density, from clusters all the way to the deep interiors of voids. On average, galaxies in the rarified environments of voids exhibit bluer colors, higher specific star formation rates, lower dust content, and more disk-like morphology than objects in denser regions. This trend persists in comparisons of samples in low vs. high-density regions with similar luminosity and morphology, thus this dependence is not simply an extension of the morphology-density relation. Large-scale modulation of the halo mass function and the temperature of the intergalactic medium might explain this dependence of galaxy evolution on the large-scale environment.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Cosmic Microwave Background observations and cosmology

Observations of the Cosmic Microwave Background (CMB) play a pivotal role in the development of modern cosmology. Most of the predictions of the Hot Big Bang model are now experimentally confirmed, and the parameters of the model have recently been constrained to good accuracy. The so-called Concordance cosmology, derived from observations of the CMB spectrum and anisotropy, from measurements of the large-scale distribution of galaxies, of the expansion of the Universe and of the abundance of light elements, requires significant contributions from dark matter (Ω m ∼ 0.3) and dark energy (Ω Λ ∼ 0.65). Also, an initial inflation phase is required for the model to be self consistent. Here we focus on CMB anisotropy and CMB polarization observations and on their role in constraining the model.

Cosmology at low redshifts

It is argued that it is far more cost effective to carry out some projects with medium-sized dedicated zenith telescopes rather than large steerable telescopes, freeing the later to carry out projects that truly need them. I show that the large number of objects observed with a surveying 4-m zenith telescope allows one to carry out cosmological projects at low redshifts. Examining two case studies, I show first that a variability survey would obtain light curves for several thousands of type Ia supernovae per year up to z = 1 and easily discriminate among competing cosmological models. Finally, I discuss a second case study, consisting of a spectrophotometric survey carried out with interference filters, showing its power to discriminate among cosmological models and to study the large-scale distribution of galaxies in the Universe.

The use of luminosity effects to calculate scaling regimes in the galaxy distribution

This work is a continuation of a previous publication Thieberger and Spiegel concerning the use of two dimensional catalogues in unraveling the large scale distribution of galaxies. Since the data in two-dimensional catalogues are more plentiful than those in three dimensions, the luminosity method can be helpful in using two dimensional data to unravel the nature of the three dimensional distribution. In this work we analyse the Cfa catalogue and demonstrate that the results indicate a distribution of two dimensions in the nearer range and three dimensions in the farther range. The possibility of a different regime for very short distances is discussed.

Parkes H I observations of galaxies behind the southern Milky Way

As part of our program to map the large-scale distribution of galaxies behind the Milky Way, we used the Parkes 210 ft (64 m) radio telescope for pointed HI observations of a sample of low surface-brightness (due to heavy obscuration) spiral galaxies selected from the deep optical Zone of Avoidance (ZOA) galaxy catalog in the Hydra/Antlia region (Kraan-Korteweg 2000a). Searching a simultaneous velocity range of either 300 to 5500 km/s or 300 to 10500 km/s to an rms level of typically 2 - 4 mJy resulted in detections in 61 of the 139 pointings, leading to a total of 66 detections (an additional detection was made in a reference position, and two other pointings revealed two and four independent signals respectively). Except for 2 strong HI emitters identified in the shallow Zone of Avoidance HI survey (Henning et al. 2000), all HI detections are new. An analysis of the properties of the observed and detected galaxies prove that pointed HI observations of highly obscured galaxies allow the tracing of a population of nearby, intrinsically large and bright spiral galaxies that otherwise would not be recovered. The new data identified a previously unrecognized nearby group at l=287.5, b=-9.5, V=1700 km/s, the continuation of the Hydra/Antlia filament on the opposite side of the Galactic plane, and helped to delimit a distinct void in the ZOA centered at 2000 km/s.

Cosmological constraints from the X-ray gas mass fraction in relaxed lensing clusters observed with Chandra

We present precise measurements of the X-ray gas mass fraction for a sample of luminous, relatively relaxed clusters of galaxies observed with the Chandra observatory, for which independent confirmation of the mass results is available from gravitational lensing studies. Parametrizing the total (luminous plus dark matter) mass profiles using the model of Navarro, Frenk & White, we show that the X-ray gas mass fractions in the clusters asymptote towards an approximately constant value at a radius r2500, where the mean interior density is 2500 times the critical density of the Universe at the redshifts of the clusters. Combining the Chandra results on the X-ray gas mass fraction and its apparent redshift dependence with recent measurements of the mean baryonic matter density in the Universe and the Hubble constant determined from the Hubble Key Project, we obtain a tight constraint on the mean total matter density of the Universe, , and measure a positive cosmological constant, . Our results are in good agreement with recent, independent findings based on analyses of anisotropies in the cosmic microwave background radiation, the properties of distant supernovae, and the large-scale distribution of galaxies.

Large‐Scale Structures in the Early Sloan Digital Sky Survey: Comparison of the North and South Galactic Strips

We compare the large-scale galaxy clustering between the north and south Sloan Digital Sky Survey (SDSS) early data release (EDR) and also with the clustering in the Automatic Plate Measuring Facility (APM) Galaxy Survey. The three samples are independent and cover an area of 150, 230, and 4300 deg2, respectively. We combine SDSS data in different ways to approach the APM selection. Given the good photometric calibration of the SDSS data and the very good match of its north and south number counts, we combine them in a single sample. The joint clustering is compared with equivalent subsamples in the APM. The final sampling errors are small enough to provide an independent test for some of the results in the APM. We find evidence for an inflection in the shape of the two-point function in the SDSS that is very similar to what is found in the APM. This feature has been interpreted as evidence for nonlinear gravitational growth. By studying higher-order correlations, we can also confirm good agreement with the hypothesis of Gaussian initial conditions (and small biasing) for the structure traced by the large-scale SDSS galaxy distribution.