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The smallest dark matter halos are formed first in the early universe. According to recent studies, the central density cusp is much steeper in these halos than in larger halos and scales as ρ ∝ r−(1.5−1.3). We present results of very large cosmological N-body simulations of the hierarchical formation and evolution of halos over a wide mass range, beginning from the formation of the smallest halos. We confirmed early studies that the inner density cusps are steeper in halos at the free streaming scale. The cusp slope gradually becomes shallower as the halo mass increases. The slope of halos 50 times more massive than the smallest halo is approximately -1.3. The concentration parameter is nearly independent of halo mass, and ruling out simple power law mass-concentration relations. The steeper inner cusps of halos near the free streaming scale enhance the annihilation luminosity of a Milky Way sized halo between 12 to 67%. [ABSTRACT FROM PUBLISHER]

Understanding the internal structure and spatial distribution of cosmic voids is crucial when considering them as probes of cosmology. We present recent advances in modeling void density- and velocity-profiles in real space, as well as void two-point statistics in redshift space, by examining voids identified via the watershed transform in state-of-the-art ΛCDM n-body simulations and mock galaxy catalogs. The simple and universal characteristics that emerge from these statistics indicate the self-similarity of large-scale structure and suggest cosmic voids to be among the most pristine objects to consider for future studies on the nature of dark energy, dark matter and modified gravity. [ABSTRACT FROM PUBLISHER]

Cosmic voids are becoming key players in testing the physics of our Universe. Here we concentrate on the abundances and the dynamics of voids as these are among the best candidates to provide information on cosmological parameters. Cai, Padilla & Li (2014) use the abundance of voids to tell apart Hu & Sawicki f(R) models from General Relativity. An interesting result is that even though, as expected, voids in the dark matter field are emptier in f(R) gravity due to the fifth force expelling away from the void centres, this result is reversed when haloes are used to find voids. The abundance of voids in this case becomes even lower in f(R) compared to GR for large voids. Still, the differences are significant and this provides a way to tell apart these models. The velocity field differences between f(R) and GR, on the other hand, are the same for halo voids and for dark matter voids. Paz et al. (2013), concentrate on the velocity profiles around voids. First they show the necessity of four parameters to describe the density profiles around voids given two distinct void populations, voids-in-voids and voids-in-clouds. This profile is used to predict peculiar velocities around voids, and the combination of the latter with void density profiles allows the construction of model void-galaxy cross-correlation functions with redshift space distortions. When these models are tuned to fit the measured correlation functions for voids and galaxies in the Sloan Digital Sky Survey, small voids are found to be of the void-in-cloud type, whereas larger ones are consistent with being void-in-void. This is a novel result that is obtained directly from redshift space data around voids. These profiles can be used to remove systematics on void-galaxy Alcock-Pacinsky tests coming from redshift-space distortions. [ABSTRACT FROM PUBLISHER]

We discuss various applications of vide, the Void IDentification and Examination toolkit, an open-source Python/C++ code for finding cosmic voids in galaxy redshift surveys and $N$-body simulations. Based on a substantially enhanced version of ZOBOV, vide not only finds voids, but also summarizes their properties, extracts statistical information, and provides a Python-based platform for more detailed analysis, such as manipulating void catalogs and particle members, filtering, plotting, computing clustering statistics, stacking, comparing catalogs, and fitting density profiles. vide also provides significant additional functionality for pre-processing inputs: for example, vide can work with volume- or magnitude-limited galaxy samples with arbitrary survey geometries, or dark matter particles or halo catalogs in a variety of common formats. It can also randomly subsample inputs and includes a Halo Occupation Distribution model for constructing mock galaxy populations. vide has been used for a wide variety of applications, from discovering a universal density profile to estimating primordial magnetic fields, and is publicly available at http://bitbucket.org/cosmicvoids/vide\_public and http://www.cosmicvoids.net. [ABSTRACT FROM PUBLISHER]

Comparison of galaxy flows with those predicted from the local galaxy distribution ended as an active field after two analyses came to vastly different conclusions 25 years ago, but that was due to faulty data. All the old results are therefore suspect. With new data collected in the last several years, the problem deserves another look. The goal is to explain the 640 km/s dipole anisotropy of the CMBR. For this we analyze the gravity field inferred from the enormous data set derived from the 2MASS collection of galaxies (Huchra et al. 2005), and compare it to the velocity field derived from the well calibrated SFI++ Tully-Fisher catalog (Springob et al. 2007). Using the “Inverse Method” to minimize Malmquist biases, within 10,000 km/s the gravity field is seen to predict the velocity field (Davis et al. 2011) to remarkable consistency. This is a beautiful demonstration of linear perturbation theory and is fully consistent with standard values of the cosmological variables. [ABSTRACT FROM PUBLISHER]

We examine the infall pattern of subhaloes onto hosts in the context of the large-scale structure. We find that the infall pattern is essentially driven by the shear tensor of the ambient velocity field. Dark matter subhaloes are preferentially accreted along the principal axis of the shear tensor which corresponds to the direction of weakest collapse. We examine the dependence of this preferential infall on subhalo mass, host halo mass and redshift. Although strongest for the most massive hosts and the most massive subhaloes at high redshift, the preferential infall of subhaloes is effectively universal in the sense that its always aligned with the axis of weakest collapse of the velocity shear tensor. It is the same shear tensor that dictates the structure of the cosmic web and hence the shear field emerges as the key factor that governs the local anisotropic pattern of structure formation. Since the small (sub-Mpc) scale is strongly correlated with the mid-range (∼ 10 Mpc) scale - a scale accessible by current surveys of peculiar velocities - it follows that findings presented here open a new window into the relation between the observed large scale structure unveiled by current surveys of peculiar velocities and the preferential infall direction of the Local Group. This may shed light on the unexpected alignments of dwarf galaxies seen in the Local Group. [ABSTRACT FROM PUBLISHER]

Coherent alignments of galaxy shapes, often called“intrinsic alignments” (IA), are the most significant source of astrophysical uncertainty in weak lensing measurements. We develop the tidal alignment model of IA and demonstrate its success in describing observational data. We also describe a technique to separate IA from galaxy-galaxy lensing measurements. Applying this technique to luminous red galaxy lenses in the Sloan Digital Sky Survey, we constrain potential IA contamination from associated sources to be below a few percent. [ABSTRACT FROM PUBLISHER]

Galaxies are not randomly distributed across the universe but showing different kinds of alignment on different scales. On small scales satellite galaxies have a tendency to distribute along the major axis of the central galaxy, with dependence on galaxy properties that both red satellites and centrals have stronger alignment than their blue counterparts. On large scales, it is found that the major axes of Luminous Red Galaxies (LRGs) have correlation up to 30Mpc/h. Using hydro-dynamical simulation with star formation, we investigate the origin of galaxy alignment on different scales. It is found that most red satellite galaxies stay in the inner region of dark matter halo inside which the shape of central galaxy is well aligned with the dark matter distribution. Red centrals have stronger alignment than blue ones as they live in massive haloes and the central galaxy-halo alignment increases with halo mass. On large scales, the alignment of LRGs is also from the galaxy-halo shape correlation, but with some extent of mis-alignment. The massive haloes have stronger alignment than haloes in filament which connect massive haloes. This is contrary to the naive expectation that cosmic filament is the cause of halo alignment. [ABSTRACT FROM PUBLISHER]

Tidal torque theory and simulations of large scale structure predict spin vectors of massive galaxies should be coplanar with sheets in the cosmic web. Recently demonstrated, the giants (Ks ⩽ -22.5 mag) in the Local Volume beyond the Local Sheet have spin vectors directed close to the plane of the Local Supercluster, supporting the predictions of Tidal Torque Theory. However, the giants in the Local Sheet encircling the Local Group display a distinctly different arrangement, suggesting that the mass asymmetry of the Local Group or its progenitor torqued them from their primordial spin directions. To investigate the origin of the spin alignment of giants locally, analogues of the Local Sheet were identified in the SDSS DR9. Similar to the Local Sheet, analogues have an interacting pair of disk galaxies isolated from the remaining sheet members. Modified sheets in which there is no interacting pair of disk galaxies were identified as a control sample.Galaxies in face-on control sheets do not display axis ratios predominantly weighted toward low values, contrary to the expectation of tidal torque theory. For face-on and edge-on sheets, the distribution of axis ratios for galaxies in analogues is distinct from that in controls with a confidence of 97.6% & 96.9%, respectively. This corroborates the hypothesis that an interacting pair can affect spin directions of neighbouring galaxies. [ABSTRACT FROM PUBLISHER]

Intrinsic alignments are believed to be a major source of systematics for future generation of weak gravitational lensing surveys like Euclid or LSST. Direct measurements of the alignment of the projected light distribution of galaxies in wide field imaging data seem to agree on a contamination at a level of a few per cent of the shear correlation functions, although the amplitude of the effect depends on the population of galaxies considered. Given this dependency, it is difficult to use dark matter-only simulations as the sole resource to predict and control intrinsic alignments. We report here estimates on the level of intrinsic alignment in the cosmological hydrodynamical simulation Horizon-AGN that could be a major source of systematic errors in weak gravitational lensing measurements. In particular, assuming that the spin of galaxies is a good proxy for their ellipticity, we show how those spins are spatially correlated and how they couple to the tidal field in which they are embedded. We will also present theoretical calculations that illustrate and qualitatively explain the observed signals. [ABSTRACT FROM PUBLISHER]

Using the Horizon-AGN simulation we find a mass dependent spin orientation trend for galaxies: the spin of low-mass, rotation-dominated, blue, star-forming galaxies are preferentially aligned with their closest filament, whereas high-mass, velocity dispersion- supported, red quiescent galaxies tend to possess a spin perpendicular to these filaments. We explore the physical mechanisms driving galactic spin swings and quantify how much mergers and smooth accretion re-orient them relative to their host filaments. [ABSTRACT FROM PUBLISHER]

Filaments of the cosmic web drive spin acquisition of disc galaxies. The point process of filament-type saddle represent best this environment and can be used to revisit the Tidal Torque Theory in the context of an anisotropic peak (saddle) background split. The constrained misalignment between the tidal tensor and the Hessian of the density field generated in the vicinity of filament saddle points simply explains the corresponding transverse and longitudinal point-reflection symmetric geometry of spin distribution. It predicts in particular an azimuthal orientation of the spins of more massive galaxies and spin alignment with the filament for less massive galaxies. Its scale dependence also allows us to relate the transition mass corresponding to the alignment of dark matter halos' spin relative to the direction of their neighboring filament to this geometry, and to predict accordingly it's scaling with the mass of non linearity, as was measured in simulations. [ABSTRACT FROM PUBLISHER]

We take a closer look at the dependence of the galactic colour histogram on the environment density using a volume-limited sample of SDSS galaxies. We find that the strongest changes with environment are taking place with spiral galaxies. In dense environment, discs become considerably redder, apparently due to the shortage of gas, and less concentrated. Contrary to expectation, the mean Sérsic index of luminous elliptical galaxies decreases in denser environments. [ABSTRACT FROM PUBLISHER]

The properties of galaxies depend on their environment: red, passive elliptical galaxies are usually located in denser environments than blue, star-forming spiral galaxies. This difference in galaxy populations can be detected at all scales from groups of galaxies to superclusters. In this paper, we will discuss the effect of the large-scale environment on galaxies. Our results suggest that galaxies in superclusters are more likely to be passive than galaxies in voids even when they belong to groups with the same richness. In addition, the galaxies in superclusters are also affected by the morphology of the supercluster: filament-type superclusters contain relatively more red, passive galaxies than spider-type superclusters. These results suggest that the evolution of a galaxy is not determined by its local environment alone, but the large-scale environment also affects. [ABSTRACT FROM PUBLISHER]

We study the role of environment in the evolution of galaxies up to z = 0.7 using the final zCOSMOS-bright data set. We use the colour as a proxy for the quenched population, and measure the dependence of the red fraction of galaxies on stellar mass and two environmental indicators: the local overdensity of galaxies δ and a demarcation of galaxies to centrals and satellites. The analysis is carried out by quantifying the role of different quenching processes. We find that the measured dependence of the red fraction of galaxies on stellar mass and environment can be well described by two quenching processes: one related only to stellar mass (mass quenching) and the other related to the local environment (environment quenching). Within the errors, these processes are independent of each other, and consistent with the z ~ 0 measurement. Moreover, the red fraction of centrals fr,cen (both singleton centrals and centrals in the groups) does not show any trend with δ and more than 95% of fr,cen is consistent with being produced through the mass quenching alone. The satellite galaxies are redder than the centrals at the same stellar mass and δ, requiring additional environment quenching. Given the observed fractional distribution of satellites at different overdensities, the normalized excess in the red fraction of satellites with respect to the red fraction of centrals is consistent with a scenario in which the satellites account for most of the δ-dependences observed in the overall population of galaxies covering 0.1 < z < 0.7. [ABSTRACT FROM PUBLISHER]

Cosmological hydrodynamical simulations show that the baryonic elements that at z = 0 form the stellar populations of late-type galaxies (LTGs), display, at high z, a gaseous web-like organization, where different singular structures (walls, filaments, nodes) show up. The analysis also shows that the spheroid-to-be elements are the first to be involved in the singular structures, while thick and thin disk-to-be elements chronologically follow them. We discuss how these differences at high z can explain the differences among these three components at z = 0. [ABSTRACT FROM PUBLISHER]

We study the evolution of the gaseous components in massive simulated galaxies and show that their early formation is fuelled by cold, low entropy gas streams. At lower redshifts of z ≲ 3 the simulated galaxies are massive enough to support stable virial shocks resulting in a transition from cold to hot gas accretion. The gas accretion history of early-type galaxies is directly linked to the formation of their stellar component in the two phased formation scenario, in which the central parts of the galaxy assemble rapidly through in situ star formation and the later assembly is dominated primarily by minor stellar mergers. [ABSTRACT FROM PUBLISHER]

Using redshifts from the 2M++ redshift compilation, we reconstruct the density of galaxies within 200 h−1 Mpc, and compare the predicted peculiar velocities Tully-Fisher and SNe peculiar velocities. The comparison yields a best-fit value of β ≡ Ωm0.55/b* = 0.431 ± 0.021, suggesting Ωm0.55σ8,lin = 0.401 ± 0.024, in good agreement with other probes. The predicted peculiar velocity of the Local Group from sources within the 2M++ volume is 540 ± 40 km s−1, towards l = 268° ± 4°, b = 38° ± 6°, which is misaligned by only 10° with the Cosmic Microwave Background dipole. To account for sources outside the 2M++ volume, we fit simultaneously for β* and an external bulk flow in our analysis. The external bulk flow has a velocity of 159 ± 23 km s−1 towards l = 304° ± 11°, b6° ± 13°. [ABSTRACT FROM PUBLISHER]

Numerical simulations predict that gas accretion from the cosmic web drives star formation in disks galaxies. The process is important in low mass haloes (< 1012 M⊙), therefore, in the early universe when galaxies were low mass, but also in dwarf galaxies of the local universe. The gas that falls in is predicted to be tenuous, patchy, partly ionized, multi-temperature, and large-scale; therefore, hard to show in a single observation. One of the most compelling cases for gas accretion at work in the local universe comes from the extremely metal poor (XMP) galaxies. They show metallicity inhomogeneities associated with star-forming regions, so that large starbursts have lower metallicity than the underlying galaxy. Here we put forward the case for gas accretion from the web posed by XMP galaxies. Two other observational results are discussed too, namely, the fact that the gas consumption time-scale is shorter than most stellar ages, and the systematic morphological distortions of the HI around galaxies. [ABSTRACT FROM PUBLISHER]

The star formation history of galaxies is a complex process usually considered to be stochastic in nature, for which we can only give average descriptions such as the color-density relation. In this work we follow star-forming gas particles in a hydrodynamical N-body simulation back in time in order to study their initial spatial configuration. By keeping record of the time when a gas particle started forming stars we can produce Lagrangian gas-star isochrone surfaces delineating the surfaces of accreting gas that begin producing stars at different times. These surfaces form a complex a network of filaments in Eulerian space from which galaxies accrete cold gas. Lagrangian accretion surfaces are closely packed inside dense regions, intersecting each other, and as a result galaxies inside proto-clusters stop accreting gas early, naturally explaining the color dependence on density. The process described here has a purely gravitational / geometrical origin, arguably operating at a more fundamental level than complex processes such as AGN and supernovae, and providing a conceptual origin for the color-density relation. [ABSTRACT FROM PUBLISHER]

We have developed a new method to approach the missing baryons problem. We assume that the missing baryons reside in a form of Warm Hot Intergalactic Medium, i.e. the WHIM. Our method consists of (a) detecting the coherent large scale structure in the spatial distribution of galaxies that traces the Cosmic Web and that in hydrodynamical simulations is associated to the WHIM, (b) mapping its luminosity into a galaxy luminosity density field, (c) using numerical simulations to relate the luminosity density to the density of the WHIM, (d) applying this relation to real data to trace the WHIM using the observed galaxy luminosities in the Sloan Digital Sky Survey and 2dF redshift surveys. In our application we find evidence for the WHIM along the line of sight to the Sculptor Wall, at redshifts consistent with the recently reported X-ray absorption line detections. Our indirect WHIM detection technique complements the standard method based on the detection of characteristic X-ray absorption lines, showing that the galaxy luminosity density is a reliable signpost for the WHIM. For this reason, our method could be applied to current galaxy surveys to optimise the observational strategies for detecting and studying the WHIM and its properties. Our estimates of the WHIM hydrogen column density NH in Sculptor agree with those obtained via the X-ray analysis. Due to the additional NH estimate, our method has potential for improving the constrains of the physical parameters of the WHIM as derived with X-ray absorption, and thus for improving the understanding of the missing baryons problem. [ABSTRACT FROM PUBLISHER]

We present coupled radiation hydrodynamical simulations of the epoch of reionization, aimed at probing self-feedback on galactic scales. Unlike previous works, which assume a (quasi) homogeneous UV background, we self-consistently evolve both the radiation field and the gas to model the impact of previously unresolved processes such as spectral hardening and self-shielding. We find that the characteristic halo mass with a gas fraction half the cosmic mean, Mc(z), a quantity frequently used in semi-analytical models of galaxy formation, is significantly larger than previously assumed. While this results in an increased suppression of star formation in the early Universe, our results are consistent with the extrapolated stellar abundance matching models from Moster et al. 2013. [ABSTRACT FROM PUBLISHER]

The intergalactic medium (IGM) accounts for ≳ 90% of baryons at all epochs and yet its three dimensional distribution in the cosmic web remains mostly unknown. This is so because the only feasible way to observe the bulk of the IGM is through intervening absorption line systems in the spectra of bright background sources, which limits its characterization to being one-dimensional. Still, an averaged three dimensional picture can be obtained by combining and cross-matching multiple one-dimensional IGM information with three-dimensional galaxy surveys. Here, we present our recent and current efforts to map and characterize the IGM in the cosmic web using galaxies as tracers of the underlying mass distribution. In particular, we summarize our results on: (i) IGM around star-forming and non-star-forming galaxies; (ii) IGM within and around galaxy voids; and (iii) IGM in intercluster filaments. With these datasets, we can directly test the modern paradigm of structure formation and evolution of baryonic matter in the Universe. [ABSTRACT FROM PUBLISHER]

The accurate description of the properties of the Lyman-α forest is a spectacular success of the Cold Dark Matter theory of cosmological structure formation. After a brief review of early models, it is shown how numerical simulations have demonstrated the Lyman-α forest emerges from the cosmic web in the quasi-linear regime of overdensity. The quasi-linear nature of the structures allows accurate modeling, providing constraints on cosmological models over a unique range of scales and enabling the Lyman-α forest to serve as a bridge to the more complex problem of galaxy formation. [ABSTRACT FROM PUBLISHER]

The hydrogen Lyα forest is an important probe of the z > 2 Universe that is otherwise challenging to observe with galaxy redshift surveys, but this technique has traditionally been limited to 1D studies in front of bright quasars. However, by pushing to faint magnitudes (g > 23) with 8-10m large telescopes it becomes possible to exploit the high area density of high-redshift star-forming galaxies to create 3D tomographic maps of large-scale structure in the foreground. I describe the first pilot observations using this technique, as well discuss future surveys and the resulting science possibilities for galaxy evolution and cosmology. [ABSTRACT FROM PUBLISHER]

While recent years have seen rapid growth in the number of galaxy peculiar velocity measurements, disagreements remain about the extent to which the peculiar velocity field - a tracer of the large-scale distribution of mass - agrees with both ΛCDM expectations and with velocity field models derived from redshift surveys. The 6dF Galaxy Survey includes peculiar velocities for nearly 9 000 early-type galaxies (6dFGSv), making it the largest and most homogeneous galaxy peculiar velocity sample to date. We have used the 6dFGS velocity field to determine the amplitude and scale of large-scale cosmic flows in the local universe and test standard cosmological models. We also compare the galaxy density and peculiar velocity fields to establish the distribution of dark and luminous matter and better constrain key cosmological parameters such as the redshift-space distortion parameter. [ABSTRACT FROM PUBLISHER]

Peculiar motion introduces systematic variations in the observed luminosity distribution of galaxies. This allows one to constrain the cosmic peculiar velocity field from large galaxy redshift surveys. Using around half a million galaxies from the SDSS Data Release 7 at z ~ 0.1, we demonstrate the applicability of this approach to large datasets and obtain bounds on peculiar velocity moments and σ8, the amplitude of the linear matter power spectrum. Our results are in good agreement with the ΛCDM model and consistent with the previously reported ~ 1% zero-point tilt in the SDSS photometry. Finally, we discuss the prospects of constraining the growth rate of density perturbations by reconstructing the full linear velocity field from the observed galaxy clustering in redshift space. [ABSTRACT FROM PUBLISHER]

We describe preliminary measurements of the pairwise velocity dispersion (PVD) of galaxies in the Galaxy and Mass Assembly (GAMA) survey as a function of projected separation and galaxy luminosity. Due to the faint magnitude limit (r < 19.8) and highly-complete spectroscopic sampling of the GAMA survey, we are able to measure the PVD to smaller scales and for lower-luminosity galaxies than previous SDSS-based work. We see no strong scale-dependence at most luminosities in the quasi-linear regime. We observe an apparent drop in PVD towards very small scales (below ≈ 0.1h−1 Mpc), but this could in part be due to a restriction of the streaming model employed. At intermediate scales, the PVD is highest (~ 500 km/s) at intermediate luminosities, dropping at both fainter and brighter luminosities. [ABSTRACT FROM PUBLISHER]

We present results of analysis of the dark matter (DM) pairwise velocity statistics in different Cosmic Web environments. We use the DM velocity and density field from the Millennium 2 simulation together with the NEXUS+ algorithm to segment the simulation volume into voxels uniquely identifying one of the four possible environments: nodes, filaments, walls or cosmic voids. We show that the PDFs of the mean infall velocities v12 as well as its spatial dependence together with the perpendicular and parallel velocity dispersions bear a significant signal of the large-scale structure environment in which DM particle pairs are embedded. The pairwise flows are notably colder and have smaller mean magnitude in wall and voids, when compared to much denser environments of filaments and nodes. We discuss on our results, indicating that they are consistent with a simple theoretical predictions for pairwise motions as induced by gravitational instability mechanism. Our results indicate that the Cosmic Web elements are coherent dynamical entities rather than just temporal geometrical associations. In addition it should be possible to observationally test various Cosmic Web finding algorithms by segmenting available peculiar velocity data and studying resulting pairwise velocity statistics. [ABSTRACT FROM PUBLISHER]

A compendium of over 8000 galaxy distances has been accumulated. Distance measurements permit the separation of observed velocities into cosmic expansion and peculiar velocity components. Only the radial component of peculiar velocities can be measured and individual errors are large, but a Wiener Filter procedure permits the reconstruction of three-dimensional motions and the density field that is responsible for these motions. A coherent flow pervades the entire domain of ± 15,000 km/s. Techniques are discussed for the separation of local and tidal components of the flow. Laniakea supercluster is identified as a region of contiguous infalling flows. [ABSTRACT FROM PUBLISHER]

Our recent estimates of galaxy counts and the luminosity density in the near-infrared (Keenan et al. 2010, 2012) indicated that the local universe may be under-dense on radial scales of several hundred megaparsecs. Such a large-scale local under-density could introduce significant biases in the measurement and interpretation of cosmological observables, such as the inferred effects of dark energy on the rate of expansion. In Keenan et al. (2013), we measured the K-band luminosity density as a function of distance from us to test for such a local under-density. We made this measurement over the redshift range 0.01 < z < 0.2 (radial distances D ~ 50 - 800 h70−1 Mpc). We found that the shape of the K-band luminosity function is relatively constant as a function of distance and environment. We derive a local (z < 0.07, D < 300 h70−1 Mpc) K-band luminosity density that agrees well with previously published studies. At z > 0.07, we measure an increasing luminosity density that by z ~ 0.1 rises to a value of ~ 1.5 times higher than that measured locally. This implies that the stellar mass density follows a similar trend. Assuming that the underlying dark matter distribution is traced by this luminous matter, this suggests that the local mass density may be lower than the global mass density of the universe at an amplitude and on a scale that is sufficient to introduce significant biases into the measurement of basic cosmological observables. At least one study has shown that an under-density of roughly this amplitude and scale could resolve the apparent tension between direct local measurements of the Hubble constant and those inferred by Planck team. Other theoretical studies have concluded that such an under-density could account for what looks like an accelerating expansion, even when no dark energy is present. [ABSTRACT FROM PUBLISHER]

Analysis of three-dimensional cosmological surveys has the potential to answer outstanding questions on the initial conditions from which structure appeared, and therefore on the very high energy physics at play in the early Universe. We report on recently proposed statistical data analysis methods designed to study the primordial large-scale structure via physical inference of the initial conditions in a fully Bayesian framework, and applications to the Sloan Digital Sky Survey data release 7. We illustrate how this approach led to a detailed characterization of the dynamic cosmic web underlying the observed galaxy distribution, based on the tidal environment. [ABSTRACT FROM PUBLISHER]

One of the main goals in cosmology is to understand how the Universe evolves, how it forms structures, why it expands, and what is the nature of dark matter and dark energy. Next decade large and expensive observational projects will bring information on the structure and the distribution of many millions of galaxies at different redshifts enabling us to make great progress in answering these questions. However, these data require a very special and complex set of analysis tools to extract the maximum valuable information. Statistical inference techniques are being developed, bridging the gaps between theory, simulations, and observations. In particular, we discuss the efforts to address the question: What is the underlying nonlinear matter distribution and dynamics at any cosmic time corresponding to a set of observed galaxies in redshift space?An accurate reconstruction of the initial conditions encodes the full phase-space information at any later cosmic time (given a particular structure formation model and a set of cosmological parameters). We present advances to solve this problem in a self-consistent way with Big Data techniques of the Cosmic Web. [ABSTRACT FROM PUBLISHER]

We describe the Bayesian \barcode\ formalism that has been designed towards the reconstruction of the Cosmic Web in a given volume on the basis of the sampled galaxy cluster distribution. Based on the realization that the massive compact clusters are responsible for the major share of the large scale tidal force field shaping the anisotropic and in particular filamentary features in the Cosmic Web. Given the nonlinearity of the constraints imposed by the cluster configurations, we resort to a state-of-the-art constrained reconstruction technique to find a proper statistically sampled realization of the original initial density and velocity field in the same cosmic region. Ultimately, the subsequent gravitational evolution of these initial conditions towards the implied Cosmic Web configuration can be followed on the basis of a proper analytical model or an N-body computer simulation. The BARCODE formalism includes an implicit treatment for redshift space distortions. This enables a direct reconstruction on the basis of observational data, without the need for a correction of redshift space artifacts. In this contribution we provide a general overview of the the Cosmic Web connection with clusters and a description of the Bayesian BARCODE formalism. We conclude with a presentation of its successful workings with respect to test runs based on a simulated large scale matter distribution, in physical space as well as in redshift space. [ABSTRACT FROM PUBLISHER]

Galaxy number or luminosity density serves as a basis for many structure classification algorithms. Several methods are used to estimate this density. Among them kernel methods have probably the best statistical properties and allow also to estimate the local sample errors of the estimate. We introduce a kernel density estimator with an adaptive data-driven anisotropic kernel, describe its properties and demonstrate the wealth of additional information it gives us about the local properties of the galaxy distribution. [ABSTRACT FROM PUBLISHER]

One of the most successful method in quantifying the structures in the Cosmic Web is the Minkowski Functionals. In 3D, there are four minkowski Functionals: Area, Volume, Integrated Mean Curvature and the Integrated Gaussian Curvature. For defining the Minkowski Functionals one should define a surface. We have developed a method based on Marching cube 33 algorithm to generate a surface from a discrete data sets. Next we calculate the Minkowski Functionals and Shapefinder from the triangulated polyhedral surface. Applying this methodology to different data sets , we obtain interesting results related to geometry, morphology and topology of the large scale structure [ABSTRACT FROM PUBLISHER]

In 1970 Zel'dovich published a far-reaching paper presenting a simple equation describing the nonlinear growth of primordial density inhomogeneities. The equation was remarkably successful in explaining the large scale structure in the Universe that we observe: a Universe in which the structure appears to be delineated by filaments and clusters of galaxies surrounding huge void regions. In order to concretise this impression it is necessary to define these structural elements through formal techniques with which we can compare the Zel'dovich model and N-body simulations with the observational data.We present an overview of recent efforts to identify voids, filaments and clusters in both the observed galaxy distribution and in numerical simulations of structure formation. We focus, in particular, on methods that involve no fine-tuning of parameters and that handle scale dependence automatically. It is important that these techniques should result in finding structures that relate directly to the dynamical mechanism of structure formation. [ABSTRACT FROM PUBLISHER]

Galaxies form chains (filaments) that connect groups and clusters of galaxies. The filamentary network includes nearly half of the galaxies and is visually the most striking feature in cosmological maps. We study the distribution of galaxies along such a filamentary network, trying to find specific patterns. Our galaxy filaments are defined using the Bisous process. We use the two-point correlation function and the Rayleigh $Z$-squared statistic to study how the galaxies are distributed along the filaments. We show that galaxies and galaxy groups are not uniformly distributed along filaments, but tend to form a regular pattern. The characteristic length of the pattern is 7~$h^{-1}$Mpc. A slightly smaller characteristic length 4~$h^{-1}$Mpc can also be found, using the $Z$-squared statistic. One can say that galaxy filaments are like pearl necklaces, where the pearls are galaxy groups distributed more or less regularly along the filaments. We propose that this well defined characteristic scale could be used as a cosmological test. [ABSTRACT FROM PUBLISHER]

We study the cosmic web at redshifts 1.0 ≤ ≤ 1.8 using quasar systems based on quasar data from the SDSS DR7 QSO catalogue. Quasar systems were determined with a friend-of-friend (FoF) algorithm at a series of linking lengths. At the linking lengths l ≤ 30 h-1 Mpc the diameters of quasar systems are smaller than the diameters of random systems, and are comparable to the sizes of galaxy superclusters in the local Universe. The mean space density of quasar systems is close to the mean space density of local rich superclusters. At larger linking lengths the diameters of quasar systems are comparable with the sizes of supercluster complexes in our cosmic neighbourhood. The richest quasar systems have diameters exceeding 500h Mpc. Very rich systems can be found also in random distribution but the percolating system which penetrate the whole sample volume appears in quasar sample at smaller linking length than in random samples showing that the large-scale distribution of quasar systems differs from random distribution. Quasar system catalogues at our web pages (http://www.aai.ee/maret/QSOsystems.html) serve as a database to search for superclusters of galaxies and to trace the cosmic web at high redshifts. [ABSTRACT FROM PUBLISHER]

In the Century since Slipher's first observations, roughly three million galaxy redshifts have been measured. The resulting maps of large-scale structure have taught us much of central importance in cosmology, ranging from the matter content of the universe to the study of the primordial density fluctuations. This talk aims to review some of the key observational and theoretical milestones on this journey, and to speculate about what the future may bring. [ABSTRACT FROM PUBLISHER]

We study the large-scale structure with superclusters from the REFLEX X-ray cluster survey together with cosmological N-body simulations. It is important to construct superclusters with criteria such that they are homogeneous in their properties. We lay out our theoretical concept considering future evolution of superclusters in their definition, and show that the X-ray luminosity and halo mass functions of clusters in superclusters are found to be top-heavy, different from those of clusters in the field. We also show a promising aspect of using superclusters to study the local cluster bias and mass scaling relation with simulations. [ABSTRACT FROM PUBLISHER]

Weak lensing detections and measurements of filaments have been elusive for a long time. The reason is that the low density contrast of filaments generally pushes the weak lensing signal to unobservably low scales. To nevertheless map the dark matter in filaments exquisite data and unusual systems are necessary. SuprimeCam observations of the supercluster system Abell 222/223 provided the required combination of excellent seeing images and a fortuitous alignment of the filament with the line-of-sight. This boosted the lensing signal to a detectable level and led to the first weak lensing mass measurement of a large-scale structure filament. The filament connecting Abell 222 and Abell 223 is now the only one traced by the galaxy distribution, dark matter, and X-ray emission from the hottest phase of the warm-hot intergalactic medium. The combination of these data allows us to put the first constraints on the hot gas fraction in filaments. [ABSTRACT FROM PUBLISHER]

Bootes filament of galaxies is a dispersed chain of groups residing on sky between the Local Void and the Virgo cluster. We consider a sample of 361 galaxies inside the sky area of RA = 13h0...18h.5 and Dec = .5°... + 10° with radial velocities VLG < 2000 km/s to clarify its structure and kinematics. In this region, 161 galaxies have individual distance estimates. We use these data to draw the Hubble relation for galaxy groups, pairs as well as the field galaxies, and to examine the galaxy distribution on peculiar velocities. Our analysis exposes the known Virgo-centric infall at RA < 14h and some signs of outflow from the Local Void at RA > 17h. According to the galaxy grouping criterion, this complex contains the members of 13 groups, 11 pairs and 140 field galaxies. The most prominent group is dominated by NGC 5846. The Bootes filament contains the total stellar mass of 2.7 ×1012M⊙ and the total virial mass of 9.07×1013M⊙, having the average density of dark matter to be Ωm = 0.09, i.e. a factor three lower than the global cosmic value. [ABSTRACT FROM PUBLISHER]

We use the Bolshoi Simulation to find the most probable location of the Local Group (LG) in the cosmic web. Our LG simulacra are pairs of halos with isolation and kinematic properties consistent with observations. The cosmic web is defined using a tidal tensor approach. We find that the LG's preferred location is regions with a dark matter overdensity close to the cosmic average. This makes filaments and sheets the preferred environment. We also find a strong alignment between the LG and the cosmic web. The orbital angular momentum is preferentially perpendicular to the smallest tidal eigenvector, while the vector connecting the two halos is strongly aligned along the the smallest tidal eigenvector and perpendicular to the largest tidal eigenvector; the pair lies and moves along filaments and sheets. We do not find any evidence for an alignment between the spin of each halo in the pair and the cosmic web. [ABSTRACT FROM PUBLISHER]

I consider a sample of ‘Updated Nearby Galaxy Catalog’ that contains eight hundred objects within 11 Mpc. Environment of each galaxy is characterized by a tidal index Θ1 depending on separation and mass of the galaxy Main Disturber (=MD). The UNGC galaxies with a common MD are ascribed to its ‘suite’ and ranked according to their Θ1. Fifteen the most populated suites contain more than half of the UNGC sample. The fraction of MDs among the brightest galaxies is almost 100% and drops to 50% at M_B = -18 mag. The observational properties of galaxies accumulated in UNGC are used to derive orbital masses of giant galaxies via motions of their satellites. The average orbital-to-stellar mass ratio for them is MorbM* ≃ 30, corresponding to the mean local density of matter Ωm ≃ 0.09, i.e 1/3 of the global cosmic one. The dark-to-stellar mass ratio for the Milky Way and M31 is typical for other neighboring giant galaxies. [ABSTRACT FROM PUBLISHER]

Our view of the low-redshift Cosmic Web has been revolutionized by galaxy redshift surveys such as 6dFGS, SDSS and 2MRS. However, the trade-off between depth and angular coverage limits a systematic three-dimensional account of the entire sky beyond the Local Volume (z < 0.05). In order to reliably map the Universe to cosmologically significant depths over the full celestial sphere, one must draw on multiwavelength datasets and state-of-the-art photometric redshift techniques. We have undertaken a dedicated program of cross-matching the largest photometric all-sky surveys – 2MASS, WISE and SuperCOSMOS – to obtain accurate redshift estimates of millions of galaxies. The first outcome of these efforts – the 2MASS Photometric Redshift catalog (2MPZ, Bilicki et al. 2014a) – has been publicly released and includes almost 1 million galaxies with a mean redshift of z=0.08. Here we summarize how this catalog was constructed and how using the WISE mid-infrared sample together with SuperCOSMOS optical data allows us to push to redshift shells of z∼ 0.2 –0.3 on unprecedented angular scales. Our catalogs, with ∼ 20 million sources in total, provide access to cosmological volumes crucial for studies of local galaxy flows (clustering dipole, bulk flow) and cross-correlations with the cosmic microwave background such as the integrated Sachs-Wolfe effect or lensing studies. [ABSTRACT FROM PUBLISHER]

The powerful Lagrangian view of structure formation was essentially introduced to cosmology by Zel'dovich. In the current cosmological paradigm, a dark-matter-sheet 3D manifold, inhabiting 6D position-velocity phase space, was flat (with vanishing velocity) at the big bang. Afterward, gravity stretched and bunched the sheet together in different places, forming a cosmic web when projected to the position coordinates.Here, I explain some properties of an origami approximation, in which the sheet does not stretch or contract (an assumption that is false in general), but is allowed to fold. Even without stretching, the sheet can form an idealized cosmic web, with convex polyhedral voids separated by straight walls and filaments, joined by convex polyhedral nodes. The nodes form in ‘polygonal’ or ‘polyhedral’ collapse, somewhat like spherical/ellipsoidal collapse, except incorporating simultaneous filament and wall formation. The origami approximation allows phase-space geometries of nodes, filaments, and walls to be more easily understood, and may aid in understanding spin correlations between nearby galaxies. This contribution explores kinematic origami-approximation models giving velocity fields for the first time. [ABSTRACT FROM PUBLISHER]

The VIMOS Public Extragalactic Redshift Survey (VIPERS) is the largest redshift survey ever conducted with the ESO telescopes. It has used the Very Large Telescope to collect nearly 100,000 redshifts from the general galaxy population at 0.5 < z < 1.2. With a combination of volume and high sampling density that is unique for these redshifts, it allows statistical measurements of galaxy clustering and related cosmological quantities to be obtained on an equal footing with classic results from local redshift surveys. At the same time, the simple magnitude-limited selection and the wealth of ancillary photometric data provide a general view of the galaxy population, its physical properties and the relation of the latter to large-scale structure. This paper presents an overview of the galaxy clustering results obtained so far, together with their cosmological implications. Most of these are based on the ∼ 55,000 galaxies forming the first public data release (PDR-1). As of January 2015, observations and data reduction are complete and the final data set of more than 90,000 redshifts is being validated and made ready for the final investigations. [ABSTRACT FROM PUBLISHER]

There is now no doubt that neutrinos are massive particles fully involved in the non-linear growth of the large-scale structure of the universe. A problem is that they are particularly difficult to include in cosmological models because the equations describing their behavior in the non-linear regime are cumbersome and difficult to handle. In this manuscript I present a new method allowing to deal with massive neutrinos in a very simple way, based on basic conservation laws. This method is still valid in the non-linear regime. The key idea is to describe neutrinos as a collection of single-flow fluids instead of seeing them as a single hot multi-flow fluid. In this framework, the time evolution of neutrinos is encoded in fluid equations describing macroscopic fields, just as what is done for cold dark matter. Although valid up to shell-crossing only, this approach is a further step towards a fully non-linear treatment of the dynamical evolution of neutrinos in the framework of large-scale structure growth. [ABSTRACT FROM PUBLISHER]

The cosmic web is a complex spatial pattern of walls, filaments, cluster nodes and underdense void regions. It emerged through gravitational amplification from the Gaussian primordial density field. Here we infer analytical expressions for the spatial statistics of caustics in the evolving large-scale mass distribution. In our analysis, following the quasi-linear Zel'dovich formalism and confined to the 1D and 2D situation, we compute number density and correlation properties of caustics in cosmic density fields that evolve from Gaussian primordial conditions. The analysis can be straightforwardly extended to the 3D situation. We moreover, are currently extending the approach to the non-linear regime of structure formation by including higher order Lagrangian approximations and Lagrangian effective field theory. [ABSTRACT FROM PUBLISHER]