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The cosmic web is one of the most striking features of the distribution of galaxies and dark matter on the largest scales in the Universe. It is composed of dense regions packed full of galaxies, long filamentary bridges, flattened sheets and vast low density voids. The study of the cosmic web has focused primarily on the identification of such features, and on understanding the environmental effects on galaxy formation and halo assembly. As such, a variety of different methods have been devised to classify the cosmic web -- depending on the data at hand, be it numerical simulations, large sky surveys or other. In this paper we bring twelve of these methods together and apply them to the same data set in order to understand how they compare. In general these cosmic web classifiers have been designed with different cosmological goals in mind, and to study different questions. Therefore one would not {it a priori} expect agreement between different techniques however, many of these methods do converge on the identification of specific features. In this paper we study the agreements and disparities of the different methods. For example, each method finds that knots inhabit higher density regions than filaments, etc. and that voids have the lowest densities. For a given web environment, we find substantial overlap in the density range assigned by each web classification scheme. We also compare classifications on a halo-by-halo basis; for example, we find that 9 of 12 methods classify around a third of group-mass haloes (i.e. $M_{rm halo}sim10^{13.5}h^{-1}M_{odot}$) as being in filaments. Lastly, so that any future cosmic web classification scheme can be compared to the 12 methods used here, we have made all the data used in this paper public.
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We introduce the NEXUS algorithm for the identification of Cosmic Web environments: clusters, filaments, walls and voids. This is a multiscale and automatic morphological analysis tool that identifies all the cosmic structures in a scale free way, without preference for a certain size or shape. We develop the NEXUS method to incorporate the density, tidal field, velocity divergence and velocity shear as tracers of the Cosmic Web. We also present the NEXUS+ procedure which, taking advantage of a novel filtering of the density in logarithmic space, is very successful at identifying the filament and wall environments in a robust and natural way. To asses the algorithms we apply them to an N-body simulation. We find that all methods correctly identify the most prominent filaments and walls, while there are differences in the detection of the more tenuous structures. In general, the structures traced by the density and tidal fields are clumpier and more rugged than those present in the velocity divergence and velocity shear fields. We find that the NEXUS+ method captures much better the filamentary and wall networks and is successful in detecting even the fainter structures. We also confirm the efficiency of our methods by examining the dark matter particle and halo distributions.
We trace the cosmic web at redshifts 1.0 <= z <= 1.8 using the quasar data from the SDSS DR7 QSO catalogue (Schneider et al. 2010). We apply a friend-of-friend (FoF) algorithm to the quasar and random catalogues to determine systems at a series of linking lengths, and analyse richness and sizes of these systems. At the linking lengths l <= 30 Mpc/h the number of quasar systems is larger than the number of systems detected in random catalogues, and systems themselves have smaller diameters than random systems. The diameters of quasar systems are comparable to the sizes of poor galaxy superclusters in the local Universe, the richest quasar systems have four members. The mean space density of quasar systems is close to the mean space density of local rich superclusters. At intermediate linking lengths (40 <= l <= 70 Mpc/h) the richness and length of quasar systems are similar to those derived from random catalogues. Quasar system diameters are similar to the sizes of rich superclusters and supercluster chains in the local Universe. At the linking length 70 Mpc/h the richest systems of quasars have diameters exceeding 500 Mpc/h. The percolating system which penetrate the whole sample volume appears in quasar sample at smaller linking length than in random samples (85 Mpc/h). Quasar luminosities in systems are not correlated with the system richness. 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.
We investigate the characteristics and the time evolution of the cosmic web from redshift, z=2, to present time, within the framework of the NEXUS+ algorithm. This necessitates the introduction of new analysis tools optimally suited to describe the very intricate and hierarchical pattern that is the cosmic web. In particular, we characterize filaments (walls) in terms of their linear (surface) mass density. This is very good in capturing the evolution of these structures. At early times the cosmos is dominated by tenuous filaments and sheets, which, during subsequent evolution, merge together, such that the present day web is dominated by fewer, but much more massive, structures. We also show that voids are more naturally described in terms of their boundaries and not their centres. We illustrate this for void density profiles, which, when expressed as a function of the distance from void boundary, show a universal profile in good qualitative agreement with the theoretical shell-crossing framework of expanding underdense regions.
This is the report of the Ultraviolet-Optical Working Group (UVOWG) commissioned by NASA to study the scientific rationale for new missions in ultraviolet/optical space astronomy approximately ten years from now, when the Hubble Space Telescope (HST) is de-orbited. The UVOWG focused on a scientific theme, The Emergence of the Modern Universe, the period from redshifts z = 3 to 0, occupying over 80% of cosmic time and beginning after the first galaxies, quasars, and stars emerged into their present form. We considered high-throughput UV spectroscopy (10-50x throughput of HST/COS) and wide-field optical imaging (at least 10 arcmin square). The exciting science to be addressed in the post-HST era includes studies of dark matter and baryons, the origin and evolution of the elements, and the major construction phase of galaxies and quasars. Key unanswered questions include: Where is the rest of the unseen universe? What is the interplay of the dark and luminous universe? How did the IGM collapse to form the galaxies and clusters? When were galaxies, clusters, and stellar populations assembled into their current form? What is the history of star formation and chemical evolution? Are massive black holes a natural part of most galaxies? A large-aperture UV/O telescope in space (ST-2010) will provide a major facility in the 21st century for solving these scientific problems. The UVOWG recommends that the first mission be a 4m aperture, SIRTF-class mission that focuses on UV spectroscopy and wide-field imaging. In the coming decade, NASA should investigate the feasibility of an 8m telescope, by 2010, with deployable optics similar to NGST. No high-throughput UV/Optical mission will be possible without significant NASA investments in technology, including UV detectors, gratings, mirrors, and imagers.
The cosmic web is the largest scale manifestation of the anisotropic gravitational collapse of matter. It represents the transitional stage between linear and non-linear structures and contains easily accessible information about the early phases of structure formation processes. Here we investigate the characteristics and the time evolution of morphological components since. Our analysis involves the application of the NEXUS Multiscale Morphology Filter (MMF) technique, predominantly its NEXUS+ version, to high resolution and large volume cosmological simulations. We quantify the cosmic web components in terms of their mass and volume content, their density distribution and halo populations. We employ new analysis techniques to determine the spatial extent of filaments and sheets, like their total length and local width. This analysis identifies cluster and filaments as the most prominent components of the web. In contrast, while voids and sheets take most of the volume, they correspond to underdense environments and are devoid of group-sized and more massive haloes. At early times the cosmos is dominated by tenuous filaments and sheets, which, during subsequent evolution, merge together, such that the present day web is dominated by fewer, but much more massive, structures. The analysis of the mass transport between environments clearly shows how matter flows from voids into walls, and then via filaments into cluster regions, which form the nodes of the cosmic web. We also study the properties of individual filamentary branches, to find long, almost straight, filaments extending to distances larger than 100Mpc/h. These constitute the bridges between massive clusters, which seem to form along approximatively straight lines.
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