We report the first statistical detection of X-ray emission from cosmic web filaments in ROSAT data. We selected 15,165 filaments at 0.2<z<0.6 ranging from 30 Mpc to 100 Mpc in length, identified in the Sloan Digital Sky Survey (SDSS) survey. We stacked the X-ray count-rate maps from ROSAT around the filaments, excluding resolved galaxy groups and clusters above the mass of ~3 * 10^13 Msun as well as the detected X-ray point sources from the ROSAT, Chandra, and XMM-Newton observations. The stacked signal results in the detection of the X-ray emission from the cosmic filaments at a significance of 4.2 sigma in the energy band of 0.56-1.21 keV. The signal is interpreted, assuming the Astrophysical Plasma Emission Code (APEC) model, as an emission from the hot gas in the filament-core regions with an average gas temperature of 0.9(+1.0-0.6) keV and a gas overdensity of ~30 at the center of the filaments. Furthermore, we show that stacking the SRG/eROSITA data for ~2,000 filaments only would lead to a ~5 sigma detection of their X-ray signal, even with an average gas temperature as low as ~0.3 keV.
We search for the lensing signal of massive filaments between 135,000 pairs of Luminous Red Galaxies (LRGs) from the Sloan Digital Sky Survey. We develop a new estimator that cleanly removes the much larger shear signal of the neighboring LRG halos, relying only on the assumption of spherical symmetry. We consider two models: a thick filament model constructed from ray-tracing simulations for $Λ$CDM model, and a thin filament model which models the filament by a string of halos along the line connecting the two LRGs. We show that the filament lensing signal is in nice agreement with the thick …
We study the stacked filaments connecting group-mass halo pairs, using dark-matter-only N-body simulations. We calculate the dark matter over-density profile of these stacked filaments at different redshifts as a function of the distance perpendicular to the filament axis. A four-parameter universal functional form, including three comoving scale radii and one amplitude parameter (core density), provides a good fit out to a radius of 20 cMpc/h for stacked filaments over a range of redshifts, lengths and masses. The scale radii are approximately independent of redshift but increase as power-law…
The large-scale structure of the Universe is characterised by a web-like structure made of voids, sheets, filaments, and knots. The structure of this so-called cosmic web is dictated by the local velocity shear tensor. In particular, the local direction of a filament should be strongly aligned with e3, the eigenvector associated with the smallest eigenvalue of the tensor. That conjecture is tested here on the basis of a cosmological simulation. The cosmic web delineated by the halo distribution is probed by a marked point process with interactions (the Bisous model), detecting filaments direct…
Diffuse filaments connect galaxy clusters to form the cosmic web. Detecting these filaments could yield information on the magnetic field strength, cosmic ray population and temperature of intercluster gas, yet, the faint and large-scale nature of these bridges makes direct detections very challenging. Using multiple independent all-sky radio and X-ray maps we stack pairs of luminous red galaxies as tracers for cluster pairs. For the first time, we detect an average surface brightness between the clusters from synchrotron (radio) and thermal (X-ray) emission with $\gtrsim 5σ$ significance, on …
The problem of detecting dark matter filaments in the cosmic web is considered. Weak lensing is an ideal probe of dark matter, and therefore forms the basis of particularly promising detection methods. We consider and develop a number of weak lensing techniques that could be used to detect filaments in individual or stacked cluster fields, and apply them to synthetic lensing data sets in the fields of clusters from the Millennium Simulation. These techniques are multipole moments of the shear and convergence, mass reconstruction, and parameterized fits to filament mass profiles using a Markov …
The main feature of the spatial large-scale galaxy distribution is its intricate network of galaxy filaments. This network is spanned by the galaxy locations that can be interpreted as a three-dimensional point distribution. The global properties of the point process can be measured by different statistical methods, which, however, do not describe directly the structure elements. The morphology of the large scale structure, on the other hand, is an important property of the galaxy distribution. Here we apply an object point process with interactions (the Bisous model) to trace and extract the …
We studied physical properties of matter in 24,544 filaments ranging from 30 to 100 Mpc in length, identified in the Sloan Digital Sky Survey (SDSS). We stacked the Comptonization y map produced by the Planck Collaboration around the filaments, excluding the resolved galaxy groups and clusters above a mass of ~3*10^13 Msun. We detected the thermal Sunyaev-Zel'dovich signal for the first time at a significance of 4.4 sigma in filamentary structures on such a large scale. We also stacked the Planck cosmic microwave background (CMB) lensing convergence map in the same manner and detected the lens…
The detection of the diffuse gas component of the cosmic web remains a formidable challenge. In this work we study synchrotron emission from the cosmic web with simulated SKA1 observations, which can represent an fundamental probe of the warm-hot intergalactic medium. We investigate radio emission originated by relativistic electrons accelerated by shocks surrounding cosmic filaments, assuming diffusive shock acceleration and as a function of the (unknown) large-scale magnetic fields. The detection of the brightest parts of large ($>10 \rm Mpc$) filaments of the cosmic web should be within rea…
The geometry of the cosmic web drives in part the spin acquisition of galaxies. This can be explained in a Lagrangian framework, by identifying the specific long-wavelength correlations within the primordial Gaussian random field which are relevant to spin acquisition. Tidal Torque Theory is revisited in the context of such anisotropic environments, biased by the presence of a filament within a wall. The point process of filament-type saddles represents it most efficiently. The constrained misalignment between the tidal and the inertia tensors in the vicinity of filament-type saddles simply ex…
Filaments and sheets are striking visual patterns in cosmic web. The maximum extent of these large-scale structures are difficult to determine due to their structural variety and complexity. We construct a volume-limited sample of galaxies in a cubic region from the SDSS, divide it into smaller subcubes and shuffle them around. We quantify the average filamentarity and planarity in the three-dimensional galaxy distribution as a function of the density threshold and compare them with those from the shuffled realizations of the original data. The analysis is repeated for different shuffling leng…
The continuous flow of gas and dark matter across scales in the cosmic web can generate correlated dynamical properties of haloes and filaments (and the magnetic fields they contain). With this work, we study the halo spin properties and orientation with respect to filaments, and the morphology of the magnetic field around these objects, for haloes with masses in the range 1e8-1e14 Msun and filaments up to 8 Mpc long. Furthermore, we study how these properties vary in presence, or lack thereof, of different (astro)physical processes and with different magnetic initial conditions. We perform co…
Using the publicly available eROSITA Final Equatorial Depth Survey (eFEDS) data, we detected the stacked X-ray emissions at the position of 463 filaments at a significance of 3.8 sigma based on the combination of all energy bands. In parallel, we found that the probability of the measurement under the null hypothesis is ~0.0017. The filaments were identified with galaxies in the Sloan Digital Sky Survey survey, ranging from 30 Mpc to 100 Mpc in length at 0.2 < z < 0.6. The stacking of the filaments was performed with the eFEDS X-ray count-rate maps in the energy range between 0.4 - 2.3 keV aft…
The large scale galaxy and matter distribution is often described by means of the cosmic web made up of voids, sheets, filaments and knots. Many different recipes exist for identifying this cosmic web. Here we focus on a sub-class of cosmic web identifiers, based on the analysis of the Hessian matrix, and proposed a method, called COsmic Web Skeleton (COWS), of separating a set of filaments cells into an ensemble of individual discreet filaments. Specifically, a thinning algorithm is applied to velocity shear tensor based cosmic web (V-web) to identify the spine of the filaments. This results …
Filament-Arm Node Systems (FANS): A new probe of the winding and chirality of the cosmic web
Center for Cosmology and Computational Astrophysics, Institute for Advanced Study in Physics, Zhejiang University（浙江大学高等物理研究院宇宙学与计算天体物理学中心）；Institute of Astronomy, School of Physics, Zhejiang University（浙江大学物理学院天文学研究所）
In the standard model of non-linear structure formation, a cosmic web of dark-matter dominated filaments connects dark matter halos. In this paper, we stack the weak lensing signal of an ensemble of filaments between groups and clusters of galaxies. Specifically, we detect the weak lensing signal, using CFHTLenS galaxy ellipticities, from stacked filaments between SDSS-III/BOSS luminous red galaxies (LRGs). As a control, we compare the physical LRG pairs with projected LRG pairs that are more widely separated in redshift space. We detect the excess filament mass density in the projected pairs …
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 c…
It is possible to visualize the Cosmic Web as an interconnected network of one-dimensional filaments, two-dimensional sheets and three-dimensional volume-filling structures which we refer to as clusters. We have used the Local Dimension D, which takes values D = 1, 2 and 3 for filaments, sheets and clusters, respectively, to analyse the Cosmic Web in a three-dimensional volume-limited galaxy sample from the Sloan Digital Sky Survey Data Release 7. The analysis was carried out separately using three different ranges of length-scales: 0.5-5, 1-10 and 5-50 $h^{-1} {\rm Mpc}$. We find that there i…
The weak lensing of cosmic web filaments is investigated in modified gravity (MOG) and it is demonstrated that the detected galaxies and baryonic gas of order $5-15 \%$ in filaments can with the enhanced value of the strength of gravity agree with the lensing data for filaments without dark matter.
Strong accretion shocks are expected to illuminate the warm-hot inter-galactic medium encompassed by the filaments of the cosmic web, through synchrotron radio emission. Given their high sensitivity, low-frequency large radio facilities may already be able to detect signatures of this extended radio emission from the region in between two close and massive galaxy clusters. In this work we exploit the non-detection of such diffuse emission by deep observations of two pairs of relatively close ($\simeq 10$ Mpc) and massive ($M_{500}\geq 10^{14}M_\odot$) galaxy clusters using the LOw-Frequency AR…
We analyze the structure and connectivity of the distinct morphologies that define the Cosmic Web. With the help of our Multiscale Morphology Filter (MMF), we dissect the matter distribution of a cosmological $Λ$CDM N-body computer simulation into cluster, filaments and walls. The MMF is ideally suited to adress both the anisotropic morphological character of filaments and sheets, as well as the multiscale nature of the hierarchically evolved cosmic matter distribution. The results of our study may be summarized as follows: i).- While all morphologies occupy a roughly well defined range in den…
On the largest scales, galaxies are pulled together by gravity to form clusters, which are connected by filaments making a web-like pattern. Radio emission is predicted from this cosmic web, which should originate from the strong accretion shocks around the cosmic structures. We present the first observational evidence that Fermi-type acceleration from strong shocks surrounding the filaments of the cosmic web, as well as in peripherals of low-mass clusters, is at work in the Universe. Using all-sky radio maps and stacking on clusters and filaments, we have detected the polarization signature o…
We generate probabilistic predictions of the low-redshift ($z < 0.2$) synchrotron Cosmic Web for half of the Northern Sky. In particular, we predict the contribution to the specific intensity function at $ν_\mathrm{obs} = 150\ \mathrm{MHz}$ from merger shocks in clusters and accretion shocks in filaments, both of which arise during large-scale structure formation. We assume a primordial magnetogenesis scenario, but our method is general enough to allow for an exploration of alternative magnetogenesis scenarios - and even alternative radiation mechanisms and spectral windows. In the future, by …
The nature versus nurture scenario in galaxy and group evolution is a long-standing problem not yet fully understood on cosmological scales. We study the properties of groups and their central galaxies in different large-scale environments defined by the luminosity density field and the cosmic web filaments. We use the luminosity density field constructed using 8 Mpc/h smoothing to characterize the large-scale environments and the Bisous model to extract the filamentary structures in different large-scale environments. We find differences in the properties of central galaxies and their groups …
We produce the first low to mid frequency radio simulation that incorporates both traditional extragalactic radio sources as well as synchrotron cosmic web emission. The FIlaments \& GAlactic RadiO (FIGARO) simulation includes ten unique \SI{4x4}{\degree} fields, incorporating active galactic nucleii (AGNs), star forming galaxies (SFGs) and synchrotron cosmic web emission out to a redshift of $z = 0.8$ and over the frequency range 100-1400 MHz. To do this, the simulation brings together a recent $100^3$ Mpc$^3$ magneto-hydrodynamic simulation (Vazza et al., 2019), calibrated to match observed …
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 …
We investigate the spin evolution of dark matter haloes and their dependence on the number of connected filaments from the cosmic web at high redshift (spin-filament relation hereafter). To this purpose, we have simulated $5000$ haloes in the mass range $5\times10^{9}h^{-1}M_{\odot}$ to $5\times10^{11}h^{-1}M_{\odot}$ at $z=3$ in cosmological N-body simulations. We confirm the relation found by Prieto et al. 2015 where haloes with fewer filaments have larger spin. We also found that this relation is more significant for higher halo masses, and for haloes with a passive (no major mergers) assem…
We use a 64$h^{-1}$Mpc dark matter (DM) only cosmological simulation to examine the large scale orientation of haloes and substructures with respect the cosmic web. A web classification scheme based on the velocity shear tensor is used to assign to each halo in the simulation a web type: knot, filament, sheet or void. Using $\sim10^6$ haloes that span ~3 orders of magnitude in mass the orientation of the halo's spin and the orbital angular momentum of subhaloes with respect to the eigenvectors of the shear tensor is examined. We find that the orbital angular momentum of subhaloes tends to alig…
The close relationship between mergers and the reorientation of the spin for galaxies and their host dark haloes is investigated using a cosmological hydrodynamical simulation (Horizon-AGN). Through a statistical analysis of merger trees, we show that spin swings are mainly driven by mergers along the filamentary structure of the cosmic web, and that these events account for the preferred perpendicular orientation of massive galaxies with respect to their nearest filament. By contrast, low-mass galaxies (M_s<10^10 M_sun at redshift 1.5) undergoing very few mergers, if at all, tend to possess a…
We present the first detection of CO emission lines in the Halpha filaments at distances as far as 50 kpc from the centre of the galaxy NGC 1275. This gas is probably dense (>=10E3 cm-3). However, it is not possible to accurately determine the density and the kinetic temperature of this relatively warm gas (Tkin~20-500K) with the current data only. The amount of molecular gas in the filaments is large 10E9 Msun (assuming a Galactic N(H2)/Ico ratio). This is 10% of the total mass of molecular gas detected in this cD galaxy. This gas has large-scale velocities comparable to those seen in Halpha.…
The detection of filaments in the cosmic web will be crucial to distinguish between the possible magnetogenesis scenarios and future large polarization surveys will be able to shed light on their magnetization level. In this work, we use numerical simulations of galaxy clusters to investigate their possible detection. We compute the Faraday Rotation signal in intracluster filaments and compare it to its surrounding environment. We find that the expected big improvement in sensitivity with the SKA-MID will in principle allow the detection of a large fraction of filaments surrounding galaxy clus…
We explore the evolution of halo spins in the cosmic web using a very large sample of dark matter haloes in the $Λ$CDM Planck-Millennium N-body simulation. We use the NEXUS+ multiscale formalism to identify the hierarchy of filaments and sheets of the cosmic web at several redshifts. We find that at all times the magnitude of halo spins correlates with the web environment, being largest in filaments, and, for the first time, we show that it also correlates with filament thickness as well as the angle between spin-orientation and the spine of the host filament. For example, massive haloes in th…
The cosmic web that characterizes the large-scale structure of the Universe can be quantified by a variety of methods. For example, large redshift surveys can be used in combination with point process algorithms to extract long curvilinear filaments in the galaxy distribution. Alternatively, given a full 3D reconstruction of the velocity field, kinematic techniques can be used to decompose the web into voids, sheets, filaments and knots. In this paper we look at how two such algorithms - the Bisous model and the velocity shear web - compare with each other in the local Universe (within 100 Mpc…
We study the formation and evolution of the cosmic web, using the high-resolution CosmoGrid \$Λ\$CDM simulation. In particular, we investigate the evolution of the large-scale structure around void halo groups, and compare this to observations of the VGS-31 galaxy group, which consists of three interacting galaxies inside a large void. The structure around such haloes shows a great deal of tenuous structure, with most of such systems being embedded in intra-void filaments and walls. We use the Nexus+ algorithm to detect walls and filaments in CosmoGrid, and find them to be present and detectab…
We investigate the possible presence of neutral hydrogen (HI) in intergalactic filaments at very low redshift ($z\sim 0.08$), by stacking a set of 274,712 2dFGRS galaxy pairs over 21-cm maps obtained with dedicated observations conducted with the Parkes radio telescope, over a total sky area of approximately 1,300 square degrees covering two patches in the northern and in the southern Galactic hemispheres. The stacking is performed by combining local maps in which each pair is brought to a common reference frame; the resulting signal from the edge galaxies is then removed to extract the filame…
Topological connections in the single-streaming voids and multistreaming filaments and walls reveal a cosmic web structure different from traditional mass density fields. A single void structure not only percolates the multistream field in all the directions, but also occupies over 99 per cent of all the single-streaming regions. Sub-grid analyses on scales smaller than simulation resolution reveal tiny pockets of voids that are isolated by membranes of the structure. For the multistreaming excursion sets, the percolating structure is significantly thinner than the filaments in over-density ex…
We investigate whether the satellite luminosity function (LF) of primary galaxies identified in the Sloan Digital Sky Survey (SDSS) depends on whether the host galaxy is in a filament or not. Isolated primary galaxies are identified in the SDSS spectroscopic sample while potential satellites (that are up to 4 magnitudes fainter than their hosts) are searched for in the much deeper photometric sample. Filaments are constructed from the galaxy distribution by the "Bisous" process. Isolated primary galaxies are divided into two subsamples: those in filaments and those not in filaments. We examine…
We construct a catalogue for filaments using a novel approach called SCMS (subspace constrained mean shift; Ozertem & Erdogmus 2011; Chen et al. 2015). SCMS is a gradient-based method that detects filaments through density ridges (smooth curves tracing high-density regions). A great advantage of SCMS is its uncertainty measure, which allows an evaluation of the errors for the detected filaments. To detect filaments, we use data from the Sloan Digital Sky Survey, which consist of three galaxy samples: the NYU main galaxy sample (MGS), the LOWZ sample and the CMASS sample. Each of the three data…
The network of filaments with embedded clusters surrounding voids seen in maps derived from redshift surveys and reproduced in simulations has been referred to as the cosmic web. A complementary description is provided by considering the shear in the velocity field of galaxies. The eigenvalues of the shear provide information on whether a region is collapsing in three dimensions, the condition for a knot, expanding in three-dimensions, the condition for a void, or in the intermediate condition of a filament or sheet. The structures that are quantitatively defined by the eigenvalues can be appr…
We study the effect of baryons on the cosmic web -- halos, filaments, walls, and voids. To do so, we apply a modified version of NEXUS, a cosmic web morphological analysis algorithm, to the IllustrisTNG simulations. We find that halos lose more than $10\%$ of their mass due to baryons, mostly to filaments and a small portion to walls and voids. However, the mass transfer does not significantly shift the boundaries of structures, leaving the volume fractions of the cosmic structures largely unaffected. We quantify the effects of baryonic feedback on the power spectrum and the probability densit…
We report the results of the first study of the multi-stream environment of dark matter haloes in cosmological N-body simulations in the LCDM cosmology. The full dynamical state of dark matter can be described as a three-dimensional sub-manifold in six dimensional phase space - the dark matter sheet. In our study we use a Lagrangian sub-manifold x = x(q,t) (where x and q are co-moving Eulerian and Lagrangian coordinates respectively), which is dynamically equivalent to the dark matter sheet but is more convenient for numerical analysis. Our major results can be summarized as follows. At the re…
We use the Millennium simulation to show that halo clustering varies significantly with cosmic web type. Halos are classified as node, filament, sheet and void halos based on the eigenvalue decomposition of the velocity shear tensor. The velocity field is sampled by the peculiar velocities of a fixed number of neighbouring halos and spatial derivatives are computed using a kernel borrowed from smoothed particle hydrodynamics. The classification scheme is used to examine the clustering of halos as a function of web type for halos with masses larger than $10^{11}$. We find that node halos show p…
We study the alignment of dark matter haloes with the cosmic web characterized by the tidal and velocity shear fields. We focus on the alignment of their shape, angular momentum and peculiar velocities. We use a cosmological N-body simulation that allows to study dark matter halos spanning almost five orders of magnitude in mass ($10^{9}$-$10^{14}$) $h^{-1}$$M_{\odot}$ and spatial scales of $(0.5$-$1.0)$ $h^{-1}$ Mpc to define the cosmic web. We find that the halo shape presents the strongest alignment along the smallest tidal eigenvector, e.g. along filaments and walls, with a signal that get…
We explore the possibility of detecting radio emission in the \emph{cosmic web} by analyzing shock waves in the MareNostrum cosmological simulation. This requires a careful calibration of shock finding algorithms in Smoothed-Particle Hydrodynamics simulations, which we present here. Moreover, we identify the elements of the cosmic web, namely voids, walls, filaments and clusters with the use of the SpineWeb technique, a procedure that classifies the structure in terms of its topology. Thus, we are able to study the Mach number distribution as a function of its environment. We find that the med…
Cosmological simulations predict the Universe contains a network of intergalactic gas filaments, within which galaxies form and evolve. However, the faintness of any emission from these filaments has limited tests of this prediction. We report the detection of rest-frame ultraviolet Lyman-alpha radiation from multiple filaments extending more than one megaparsec between galaxies within the SSA 22 proto-cluster at a redshift of 3.1. Intense star formation and supermassive black-hole activity is occurring within the galaxies embedded in these structures, which are the likely sources of the eleva…
The universe in large scales is structured as a network known as cosmic web. Filaments are one of the structural components of this web, which can be introduced as a novel probe to study the formation and evolution of structures and as a probe to study the cosmological models and to address the missing baryon problem. The aim of this work is to introduce an analytical framework to study the statistics of filaments such as number density of them and also to obtain the length-mass relation. For this objective, we model filaments as collapsed objects which have an extension in one direction, acco…
Galaxy cluster mass halos ("clusters") in a dark matter simulation are matched to nodes in several different cosmic webs found using the Disperse cosmic web finder. The webs have different simulation smoothings and Disperse parameter choices; for each, 4 methods are considered for matching Disperse nodes to clusters. For most of the webs, Disperse nodes outnumber clusters, but not every cluster has a Disperse node match (and sometimes $>1$ cluster matches to the same Disperse node). The clusters frequently lacking a matching Disperse node have a different distribution of local shear trends and…
The standard cosmological model ($Λ$CDM) predicts the existence of the cosmic web: a distribution of matter into sheets and filaments connecting massive halos. However, observational evidence has been elusive due to the low surface brightness of the filaments. Recent deep MUSE/VLT data and upcoming observations offer a promising avenue for Ly$α$ detection, motivating the development of modern theoretical predictions. We use hydrodynamical cosmological simulations run with the AREPO code to investigate the potential detectability of large-scale filaments, excluding contributions from the halos …
We investigate the alignment of haloes with the filaments of the cosmic web using an unprecedently large sample of dark matter haloes taken from the P-Millennium $Λ$CDM cosmological N-body simulation. We use the state-of-the-art NEXUS morphological formalism which, due to its multiscale nature, simultaneously identifies structures at all scales. We find strong and highly significant alignments, with both the major axis of haloes and their peculiar velocity tending to orient along the filament. However, the spin - filament alignment displays a more complex trend changing from preferentially par…
Dark matter haloes in cosmological filaments and walls have their spin vector aligned (in average) with their host structure. While haloes in walls are aligned with the plane of the wall independently of their mass, haloes in filaments present a mass dependent two-regime orientation. Here we show that the transition mass determining the change in the alignment regime (from parallel to perpendicular) depends on the hierarchical level in which the halo is located, reflecting the hierarchical nature of the Cosmic Web. By explicitly exposing this hierarchy we are able to identify the contributions…
