Cosmic Web Structure Research Articles

THE HYDRODYNAMIC STABILITY OF GASEOUS COSMIC FILAMENTS

ABSTRACT Virial shocks at the edges of cosmic-web structures are a clear prediction of standard structure formation theories. We derive a criterion for the stability of the post-shock gas and of the virial shock itself in spherical, filamentary, and planar infall geometries. When gas cooling is important, we find that shocks become unstable, and gas flows uninterrupted toward the center of the respective halo, filament, or sheet. For filaments, we impose this criterion on self-similar infall solutions. We find that instability is expected for filament masses between 1011 and 1013 Mpc−1. Using a simplified toy model, we then show that these filaments will likely feed halos with 1010 M ⊙ ≲ M halo ≲ 1013 M ⊙ at redshift z = 3, as well as 1012 M ⊙ ≲ M halo ≲ 1015 M ⊙ at z = 0. The instability will affect the survivability of the filaments as they penetrate gaseous halos in a non-trivial way. Additionally, smaller halos accreting onto non-stable filaments will not be subject to ram pressure inside the filaments. The instreaming gas will continue toward the center and stop either once its angular momentum balances the gravitational attraction, or when its density becomes so high that it becomes self-shielded to radiation.

Mean maps for cosmic web structures in cosmological initial conditions

Halos, filaments, sheets and voids in the cosmic web can be defined in terms of the eigenvalues of the smoothed shear tensor and a threshold $\lambda_{\rm th}$. Using analytic methods, we construct mean maps centered on these types of structures for Gaussian random fields corresponding to cosmological initial conditions. Each map also requires a choice of shear at the origin; we consider three possibilities. We find characteristic sizes, shapes and other properties of the central objects in these mean maps and explore how these properties change with varying the threshold and smoothing scale, i.e. varying the separation of the cosmic web into different kinds of components. The mean maps become increasingly complex as the threshold $\lambda_{\rm th}$ decreases to zero. We also describe scatter around these mean maps, subtleties which can arise in their construction, and some comparisons between halos in the maps and collapsed halos at final times.

Smooth halos in the cosmic web

Dark matter halos can be defined as smooth distributions of dark matter placed in a non-smooth cosmic web structure. This definition of halos demands a precise definition of smoothness and a characterization of the manner in which the transition from smooth halos to the cosmic web takes place. We introduce entropic measures of smoothness, related to measures of inequality previously used in economy and with the advantage of being connected with standard methods of multifractal analysis already used for characterizing the cosmic web structure in cold dark matter N-body simulations. These entropic measures provide us with a quantitative description of the transition from the small scales portrayed as a distribution of halos to the larger scales portrayed as a cosmic web and, therefore, allow us to assign definite sizes to halos. However, these ``smoothness sizes'' have no direct relation to the virial radii. Finally, we discuss the influence of N-body discreteness parameters on smoothness.

One dark matter mystery: halos in the cosmic web

The current cold dark matter cosmological model explains the large scale cosmic web structure but is challenged by the observation of a relatively smooth distribution of matter in galactic clusters. We consider various aspects of modeling the dark matter around galaxies as distributed in smooth halos and, especially, the smoothness of the dark matter halos seen in N-body cosmological simulations. We conclude that the problems of the cold dark matter cosmology on small scales are more serious than normally admitted.

KINEMATIC MORPHOLOGY OF LARGE-SCALE STRUCTURE: EVOLUTION FROM POTENTIAL TO ROTATIONAL FLOW

As an alternative way of describing the cosmological velocity field, we discuss the evolution of rotational invariants constructed from the velocity gradient tensor. Compared with the traditional divergence-vorticity decomposition, these invariants, defined as coefficients of characteristic equation of the velocity gradient tensor, enable a complete classification of all possible flow patterns in the dark-matter comoving frame, including both potential and vortical flows. We show that this tool, first introduced in turbulence two decades ago, proves to be very useful in understanding the evolution of the cosmic web structure, and in classifying its morphology. Before shell-crossing, different categories of potential flow are highly associated with cosmic web structure, because of the coherent evolution of density and velocity. This correspondence is even preserved at some level when vorticity is generated after shell-crossing. The evolution from the potential to vortical flow can be traced continuously by these invariants. With the help of this tool, we show that the vorticity is generated in a particular way that is highly correlated with the large-scale structure. This includes a distinct spatial distribution and different types of alignment between cosmic web and vorticity direction for various vortical flows. Incorporating shell-crossing into closed dynamical systems is highly non-trivial, but we propose a possible statistical explanation for some of these phenomena relating to the internal structure of the three-dimensional invariants space.

Halo Models of Large Scale Structure and Reliability of Cosmological N-Body Simulations

Halo models of the large scale structure of the Universe are critically examined, focusing on the definition of halos as smooth distributions of cold dark matter. This definition is essentially based on the results of cosmological N-body simulations. By a careful analysis of the standard assumptions of halo models and N-body simulations and by taking into account previous studies of self-similarity of the cosmic web structure, we conclude that N-body cosmological simulations are not fully reliable in the range of scales where halos appear. Therefore, to have a consistent definition of halos is necessary either to define them as entities of arbitrary size with a grainy rather than smooth structure or to define their size in terms of small-scale baryonic physics.

The initial conditions of the Universe from constrained simulations

Abstract I present a new approach to recover the primordial density fluctuations and the cosmic web structure underlying a galaxy distribution. The method is based on sampling Gaussian fields which are compatible with a galaxy distribution and a structure formation model. This is achieved by splitting the inversion problem into two Gibbs-sampling steps: the first being a Gaussianization step transforming a distribution of point sources at Lagrangian positions – which are not a priori given– into a linear alias-free Gaussian field. This step is based on Hamiltonian sampling with a Gaussian–Poisson model. The second step consists on a likelihood comparison in which the set of matter tracers at the initial conditions is constrained on the galaxy distribution and the assumed structure formation model. For computational reasons second-order Lagrangian perturbation theory is used. However, the presented approach is flexible to adopt any structure formation model. A semi-analytic halo-model-based galaxy mock catalogue is taken to demonstrate that the recovered initial conditions are closely unbiased with respect to the actual ones from the corresponding N-body simulation down to scales of a ∼5 Mpc h−1. The cross-correlation between them shows a substantial gain of information, being at k ∼ 0.3 h Mpc−1 more than doubled. In addition the initial conditions are extremely well Gaussian distributed and the power spectra follow the shape of the linear power spectrum being very close to the actual one from the simulation down to scales of k ∼ 1 h Mpc−1.

DISCOVERY OF THE FIRST GIANT DOUBLE RADIO RELIC IN A GALAXY CLUSTER FOUND IN THE PLANCK SUNYAEV-ZEL'DOVICH CLUSTER SURVEY: PLCK G287.0+32.9

We report the discovery of large scale diffuse non-thermal radio emission in PLCK G287.0+32.9, an exceptionally hot (T ~ 13 keV), massive and luminous galaxy cluster, strongly detected by the PLANCK satellite in a recent, all-sky blind search for new clusters through Sunyaev-Zel'dovich effect. Giant Metrewave Radio telescope 150 MHz and Very Large Array 1.4 GHz radio data reveal a pair of giant (>1 Mpc) "arc" shaped peripheral radio-relics (signatures of shock waves) of unprecedented scale (linear separation ~4.4 Mpc at redshift 0.39), located at distances from the cluster center which are about 0.7 and 1.3 of the cluster's virial radius. Another possible giant relic and a radio-halo is detected closer to the cluster center. These relic sources are unique "signposts" of extremely energetic mergers and shocks (both ongoing and past), that are assembling and heating up this very massive galaxy cluster. They are also a probe of the filamentary cosmic-web structure beyond the cluster virial radius. Optical imaging with the IUCAA 2 meter telescope and XMM-Newton X-ray data confirm a very rich galaxy cluster with a morphologically disturbed core region, suggesting a dynamically perturbed merging system.

Bayesian non-linear large-scale structure inference of the Sloan Digital Sky Survey Data Release 7

In this work, we present the first non-linear, non-Gaussian full Bayesian large-scale structure analysis of the cosmic density field conducted so far. The density inference is based on the Sloan Digital Sky Survey (SDSS) Data Release 7, which covers the northern galactic cap. We employ a novel Bayesian sampling algorithm, which enables us to explore the extremely high dimensional non-Gaussian, non-linear lognormal Poissonian posterior of the three-dimensional density field conditional on the data. These techniques are efficiently implemented in the Hamiltonian Density Estimation and Sampling (hades) computer algorithm and permit the precise recovery of poorly sampled objects and non-linear density fields. The non-linear density inference is performed on a 750-Mpc cube with roughly 3-Mpc grid resolution, while accounting for systematic effects, introduced by survey geometry and selection function of the SDSS, and the correct treatment of a Poissonian shot noise contribution. Our high-resolution results represent remarkably well the cosmic web structure of the cosmic density field. Filaments, voids and clusters are clearly visible. Further, we also conduct a dynamical web classification and estimate the web-type posterior distribution conditional on the SDSS data.

Fractal analysis of the dark matter and gas distributions in the Mare-Nostrum universe

We develop a method of multifractal analysis of N-bodycosmological simulations that improves on the customary counts-in-cells methodby taking special care of the effects of discreteness and large scalehomogeneity. The analysis of the Mare-Nostrum simulation with our methodprovides strong evidence of self-similar multifractal distributions of darkmatter and gas, with a halo mass function that is of Press-Schechter type buthas a power-law exponent -2, as corresponds to a multifractal. Furthermore,our analysis shows that the dark matter and gas distributions areindistinguishable as multifractals. To determine if there is any gas biasing,we calculate the cross-correlation coefficient, with negative but inconclusiveresults. Hence, we develop an effective Bayesian analysis connected withinformation theory, which clearly demonstrates that the gas is biased in along range of scales, up to the scale of homogeneity. However, entropicmeasures related to the Bayesian analysis show that this gas bias is small (ina precise sense) and is such that the fractal singularities of bothdistributions coincide and are identical. We conclude that this commonmultifractal cosmic web structure is determined by the dynamics and isindependent of the initial conditions.

Statistics and geometry of cosmic voids

We introduce new statistical methods for the study of cosmic voids,focusing on the statistics of largest size voids. We distinguish threedifferent types of distributions of voids, namely, Poisson-like,lognormal-like and Pareto-like distributions. The last two distributions areconnected with two types of fractal geometry of the matterdistribution. Scaling voids with Pareto distribution appear in fractaldistributions with box-counting dimension smaller than three (its maximumvalue), whereas the lognormal void distribution corresponds to multifractalswith box-counting dimension equal to three. Moreover, voids of the formertype persist in the continuum limit, namely, as the number density ofobservable objects grows, giving rise to lacunar fractals, whereas voids ofthe latter type disappear in the continuum limit, giving rise to non-lacunar(multi)fractals. We propose both lacunar and non-lacunar multifractal modelsof the cosmic web structure of the Universe. A non-lacunar multifractal modelis supported by current galaxy surveys as well as cosmological N-bodysimulations. This model suggests, in particular, that small dark matter halosand, arguably, faint galaxies are present in cosmic voids.

Cosmic cartography of the large-scale structure with Sloan Digital Sky Survey data release 6

We present the largest Wiener reconstruction of the cosmic density field made to date. The reconstruction is based on the Sloan Digital Sky Survey data release 6 covering the northern Galactic cap. We use a novel supersampling algorithm to suppress aliasing effects and a Krylov-space inversion method to enable high performance with high resolution. These techniques are implemented in the ARGO computer code. We reconstruct the field over a 500 Mpc cube with Mpc grid-resolution while accounting both for the angular and radial selection functions of the SDSS, and the shot noise giving an effective resolution of the order of ~10 Mpc. In addition, we correct for the redshift distortions in the linear and nonlinear regimes in an approximate way. We show that the commonly used method of inverse weighting the galaxies by the corresponding selection function heads to excess noise in regions where the density of the observed galaxies is small. It is more accurate and conservative to adopt a Bayesian framework in which we model the galaxy selection/detection process to be Poisson-binomial. This results in heavier smoothing in regions of reduced sampling density. Our results show a complex cosmic web structure with huge void regions indicating that the recovered matter distribution is highly non-Gaussian. Filamentary structures are clearly visible on scales up to ~20 Mpc. We also calculate the statistical distribution of density after smoothing the reconstruction with Gaussian kernels of different radii r_S and find good agreement with a log-normal distribution for ~10 Mpc < r_S < ~30 Mpc.