Dmitri Pogosyan

How filaments of galaxies are woven into the cosmic web

LARGE-SCALE structure in the distribution of galaxies is thought to have evolved through gravitational instabilities from small density fluctuations in the (largely homogeneous) early Universe. This structure of galaxies consists of rich and poor clusters, connected by filaments and sheets, with regions largely devoid of galaxies (voids) in between1. Numerical simulations of the growth of initial density fluctuations through a nonlinear regime, motivated by the likely physics of the early Universe, also show a network of filaments and voids2,3,18, but the origin of this picture of filaments as the dominant structure was not well understood. Here we show that the web of filaments that defines the final state in these simulations is present in the initial density fluctuations; the pattern of the web is defined largely by the rare density peaks in the initial fluctuations, with the subsequent nonlinear evolution of the structure bringing the filamentary network into sharper relief. Applying these results to the observed galaxy distribution, we suggest that superclusters are filamentary cluster–cluster bridges, and we predict that the most pronounced filaments will be found between clusters of galaxies that are aligned with each other and close together.

Coherent structures in the universe and the adhesion model

An adhesion model is used to study the formation process of large-scale structures due to nonlinear gravitational growth of small initial fluctuations in the universe dominated by dark matter. The model is compared with 2D N-body simulations with initial power-law spectral indices n = -2, 0, +2, and various cutoffs. It is found that the adhesion model imitates the skeleton of the structure extremely well for the parameters of the initial spectra until the stage when the nonlinear scale reaches the correlation length R(phi) of the initial gravitational potential. The model explains the origin of large-scale coherent sructures, such as superpancakes and superfilaments, as a result of coherent motion of clumps due to large-scale inhomogeneities in the initial gravitational potential. It is found that clumps of mass identified in the N-body simulations correspond to several knots in the adhesion model, which influences the way of calculating the mass distribution function. The distribution functions of velocities and masses of clumps and areas of cells in the adhesion model satisfy self-similar scaling laws of the n = 2 model.

EMISSIVITY STATISTICS IN TURBULENT COMPRESSIBLE MAGNETOHYDRODYNAMIC FLOWS AND THE DENSITY-VELOCITY CORRELATION

In this paper we test the results of a recent analytical study by Lazarian and Pogosyan on the statistics of emissivity in velocity channel maps, in the case of realistic density and velocity —elds obtained from numerical simulations of magnetohydrodynamic turbulence in the interstellar medium. To compensate for the lack of well-developed inertial ranges in the simulations owing to the limited resolution, we apply a procedure for modifying the spectral slopes of the —elds while still preserving the spatial structures. We —nd that the density and velocity are moderately correlated in space, and we prove that the analytical results by Lazarian and Pogosyan hold in the case when these —elds obey the —uid conservation equations. Our results imply that the spectra of velocity and density can be safely recovered from the position-position-velocity (PPV) data cubes available through observations and con—rm that the relative contributions of the velocity and density —uctuations to those of the emissivity depend on the velocity resolution used and on the steepness of the density spectral index. Furthermore, this paper supports previous reports that an interpretation of the features in the PPV data cubes as simple density enhancements (i.e., ii clouds ˇˇ) can often be erroneous, as we observe that changes in the velocity statistics substantially modify the emissivity statistics within the velocity data cubes. & ,

Gravitational instability in the strongly non-linear regime: a study of various approximations

We study the development of gravitational instability in the strongly non-linear regime. For this purpose we use a number of statistical indicators such as filamentary statistics, the spectrum of overdense/underdense regions and the void probability function, each of which probes a particular aspect of gravitational clustering. We use these statistical indicators to discriminate between different approximations to gravitational instability which we test against N-body simulations. The approximations that we test are the truncated Zeldovich approximation (TZ), the adhesion approximation (AA), and the frozen flow (FF) and linear potential (LP) approximations. Of these we find that FF and LP break down relatively early, soon after the non-linear length scale exceeds R_* - the mean distance between peaks of the gravitational potential. The reason for this breakdown is easy to understand: particles in FF are constrained to follow the streamlines of the initial velocity field. Shell crossing is absent in this case and structure gradually freezes as particles begin to collect near minima of the gravitational potential. In LP, particles follow the lines of force of the primordial potential, oscillating about its minima at late times when the non-linear length scale k^-1_NL~=R_*. Unlike FF and LP, the adhesion model (and to some extent TZ) continues to give accurate results even at late times when k^-1_NL>=R_*. This is because both AA and TZ use the presence of long-range modes in the gravitational potential to move particles. Thus, as long as the initial potential has sufficient long-range power to initiate large-scale coherent motions, TZ and AA will remain approximately valid. In relation to AA, TZ suffers from a single major drawback - it underestimates the presence of small clumps. Similarly, it predicts the right mean density in large voids but misses subcondensations within them. The reason for this is clear: the artificial removal of power on scales smaller than k^-1_NL in the initial potential in TZ, designed to prevent shell crossing, causes a substantial fraction of matter (which would have been clustered in N-body simulations) to lie within low-density regions at all epochs. On the other hand, TZ is very fast to implement and more accurately predicts the location of large objects at late times; AA more correctly represents the subcondensations but does not always accurately predict their positions.

Mixed dark matter in halos of clusters

We discuss the structure of clusters in a class of flat cosmological models with the fraction of mass Omega_{CDM} ~0.8 in cold dark matter, and the rest in hot dark matter in the form of massive neutrinos. We consider such Cold+Hot Dark Matter (CHDM) models with one, two or three massive neutrinos, with total mass ~4.6eV. Neutrinos of such low mass cannot constitute halos of galaxies and groups, but only of clusters of galaxies. The limit on the density of neutrinos in the central parts of galaxy clusters is estimated from the phase space den- sity constraints. The ratio of the neutrino density to that of CDM through the cluster is found analytically. It appears that the density of neutrinos is suppressed within the Abell radius. However, neutrinos contribute ~20% of the mass density to the cluster halo. nOur numerical simulations match analytical results. The simulations indicate that the cluster halo dark matter density profile has the power-law slope ~-2.5 which is close to that in the model with cosmological constant. We also found that in the CHDM models the velocity dispersion is almost constant across the cluster. This is quite different from the model with cosmological constant or the open model where the velocity dispersion falls in the cluster outskirts. nWe discuss X-ray emission and weak gravitational lensing by clusters in the model. We input the found spherically symmetrical fit to the CHDM mass density profile and the X-ray surface brightness for the cluster A2256 into simple equation of hydrostatic equilibrium of the hot gas. X-ray temperature derived this way departures from both the data and actual prediction of the model, which give almost constant temperature. We found also that the problem of high baryonic fraction in clusters is not resolved in the CHDM models.

COMPUTING CMB ANISOTROPY IN COMPACT HYPERBOLIC SPACES

The measurements of CMB anisotropy have opened up a window for probing the global topology of the universe on length scales comparable to and beyond the Hubble radius. For compact topologies, the two main effects on the CMB are: (i) the breaking of statistical isotropy in characteristic patterns determined by the photon geodesic structure of the manifold and (ii) an infrared cut-off in the power spectrum of perturbations imposed by the finite spatial extent. We present a completely general scheme using the regularized method of images for calculating CMB anisotropy in models with non-trivial topology, and apply it to the computationally challenging compact hyperbolic topologies. This new technique eliminates the need for the difficult task of spatial eigenmode decomposition on these spaces. We estimate a Bayesian probability for a selection of models by confronting the theoretical pixel-pixel temperature correlation function with the COBE-DMR data. Our results demonstrate that strong constraints on compactness arise: if the universe is small compared to the `horizon size, correlations appear in the maps that are irreconcilable with the observations. If the universe is of comparable size, the likelihood function is very dependent upon orientation of the manifold with respect to the sky. While most orientations may be strongly ruled out, it sometimes happens that for a specific orientation the predicted correlation patterns are preferred over the conventional infinite models.

Beyond Kaiser bias: mildly non-linear two-point statistics of densities in distant spheres

Simple parameter-free analytic bias functions for the two-point correlation of densities in spheres at large separation are presented. These bias functions generalize the so-called Kaiser bias to the mildly non-linear regime for arbitrary density contrasts as b(ρ) − b(1) ∝ (1 − ρ

INTERSTELLAR FILAMENTS AND THE STATISTICS OF GALACTIC H I

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Non Gaussian Minkowski functionals and extrema counts for CMB maps

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Cosmic Web: Origin and Observables

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