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The present day universe consists of galaxies, galaxy clusters, one-dimensional filaments and two-dimensional sheets or pancakes, all of which combine to form the cosmic web. The so called Zeldovich pancakes, are very difficult to observe, because their overdensity is only slightly greater than the average density of the universe. Falco et al (2014) presented a method to identify Zeldovich pancakes in observational data, and these were used as a tool for estimating the mass of galaxy clusters. Here we expand and refine that observational detection method. We study two pancakes on scales of 10 Mpc, identified from spectroscopically observed galaxies near the Coma cluster, and compare with twenty numerical pancakes. We find that the observed structures have velocity dispersion about 100 km/sec, which is relatively low compared to typical groups and filaments. These velocity dispersions are consistent with those found for the numerical pancakes. We also confirm that the identified structures are in fact two-dimensional structures. Finally, we estimate the stellar to total mass of the observational pancakes to be $2 times 10^{-4}$, within one order of magnitude, which is smaller than that of clusters of galaxies.
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If the large scale structure of the Universe was created, even partially, via Zeldovich pancakes, than the fluctuations of the CMB radiation should be formed due to bulk comptonization of black body spectrum on the contracting pancake. Approximate formulas for the CMB energy spectrum after bulk comptonization are obtained. The difference between comptonized energy spectra of the CMB due to thermal and bulk comptonization may be estimated by comparison of the plots for the spectra in these two cases.
One of the components of the cosmic web are sheets, which are commonly referred to as Zeldovich pancakes. These are structures which have only collapsed along one dimension, as opposed to filaments or galaxies and cluster, which have collapsed along two or three dimensions. These pancakes have recently received renewed interest, since they have been shown to be useful tools for an independent method to determine galaxy cluster masses. We consider sheet-like structures resulting from cosmological simulations, which were previously used to establish the cluster mass determination method, and we show through their level of equilibration, that these structures have indeed only collapsed along the one dimension. We also extract the density profiles of these pancake, which agrees acceptably well with theoretical expectations. We derive the observable velocity distribution function (VDF) analytically by generalizing the Eddington method to one dimension, and we compare with the distribution function from the numerical simulation.
The evolution of a planar perturbation in a Einstein-de Sitter Universe is studied using a previously introduced Lagrangian scheme. An approximate discrete dynamical system is derived, which describes the mass agglomeration process qualitatively. Quantitative predictions for the late density profile are obtained therefrom, and validated by numerical simulations. The main result is a scaling regime for the density profile of a collapsing object of mass $M$ around cosmological coordinate $r^*$, $rho(r)sim frac{M}{d}(frac{|r-r^*|}{d})^{-{1/4}}$. The characteristic scale of the agglomeration, $dsim (t/t_0)^{{4/9}}$, is an increasing function of cosmological time $t$. The major part of the mass hence always lies in a region with decreasing mass density. This shows that one-dimensional self-gravitating motion is not sufficient to effectively drive structure formation in an Einstein-de Sitter Universe. These results are compared with analogous investigations for the adhesion model (Burgers equation with positive viscosity), where the agglomeration is faster, and one-dimensional dynamics is effective. We further study the mutual motion of two mass agglomerations, and show that they oscillate around each other for long times, like two ``heavy particles. Individual particles in the two agglomerations do not mix effectively on the time scale of the interagglomeration motion.
We explore the potential of the kinetic Sunyaev-Zeldovich (kSZ) effect as the cornerstone of a future observational probe for halo spin bias, the secondary dependence of halo clustering on halo spin at fixed halo mass. Using the IllustrisTNG magneto-hydrodynamical cosmological simulation, we measure both the kSZ and the thermal SZ (tSZ) effects produced by the baryonic content of more than 50,000 haloes within the halo mass range $11 < log_{10} ({rm M_{vir}}/ h^{-1} {rm M_{odot}}) lesssim 14.5$. First, we confirm that the magnitude of both effects depends strongly on the total gas and virial mass of the haloes, and that the integrated kSZ signal displays a significant correlation with the angular momentum of the intra-halo gas, particularly for massive haloes. Second, we show that both the integrated kSZ signal and the ratio of the integrated kSZ and tSZ signals trace total halo spin, even though significant scatter exists. Finally, we demonstrate that, in the absence of observational and instrumental uncertainties, these SZ-related statistics can be used to recover most of the underlying IllustrisTNG halo spin bias signal. Our analysis represents the first attempt to develop a future observational probe for halo spin bias, bringing forward alternative routes for measuring the secondary bias effects.
The physical and evolutionary relation between growing supermassive black holes (AGN) and host galaxies is currently the subject of intense research activity. Nevertheless, a deep theoretical understanding of such a relation is hampered by the unique multi-scale nature of the combined AGN-galaxy system, which defies any purely numerical, or semi-analytic approach. Various physical process active on different scales have signatures in different parts of the electromagnetic spectrum; thus, observations at different wavelengths and theoretical ideas all should contribute towards a large dynamic range view of the AGN phenomenon. As an example, I will focus in this review on two major recent observational results on the cosmic evolution of supermassive black holes, focusing on the novel contribution given to the field by the COSMOS survey. First of all, I will discuss the evidence for the so-called downsizing in the AGN population as derived from large X-ray surveys. I will then present new constraints on the evolution of the black hole-galaxy scaling relation at 1<z<2 derived by exploiting the full multi-wavelength coverage of the survey on a complete sample of ~90 type 1 AGN.
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