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Aims. Our goal is to determine how the spatial correlation function of galaxies describes biasing and fractal properties of the cosmic web. Methods. We calculated spatial correlation functions of galaxies, ξ(r), structure functions, g(r) = 1 + ξ(r), gradient functions, γ(r) = d log g(r)/d log r, and fractal dimension functions, D(r) = 3 + γ(r), using dark matter particles of the biased Λ cold dark matter (CDM) simulation, observed galaxies of the Sloan Digital Sky Survey (SDSS), and simulated galaxies of the Millennium and EAGLE simulations. We analysed how these functions describe fractal and biasing properties of the cosmic web. Results. The correlation functions of the biased ΛCDM model samples at small distances (particle and galaxy separations), r ≤ 2.25 h−1 Mpc, describe the distribution of matter inside dark matter halos. In real and simulated galaxy samples, only the brightest galaxies in clusters are visible, and the transition from clusters to filaments occurs at a distance r ≈ 0.8−1.5 h−1 Mpc. At larger separations, the correlation functions describe the distribution of matter and galaxies in the whole cosmic web. The effective fractal dimension of the cosmic web is a continuous function of the distance (separation). Real and simulated galaxies of low luminosity, Mr ≥ −19, have almost identical correlation lengths and amplitudes, indicating that dwarf galaxies are satellites of brighter galaxies, and do not form a smooth population in voids. Conclusions. The combination of several physical processes (e.g. the formation of halos along the caustics of particle trajectories and the phase synchronisation of density perturbations on various scales) transforms the initial random density field to the current highly non-random density field. Galaxy formation is suppressed in voids, which increases the amplitudes of correlation functions and power spectra of galaxies, and increases the large-scale bias parameter. The combined evidence leads to the large-scale bias parameter of L⋆ galaxies the value b⋆ = 1.85 ± 0.15. We find r0(L⋆) = 7.20 ± 0.19 for the correlation length of L⋆ galaxies. [ABSTRACT FROM AUTHOR] more...

Context. Active galactic nuclei (AGN) are known to account for a major portion, if not all, of the cosmic X-ray background (CXB) radiation. The dominant sharp spectral feature in their spectra is the 6.4 keV fluorescent line of iron, which may contribute to as much as ~5−10% of the CXB spectral intensity at ~2−6 keV. Owing to cosmological redshift, the line photons detected at the energy E carry information about objects located at the redshift z = 6.4/E − 1. In particular, imprinted in their angular fluctuations is the information about the large-scale structure at redshift z. This opens the possibility of performing the Fe Kα line tomography of the cosmic large-scale structure. Aims. The goal of this paper is to investigate the feasibility of the Fe Kα line tomography of the large-scale structure. Methods. At any observed energy E, the 6.4 keV line photons are blended with continuum emission, which originates in objects located at many different redshifts and therefore contaminates and dilutes the tomographic signal. However, its contribution can be removed by doing observations at two nearby energy intervals and by calculating the power spectrum of the corresponding differential signal map. Results. We show that detection of the tomographic signal at ≳100σ confidence requires an all-sky survey by an instrument with an effective area of ~10 m2 and field of view of ~1deg2. The signal is strongest for objects located at the redshift z ~ 1 and at the angular scales corresponding to ℓ ~ 100−300, therefore an optimal detection can be achieved with an instrument having a rather modest angular resolution of ~0.1−0.5deg. For such an instrument, the CCD-type energy resolution of ~100−200 eV FWHM is entirely sufficient for the optimal separation of the signals coming from different redshifts. The gain in the signal strength that could potentially be achieved with energy resolution comparable to the line width is nullified by the photon counting and AGN discreteness noise. Among the currently planned and proposed missions, these requirements are best satisfied by LOFT, even though that it was proposed for an entirely different purpose. Among others, clear detection should be achieved by WFXT (~25−40σ) and ATHENA (~20−30σ). [ABSTRACT FROM AUTHOR] more...