1,721,238 research outputs found
Searching for AGN-driven Shocks in Galaxy Clusters
No full textShocks and blast waves are conceivably driven into the intracluster medium filling galaxy groups and clusters by powerful outbursts of active galactic nuclei or quasars in the member galaxies; the first footprints of shock fronts have been tentatively traced out with X-ray imaging. We show how overpressures in the blasts behind the shock can prove the case and also provide specific marks of the nuclear activity: its strength, its current stage, and the nature of its prevailing output. We propose to detect these marks with the aimed pressure probe constituted by the resolved Sunyaev-Zel'dovich effect. We compute and discuss the outcomes to be expected in nearby and distant sources at different stages of their activity
Probing Structure and History of Dark Matter Halos with Gravitational Lensing Observations
No full textWe discuss the structure and history of dark matter halos in galaxies and galaxy systems, in terms of the halo two-stage development and of the related ‘α-profiles’. We probe both with the recent extensive dataset from weak and strong lensing observations in and around the cluster A1689
A Stochastic Theory of the Hierarchical Clustering. I. Halo Mass Function
No full textWe present a new theory for the hierarchical clustering of dark matter (DM) halos, based on stochastic differential equations, that constitutes a change of perspective with respect to existing frameworks (e.g., the excursion set approach); this work is specifically focused on the halo mass function. First, we present a stochastic differential equation that describes fluctuations in the mass growth of DM halos, as driven by a multiplicative white (Gaussian) noise dependent on the spherical collapse threshold and on the power spectrum of DM perturbations. We demonstrate that such a noise yields an average drift of the halo population toward larger masses, that quantitatively renders the standard hierarchical clustering. Then, we solve the Fokker–Planck equation associated to the stochastic dynamics, and obtain the Press & Schechter mass function as a (stationary) solution. Moreover, generalizing our treatment to a mass-dependent collapse threshold, we obtain an exact analytic solution capable of fitting remarkably well the N-body mass function over a wide range in mass and redshift. All in all, the new perspective offered by the theory presented here can contribute to a better understanding of the gravitational dynamics leading to the formation, evolution, and statistics of DM halos across cosmic times
A Stochastic Theory of the Hierarchical Clustering. II. Halo Progenitor Mass Function and Large-scale Bias
No full textWe generalize the stochastic theory of hierarchical clustering presented in Paper I by Lapi & Danese to derive the (conditional) halo progenitor mass function and the related large-scale bias. Specifically, we present a stochastic differential equation that describes fluctuations in the mass growth of progenitor halos of given descendant mass and redshift, as driven by a multiplicative Gaussian white noise involving the power spectrum and the spherical collapse threshold of density perturbations. We demonstrate that, as cosmic time passes, the noise yields an average drift of the progenitors toward larger masses, which quantitatively renders the expectation from the standard extended Press and Schechter (EPS) theory. We solve the Fokker-Planck equation associated with the stochastic dynamics, and obtain as an exact, stationary solution, the EPS progenitor mass function. Then we introduce a modification of the stochastic equation in terms of a mass-dependent collapse threshold modulating the noise, and solve analytically the associated Fokker-Planck equation for the progenitor mass function. The latter is found to be in excellent agreement with the outcomes of N-body simulations; even more remarkably, this is achieved with the same shape of the collapse threshold used in Paper I to reproduce the halo mass function. Finally, we exploit the above results to compute the large-scale halo bias, and find it in pleasing agreement with the N-body outcomes. All in all, the present paper illustrates that the stochastic theory of hierarchical clustering introduced in Paper I can describe effectively not only halos' abundance, but also their progenitor distribution and their correlation with the large-scale environment across cosmic times
Self-Similar Dynamical Relaxation of Dark Matter Halos in an Expanding Universe
No full textWe investigate the structure of cold dark matter halos using advanced models of spherical collapse and accretion in an expanding universe. These are based on solving time-dependent equations for the moments of the phase-space distribution function in the fluid approximation; our approach includes non-radial random motions and, most importantly, an advanced treatment of both dynamical relaxation effects that take place in the infalling matter: phase-mixing associated with shell crossing and collective collisions related to physical clumpiness. We find self-similar solutions for the spherically averaged profiles of mass density ρ(r), pseudo phase-space density Q(r), and anisotropy parameter β(r). These profiles agree with the outcomes of state-of-the-art N-body simulations in the radial range currently probed by the latter; at smaller radii, we provide specific predictions. In the perspective provided by our self-similar solutions, we link the halo structure to its two-stage growth history and propose the following picture. During the early fast collapse of the inner region dominated by a few merging clumps, efficient dynamical relaxation plays a key role in producing closely universal mass density and pseudo phase-space density profiles; in particular, these are found to depend only weakly on the detailed shape of the initial perturbation and the related collapse times. The subsequent inside-out growth of the outer regions feeds on the slow accretion of many small clumps and diffuse matter; thus the outskirts are only mildly affected by dynamical relaxation but are more sensitive to asymmetries and cosmological variance
Dark Matter Halos: The Dynamical Basis of Effective Empirical Models
Full text linkWe investigate the dynamical basis of the classic empirical models (specifically, Sérsic-Einasto
and generalized NFW) that are widely used to describe the distributions of collisionless matter in
galaxies. We submit that such a basis is provided by our α-profiles, shown to constitute solutions
of the Jeans dynamical equilibrium with physical boundary conditions. We show how to set the
parameters of the empirical in terms of the dynamical models; we find the empirical models,
and specifically Sérsic-Einasto, to constitute a simple and close approximation to the dynamical
models. Finally, we discuss how these provide a useful baseline for assessing the impact of the
small-scale dynamics that may modulate the density slope in the central galaxy regions
Structure and history of dark matter halos probed with gravitational lensing
No full textWe test with gravitational lensing (GL) data the dark matter (DM) halos embedding the luminous baryonic component of galaxy clusters; our benchmark is provided by their two-stage cosmogonical development that we compute with its variance, and by the related "α-profiles" we derive. The latter solve the Jeans equation for the self-gravitating, anisotropic DM equilibria, and yield the radial runs of the density ρ(r) and the velocity dispersion σ2 r (r) in terms of the DM "entropy" K ≡ σ2 r /ρ2/3 r α highlighted by recent N-body simulations; the former constrains the slope to the narrow range α 1.25-1.3. These physically based α-profiles meet the overall requirements from GL observations, being intrinsically flatter at the center and steeper in the outskirts relative to the empirical Navarro, Frenk, & White formula. Specifically, we project them along the line of sight and compare with a recent extensive data set from strong and weak lensing observations in and around the cluster A1689. We find an optimal fit at both small and large scales in terms of a halo constituted by an early body with α 1.25 and by recent extensive outskirts, that make up an overall mass 1015 M ☉ with a concentration parameter c 10 consistent with the variance we compute in the ΛCDM cosmogony. The resulting structure corresponds to a potential well shallow in the outskirts as that inferred from the X rays radiated from the hot electrons and baryons constituting the intracluster plasma
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