1,721,058 research outputs found
"Ho vedute delle pitture di Giotto e di Taddeo Gaddi che non avevo mai studiate, o avvertite": il cammino verso i primitivi
No full textnell'ambito di una nuova considerazione del collezionismo dei priitivi, a distanza di 50 anni dal fondamentale saggio di Giovanni Previtali, il saggio affronta l'analisi della "sensibilità all'antico" sviluppatasi tra Settecento e Ottocento. Una particolare attenzione è rivolta alla politica museale della Galleria e alla fortuna visiva della pittura medioevale tramite la stampa di traduzione
A detailed description of the subhalo population of the Milky Way
No full textIn the standard model of cosmic structure formation, dark matter haloes form by gravitational instability. The process is hierarchical: smaller systems collapse earlier, and later merge to form larger halos. The probability that a halo of mass m at redshift z will be part of a larger halo of mass M at the present time is described by the progenitor (conditional) mass function f(m,z|M,0), according to the so called extended Press & Schechter theory. Using the progenitor mass function we calculate analytically, at redshift zero, the distribution of subhalos in mass, formation epoch and rarity of the peak of the density field at the formation epoch. That is done for a Milky Way-size system, assuming both a spherical and an ellipsoidal collapse model. Our calculation assumes that small progenitors do not loose mass due to dynamical processes after entering the parent halo, nor that they interact with other subhalos. For a LCDM power spectrum we obtain a subhalo mass function dn/dm proportional to malpha with a model-independent alpha~2, confirming what is found in N-body simulations. The inferred distributions can be used to test the feasibility of indirect detection of such a population of subhalos with modern experimental tecniques
Ellipsoidal halo finders and implications for models of triaxial halo formation
No full textWe describe an algorithm for identifying ellipsoidal haloes in numerical simulations, and quantify how the resulting estimates of halo mass and shape differ with respect to spherical halo finders. Haloes become more prolate when fit with ellipsoids, the difference being most pronounced for the more aspherical objects. Although the ellipsoidal mass is systematically larger, this is less than 10 per cent for most of the haloes. However, even this small difference in mass corresponds to a significant difference in shape. We quantify these effects also on the initial mass and deformation tensors, on which most models of triaxial collapse are based.By studying the properties of protohaloes in the initial conditions, we find that models in which protohaloes are identified in Lagrangian space by three positive eigenvalues of the deformation tensor are tenable only at the masses well above M*. The overdensity δ within almost any protohalo is larger than the critical value associated with spherical collapse (increasing as mass decreases); this is in good qualitative agreement with models which identify haloes requiring that collapse has occurred along all three principal axes, each axis having turned around from the universal expansion at a different time. The distributions of initial values are in agreement with the simplest predictions associated with ellipsoidal collapse, assuming initially spherical protohaloes, collapsed around random positions which were sufficiently overdense.However, most protohaloes are not spherical and departures from sphericity increase as protohalo mass decreases. The mass and deformation tensors are well aligned, in agreement with the fundamental assumption of ellipsoidal collapse, and with models which identify haloes with peaks in the initial density fluctuation field. But the direction of maximum initial compression coincides with the direction of what is initially the longest axis, contrary to what the peaks model predicts. By the final time, it is the shortest axis of the final object which tends to be aligned with the direction of initial maximal compression: the alignment changes during the evolution. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society
Comparing the temperatures of galaxy clusters from hydrodynamical N-body simulations to Chandra and XMM-Newton observations
No full textTheoretical studies of the physical processes guiding the formation and
evolution of galaxies and galaxy clusters in the X-ray region are mainly
based on the results of numerical hydrodynamical N-body simulations,
which in turn are often directly compared with X-ray observations.
Although trivial in principle, these comparisons are not always simple.
We demonstrate that the projected spectroscopic temperature of thermally
complex clusters obtained from X-ray observations is always lower than
the emission-weighed temperature, which is widely used in the analysis
of numerical simulations. We show that this temperature bias is mainly
related to the fact that the emission-weighted temperature does not
reflect the actual spectral properties of the observed source. This has
important implications for the study of thermal structures in clusters,
especially when strong temperature gradients, such as shock fronts, are
present. Because of this bias, in real observations shock fronts appear
much weaker than what is predicted by emission-weighted temperature
maps, and may not even be detected. This may explain why, although
numerical simulations predict that shock fronts are a quite common
feature in clusters of galaxies, to date there are very few observations
of objects in which they are clearly seen. To fix this problem we
propose a new formula, the spectroscopic-like temperature function, and
show that, for temperatures higher than 3 keV, it approximates the
spectroscopic temperature to better than a few per cent, making
simulations more directly comparable to observations
I Malvezzi
No full textIl contributo fa luce sulle vicende collezionistiche della quadreria appartenuta al nobiluomo bolognese Piriteo Malvezzi (1734-1806), divisa alla sua morte tra le figlie Teresa Ranuzzi e Maria Hercolani. I documenti conservati nell'archivio privato Hercolani di Bologna provano che il nucleo Malvezzi ereditato da Maria rimase integro fino alla sua cessione, nella seconda metà dell'Ottocento, al musicista Gioacchino Rossini. Attraverso l'analisi dei documenti d'archivio, uno dei quali inedito, è stato possibile identificare buona parte dei pezzi del pregevole nucleo di circa settanta quadri antichi celebrato dalle fonti letterarie e collocato in palazzo Hercolani a Bologna in un ambiente appositamente allestito: la Sala grande dei Quadri Antichi
Turbulent velocity fields in smoothed particle hydrodymanics simulated galaxy clusters: Scaling laws for the turbulent energy
No full textWe present a study of the turbulent velocity fields in the intracluster medium (ICM) of a sample of 21 galaxy clusters simulated by the smoothed particle hydrodynamics code GADGET2, using a new numerical scheme where the artificial viscosity is suppressed outside shocks. The turbulent motions in the ICM of our simulated clusters are detected with a novel method devised to better disentangle laminar bulk motions from chaotic ones. We focus on the scaling law between the turbulent energy content of the gas particles and the total mass, and find that the energy in the form of turbulence scales approximately with the thermal energy of clusters. We follow the evolution with time of the scaling laws and discuss the physical origin of the observed trends. The simulated data are in agreement with independent semi-analytical calculations, and the combination between the two methods allows one to constrain the scaling law over more than two decades in cluster mass. © 2006 The Authors. Journal compilation © 2006 RAS
Turbulent gas motions in galaxy cluster simulations: The role of smoothed particle hydrodynamics viscosity
No full textSmoothed particle hydrodynamics (SPH) employs an artificial viscosity to properly capture hydrodynamic shock waves. In its original formulation, the resulting numerical viscosity is large enough to suppress structure in the velocity field on scales well above the nominal resolution limit, and to damp the generation of turbulence by fluid instabilities. This could artificially suppress random gas motions in the intracluster medium (ICM), which are driven by infalling structures during the hierarchical structure formation process. We show that this is indeed the case by analysing results obtained with an SPH formulation where an individual, time-variable viscosity is used for each particle, following a suggestion by Morris & Monaghan. Using test calculations involving strong shocks, we demonstrate that this scheme captures shocks as well as the original formulation of SPH, but, in regions away from shocks, the numerical viscosity is much smaller. In a set of nine high-resolution simulations of cosmological galaxy cluster formation, we find that this low-viscosity formulation of SPH produces substantially higher levels of turbulent gas motions in the ICM, reaching a kinetic energy content in random gas motions (measured within a 1-Mpc cube) of up to 5-30 per cent of the thermal energy content, depending on cluster mass. This also has significant effects on radial gas profiles and bulk cluster properties. We find a central flattening of the entropy profile and a reduction of the central gas density in the low-viscosity scheme. As a consequence, the bolometric X-ray luminosity is decreased by about a factor of 2. However, the cluster temperature profile remains essentially unchanged. Interestingly, this tends to reduce the differences seen in SPH and adaptive mesh refinement simulations of cluster formation. Finally, invoking a model for particle acceleration by magnetohydrodynamics waves driven by turbulence, we find that efficient electron acceleration and thus diffuse radio emission can be powered in the clusters simulated with the low-viscosity scheme provided that more than 5-10 per cent of the turbulent energy density is associated with fast magneto-sonic modes. © 2005 RAS
A dynamical model for the distribution of dark matter and gas in galaxy clusters
No full textUsing the results of an extended set of high-resolution non-radiative
hydrodynamic simulations of galaxy clusters, we obtain simple analytic
formulae for the dark matter and hot gas distribution, in the spherical
approximation. Starting from the dark matter phase-space radial density
distribution, we derive fits for the dark matter density, velocity
dispersion and velocity anisotropy. We use these models to test the
dynamical equilibrium hypothesis through the Jeans equation: we find
that this is satisfied to good accuracy by our simulated clusters inside
their virial radii. This result also shows that our fits constitute a
self-consistent dynamical model for these systems.
We then extend our analysis to the hot gas component, obtaining analytic
fits for the gas density, temperature and velocity structure, with no
further hypothesis on the gas dynamical status or state equation. Gas
and dark matter show similar density profiles down to ~0.06Rv
(with Rv the virial radius), while at smaller radii the gas
flattens, producing a central core. Gas temperatures are almost
isothermal out to roughly 0.2 Rv, then steeply decrease,
reaching at the virial radius a value almost a factor of 2 lower. We
find that the gas is not at rest inside Rv: velocity
dispersions are increasing functions of the radius, motions are
isotropic to slightly tangential, and contribute non-negligibly to the
total pressure support. We test this model using a generalization of the
hydrostatic equilibrium equation, where the gas motion is properly taken
into account. Again we find that the fits provide an accurate
description of the system: the hot gas is in equilibrium and is a good
tracer of the overall cluster potential if all terms (density,
temperature and velocity) are taken into account, while simpler
assumptions cause systematic mass underestimates. In particular, we find
that using the so-called β-model underestimates the true cluster
mass by up to 50 per cent at large radii. We also find that, if gas
velocities are neglected, then a simple isothermal model fares better at
large radii than a non-isothermal one.
The shape of the gas density profile at small radii is at least
partially explained by the gas expansion caused by energy transfer from
dark matter during the collapse. In fact, when gas bulk energy is also
considered, gas and dark matter are in energy equipartition in the final
system at radii r > 0.1Rv, while at smaller radii the gas
is hotter than the dark matter. This energy imbalance is also probably
the reason of the further global halo compression compared with a pure
collisionless collapse, which we point out by comparing the dark matter
and total density profiles of our hydro-simulated clusters with a set of
identical - but pure N-body - ones. The compression has the effect of
raising the mean concentration by an amount of roughly 10 per cent
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