122,396 research outputs found
Properties of cluster satellites in hydrodynamical simulations
No full textWe analyse the dynamical and thermal evolution of dark matter and the
intracluster medium in hydrodynamical N-body simulations of galaxy
clusters. Starting from a sample of 17 high-resolution objects, with
virial mass ranging from 3 × 1014 to 1.7 ×
1015h-1 Msolar, we follow the build-up
of the systems in dark matter and hot gas through the repeated merging
of satellites along their merging history trees. We measure the
self-bound mass fraction of subhaloes as a function of time after the
merging, estimate the satellite mean orbital properties as a function of
the mass ratio with the main cluster at merging time, and study the
evolution of their internal velocity dispersion, gas temperature and
entropy as the substructure is disrupted by various dynamical processes,
eventually reaching thermodynamic equilibrium in the gravitational
potential of the main cluster. We model some relevant properties of
subhalo orbits, as the time of the first pericentric and apocentric
passages, and the typical distances and velocities at the corresponding
times. This survival study can be used to interpret the dynamics of
observed merging clusters; as an example, we apply our results to the
system 1E0657-56. We show that, in the light of our results, the most
likely interpretation of the data for this cluster points to the merger
of a small group with mass M~ 1 × 1013h-1
Msolar with a massive cluster with M~ 1.3 ×
1015h-1 Msolar
Adding long-wavelength modes to an N-body simulation
No full textWe present a new method to add long-wavelength power to an evolved N-body simulation, making use of the Zeldovich approximation to change positions and velocities of particles. We describe the theoretical framework of our technique and apply it to a P3M cosmological simulation performed on a cube of 100 Mpc on a side, obtaining a new "simulation" of 800 Mpc on a side. We study the effect of the power added by long waves by means of several statistics of the density and velocity field, and we suggest possible applications of our method to the study of the large-scale structure of the universe
Biodiversity Friend® Beekeeping: una nuova certificazione per gli apicoltori, le api ed i consumatori
No full textSerendipity versus proactive search of elusive species - the Encounter Predictability Scorecard (EPS), a new customizable tool for field researchers
No full textSince field research requires a lot of effort, time, economic and resource investment, it necessitates fact-based tools for a sound preliminary evaluation of the actual possibility to achieve its objective. Such a tool, the Encounter Predictability Scorecard (EPS), is here described for the first time. The rediscovery of the endemic Orthopteran Uromenus annae proved that field research is performed under strong biases including blind faith in previous scientific literature, and expectations about the biology and ecology of the target species. U. annae escaped field researches in the documented localities, and was rediscovered serendipitously in two new unrelated locations. This casts doubt on the capacity of field researchers to assess, even in general terms, the possibility of success of field expeditions. We conceived a method inspired by the performance management tools from the world of corporate strategy: scorecards. The most famous among closed-choice, qualitative-quantitative checklists, is the Balanced Scorecard, based on original work from the late 1980’s. We adapted those methods to the constraints of field research, and field-tested in a retrospective way for the search of U. annae. The EPS is freely available as a digital spreadsheet, and can be tested and customised at any time. Although in its infancy, the EPS looks like a promising operational tool to help saving time and money, and to identify which objectives and organizational setups are more promising. Besides providing a clearer, more rational basis for operational decisions, the straightforward compilation of an EPS may also mitigate the impact of cognitive biases
The assembly of matter in galaxy clusters
No full textWe study the merging history of dark matter haloes that end up in rich clusters, using N-body simulations of a scale-free universe. We compare the predictions of the extended Press & Schechter (P&S) formalism with several conditional statistics of the protocluster matter: the mass distribution and relative abundance of progenitor haloes at different redshifts, the infall rate of progenitors within the protocluster, the formation redshift of the most massive cluster progenitor, and the accretion rates of other haloes on to it. The high quality of our simulations allows an unprecedented resolution in the mass range of the studied distributions. We also present the global mass function for the same cosmological model. We find that the P&S formalism and its extensions cannot simultaneously describe the global evolution of clustering and its evolution in a protocluster environment. The best-fitting P&S model for the global mass function is a poor fit to the statistics of cluster progenitors. This discrepancy is in the sense of underpredicting the number of high-mass progenitors at high redshift. Although the P&S formalism can provide a good qualitative description of the global evolution of hierarchical clustering, particular attention is needed when applying the theory to the mass distribution of progenitor objects at high redshift
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
The rise and fall of satellites in galaxy clusters
No full textWe use N-body simulations to study the infall of dark matter haloes on to rich clusters of galaxies. After identification of all cluster progenitors in the simulations, we select those haloes that accrete directly on to the main cluster progenitor. We construct the mass function of these merging satellites, and calculate the main orbital parameters for the accreted lumps. The average circularity of the orbits is epsilon~=0.5, while either radial or almost circular orbits are equally avoided. More massive satellites move along slightly more eccentric orbits, with lower specific angular momentum and a smaller pericentre. We find that the infall of satellites on to the main cluster progenitor has a very anisotropic distribution. This anisotropy is to a large extent responsible for the shape and orientation of the final cluster and of its velocity ellipsoid. At the end of the simulations, the major axis of the cluster is aligned both with that of its velocity ellipsoid and with the major axis of the ellipsoid defined by the satellite infall pattern, to ~30 deg on average. We also find that, in lower mass clusters, a higher fraction of the final virial mass is provided by small, dense satellites. These sink to the centre of the parent cluster and so enhance its central density. This mechanism is found to be partially responsible for the correlation between halo masses and characteristic overdensities, recently highlighted by Navarro, Frenk & White
Collisional dark matter and the structure of dark halos
No full textWe study how the internal structure of dark halos is affected if cold dark matter particles are assumed to have a large cross section for elastic collisions. We identify a cluster halo in a large cosmological N-body simulation and resimulate its formation with progressively increasing resolution. We compare the structure found in the two cases in which dark matter is treated as collisionless or as a fluid. For the collisionless case, the overall ellipticity of the cluster, the central density cusp, and the amount of surviving substructure are all similar to those found in earlier high-resolution simulations. Collisional dark matter results in a cluster that is more nearly spherical at all radii, has a steeper central density cusp, and has less-but still substantial-surviving substructure. As in the collisionless case, these results for a ``fluid'' cluster halo are expected to carry over approximately to smaller mass systems. The observed rotation curves of dwarf galaxies then argue that self-interacting dark matter can only be viable if intermediate cross sections produce structure that does not lie between the extremes we have simulated
- …
