103 research outputs found
Collisional dark matter and the structure of dark halos
We study how the internal structure of dark halos is affected if Cold DarkMatter particles are assumed to have a large cross-section for elasticcollisions. We identify a cluster halo in a large cosmological N-bodysimulation and resimulate its formation with progressively increasingresolution. We compare the structure found in the two cases where dark matteris treated as collisionless or as a fluid. For the collisionless case ourresults agree with those of other workers. Collisional dark matter results in acluster which is more nearly spherical and has a more singular central densityprofile. Substructure within the cluster is only weakly suppressed relative tothe collisionless case. The observed structure of dwarf galaxies argues againstself-interacting dark matter if, as seems likely, intermediate cross-sectionsproduce structure lying between the extremes we have simulated
A solution to the problems of cusps and rotation curves in dark matter halos in the cosmological standard model
We discuss various aspects of the inner structure formation in virialized
dark matter (DM) halos that form as primordial density inhomogeneities evolve
in the cosmological standard model. The main focus is on the study of central
cusps/cores and of the profiles of DM halo rotation curves, problems that
reveal disagreements among the theory, numerical simulations, and observations.
A method that was developed by the authors to describe equilibrium DM systems
is presented, which allows investigating these complex nonlinear structures
analytically and relating density distribution profiles within a halo both to
the parameters of the initial small-scale inhomogeneity field and to the
nonlinear relaxation characteristics of gravitationally compressed matter. It
is shown that cosmological random motions of matter `heat up' the DM particles
in collapsing halos, suppressing cusp-like density profiles within developing
halos, facilitating the formation of DM cores in galaxies, and providing an
explanation for the difference between observed and simulated galactic rotation
curves. The analytic conclusions obtained within this approach can be confirmed
by the N-body model simulation once improved spatial resolution is achieved for
central halo regions.Comment: 44 pages, 16 figures, 1 tabl
Structure of Dark Matter Halos From Hierarchical Clustering
We investigate the structure of the dark matter halo formed in the cold dark
matter scenario using -body simulations. We simulated 12 halos with the mass
of to . In almost all
runs, the halos have density cusps proportional to developed at the
center, which is consistent with the results of recent high-resolution
calculations. The density structure evolves in a self-similar way, and is
universal in the sense that it is independent of the halo mass and initial
random realization of density fluctuation. The density profile is in good
agreement with the profile proposed by Moore et al. (1999), which has central
slope proportional to and outer slope proportional to . The
halo grows through repeated accretion of diffuse smaller halos. We argue that
the cusp is understood as a convergence slope for the accretion of tidally
disrupted matter.Comment: 34 including 23 figures, revised version, accepted for publication in
Ap
Substructures in Cold Dark Matter Haloes
We analyse the properties of substructures within dark matter halos
(subhalos) using a set of high-resolution numerical simulations of the
formation of structure in a Lambda-CDM Universe. Our simulation set includes 11
high-resolution simulations of massive clusters as well as a region of mean
density, allowing us to study the spatial and mass distribution of
substructures down to a mass resolution limit of 10^9 h^(-1)Mo. We also
investigate how the properties of substructures vary as a function of the mass
of the `parent' halo in which they are located. We find that the substructure
mass function depends at most weakly on the mass of the parent halo and is well
described by a power-law. The radial number density profiles of substructures
are steeper in low mass halos than in high mass halos. More massive
substructures tend to avoid the centres of halos and are preferentially located
in the external regions of their parent halos. We also study the mass accretion
and merging histories of substructures, which we find to be largely independent
of environment. We find that a significant fraction of the substructures
residing in clusters at the present day were accreted at redshifts z < 1. This
implies that a significant fraction of present-day `passive' cluster galaxies
should have been still outside the cluster progenitor and more active at z~1.Comment: 13 pages, 15 figure. Accepted to MNRA
The Buprestidae (Coleoptera, Buprestoidea) of the Tuscan Archipelago (Italy)
Background
Buprestidae is a group of beetles of important conservation and phytosanitary value that is poorly studied in the Tuscan Archipelago and the limited faunistic knowledge available refers to a few scant historical records.
New information
The present contribution increments the species documented in the Archipelago from 27 to 51, providing more than 300 georeferenced occurrence records, derived from both direct field research and citizen science via iNaturalist. Of particular importance is the discovery of Eurythyrea quercus on Isola d'Elba, an uncommon and localised species currently critically endangered
Giant cluster arcs as a constraint on the scattering cross-section of dark matter
We carry out ray tracing through five high resolution simulations of a galaxy
cluster to study how its ability to produce giant gravitationally lensed arcs
is influenced by the collision cross-section of its dark matter. In three cases
typical dark matter particles in the cluster core undergo between 1 and 100
collisions per Hubble time; two more explore the long (``collisionless'') and
short (``fluid'') mean free path limits. We study the size and shape
distributions of arcs and compute the cross-section for producing ``extreme''
arcs of various sizes. Even a few collisions per particle modify the core
structure enough to destroy the cluster's ability to produce long, thin arcs.
For larger collision frequencies the cluster must be scaled up to
unrealistically large masses before it regains the ability to produce giant
arcs. None of our models with self-interacting dark matter (except the
``fluid'' limit) is able to produce radial arcs; even the case with the
smallest scattering cross-section must be scaled to the upper limit of observed
cluster masses before it produces radial arcs. Apparently the elastic collision
cross-section of dark matter in clusters must be very small, below 0.1 cm^2
g^-1, to be compatible with the observed ability of clusters to produce both
radial arcs and giant arcs.Comment: 8 pages, Latex using MN style, accepted version, to appear in MNRAS.
Other figures not included in the paper can be found at
http://www.mpa-garching.mpg.de/~massimo/SIDMlens.htm
Spatial and kinematic alignments between central and satellite halos
Based on a cosmological N-body simulation we analyze spatial and kinematic
alignments of satellite halos within six times the virial radius of group size
host halos (Rvir). We measure three different types of spatial alignment: halo
alignment between the orientation of the group central substructure (GCS) and
the distribution of its satellites, radial alignment between the orientation of
a satellite and the direction towards its GCS, and direct alignment between the
orientation of the GCS and that of its satellites. In analogy we use the
directions of satellite velocities and probe three further types of alignment:
the radial velocity alignment between the satellite velocity and connecting
line between satellite and GCS, the halo velocity alignment between the
orientation of the GCS and satellite velocities and the auto velocity alignment
between the satellites orientations and their velocities. We find that
satellites are preferentially located along the major axis of the GCS within at
least 6 Rvir (the range probed here). Furthermore, satellites preferentially
point towards the GCS. The most pronounced signal is detected on small scales
but a detectable signal extends out to 6 Rvir. The direct alignment signal is
weaker, however a systematic trend is visible at distances < 2 Rvir. All
velocity alignments are highly significant on small scales. Our results suggest
that the halo alignment reflects the filamentary large scale structure which
extends far beyond the virial radii of the groups. In contrast, the main
contribution to the radial alignment arises from the adjustment of the
satellite orientations in the group tidal field. The projected data reveal good
agreement with recent results derived from large galaxy surveys. (abridged)Comment: accepted for publication in Ap
Interindividual variation and consistency of migratory behavior in the Eurasian woodcock
Diverse spatio-temporal aspects of avian migration rely on relatively rigid endogenous programs. However, flexibility in migratory behavior may allow effective coping with unpredictable variation in ecological conditions that can occur during migration. We aimed at characterizing inter- and intraindividual variation of migratory behavior in a forest-dwelling wader species, the Eurasian woodcock Scolopax rusticola, focusing on spatio-temporal consistency across repeated migration episodes. By satellite-tracking birds from their wintering sites along the Italian peninsula to their breeding areas, we disclosed a remarkable variability in migration distances, with some birds flying more than 6,000 km to Central Asian breeding grounds (up to 101\ub0E). Prebreeding migration was faster and of shorter duration than postbreeding migration. Birds moving over longer distances migrated faster during prebreeding migration, and those breeding at northernmost latitudes left their wintering areas earlier. Moreover, birds making longer migrations departed earlier from their breeding sites. Breeding site fidelity was very high, whereas fidelity to wintering areas increased with age. Migration routes were significantly consistent, both among repeated migration episodes and between pre- and postbreeding migration. Prebreeding migration departure date was not significantly repeatable, whereas arrival date to the breeding areas was highly repeatable. Hence, interindividual variation in migratory behavior of woodcocks was mostly explained by the location of the breeding areas, and spatial consistency was relatively large through the entire annual cycle. Flexibility in prebreeding migration departure date may suggest that environmental effects have a larger influence on temporal than on spatial aspects of migratory behavior
Non-radial motion and the NFW profile
The self-similar infall model (SSIM) is normally discussed in the context of
radial orbits in spherical symmetry. However it is possible to retain the
spherical symmetry while permitting the particles to move in Keplerian
ellipses, each having the squared angular momentum peculiar to their 'shell'.
The spherical 'shell', defined for example by the particles turning at a given
radius, then moves according to the radial equation of motion of a 'shell'
particle. The 'shell' itself has no physical existence except as an ensemble of
particles, but it is convenient to sometimes refer to the shells since it is
they that are followed by a shell code. In this note we find the distribution
of squared angular momentum as a function of radius that yields the NFW density
profile for the final dark matter halo. It transpires that this distribution is
amply motivated dimensionally. An effective 'lambda' spin parameter is roughly
constant over the shells. We also study the effects of angular momentum on the
relaxation of a dark matter system using a three dimensional representation of
the relaxed phase space.Comment: accepted for publication in Astronomy and Astrophysics. date
received: 31-03-03 date accepted: 10-06-0
The effect of an expanding universe on massive objects
We present some astrophysical consequences of the metric for a point mass in
an expanding universe derived in Nandra, Lasenby & Hobson, and of the
associated invariant expression for the force required to keep a test particle
at rest relative to the central mass. We focus on the effect of an expanding
universe on massive objects on the scale of galaxies and clusters. Using
Newtonian and general-relativistic approaches, we identify two important
time-dependent physical radii for such objects when the cosmological expansion
is accelerating. The first radius, , is that at which the total radial
force on a test particle is zero, which is also the radius of the largest
possible circular orbit about the central mass and where the gas pressure
and its gradient vanish. The second radius, , which is \approx r_F/1.6m\Lambda$CDM concordance model, at the
present epoch we find that these radii put a sensible constraint on the typical
sizes of both galaxies and clusters at low redshift. For galaxies, we also find
that these radii agree closely with zeroes in the radial velocity field in the
neighbourhood of nearby galaxies, as inferred by Peirani & Pacheco from recent
observations of stellar velocities. We then consider the future effect on
massive objects of an accelerating cosmological expansion driven by phantom
energy, for which the universe is predicted to end in a `Big Rip' at a finite
time in the future at which the scale factor becomes singular. In particular,
we present a novel calculation of the time prior to the Big Rip that an object
of a given mass and size will become gravitationally unbound.Comment: 16 pages, 5 tables, 6 figures; new version, to match the version
published in MNRA
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