5,910 research outputs found
The Dynamical Evolution of Substructure
The evolution of substructure embedded in non-dissipative dark halos is
studied through N-body simulations of isolated systems, both in and out of
initial equilibrium, complementing cosmological simulations of the growth of
structure. We determine by both analytic calculations and direct analysis of
the N-body simulations the relative importance of various dynamical processes
acting on the clumps, such as the removal of material by global tides,
clump-clump heating, clump-clump merging and dynamical friction. Our comparison
between merging and disruption processes implies that spiral galaxies cannot be
formed in a proto-system that contains a few large clumps, but can be formed
through the accretion of many small clumps; elliptical galaxies form in a more
clumpy environment than do spiral galaxies. Our results support the idea that
the central cusp in the density profiles of dark halos is the consequence of
self-limiting merging of small, dense halos. This implies that the collapse of
a system of clumps/substructure is not sufficient to form a cD galaxy, with an
extended envelope; plausibly subsequent accretion of large galaxies is
required. Persistent streams of material from disrupted clumps can be found in
the outer regions of the final system, and at an overdensity of around 0.75,
can cover 10% to 30% of the sky.Comment: Accepted for publication in MNRAS. 61 pages, 22 figures; figures 2-7
and 21-22 are separate gif files. Complete paper plus high resolution figures
available from http://www.stsci.edu/~mstiavel/Bing_et_al_02.htm
LIDT-DD: A new self-consistent debris disc model including radiation pressure and coupling collisional and dynamical evolution
In most current debris disc models, the dynamical and the collisional
evolutions are studied separately, with N-body and statistical codes,
respectively, because of stringent computational constraints. We present here
LIDT-DD, the first code able to mix both approaches in a fully self-consistent
way. Our aim is for it to be generic enough so as to be applied to any
astrophysical cases where we expect dynamics and collisions to be deeply
interlocked with one another: planets in discs, violent massive breakups,
destabilized planetesimal belts, exozodiacal discs, etc. The code takes its
basic architecture from the LIDT3D algorithm developed by Charnoz et al.(2012)
for protoplanetary discs, but has been strongly modified and updated in order
to handle the very constraining specificities of debris discs physics:
high-velocity fragmenting collisions, radiation-pressure affected orbits,
absence of gas, etc. In LIDT-DD, grains of a given size at a given location in
a disc are grouped into "super-particles", whose orbits are evolved with an
N-body code and whose mutual collisions are individually tracked and treated
using a particle-in-a-box prescription. To cope with the wide range of possible
dynamics, tracers are sorted and regrouped into dynamical families depending on
their orbits. The code retrieves the classical features known for debris discs,
such as the particle size distributions in unperturbed discs, the outer radial
density profiles (slope in -1.5) outside narrow collisionally active rings, and
the depletion of small grains in "dynamically cold" discs. The potential of the
new code is illustrated with the test case of the violent breakup of a massive
planetesimal within a debris disc. The main potential future applications of
the code are planet/disc interactions, and more generally any configurations
where dynamics and collisions are expected to be intricately connected.Comment: Accepted for publication in A&A. 20 pages, 17 figures. Abstract
shortened for astro-p
Numerical Simulations of the Dark Universe: State of the Art and the Next Decade
We present a review of the current state of the art of cosmological dark
matter simulations, with particular emphasis on the implications for dark
matter detection efforts and studies of dark energy. This review is intended
both for particle physicists, who may find the cosmological simulation
literature opaque or confusing, and for astro-physicists, who may not be
familiar with the role of simulations for observational and experimental probes
of dark matter and dark energy. Our work is complementary to the contribution
by M. Baldi in this issue, which focuses on the treatment of dark energy and
cosmic acceleration in dedicated N-body simulations. Truly massive dark
matter-only simulations are being conducted on national supercomputing centers,
employing from several billion to over half a trillion particles to simulate
the formation and evolution of cosmologically representative volumes (cosmic
scale) or to zoom in on individual halos (cluster and galactic scale). These
simulations cost millions of core-hours, require tens to hundreds of terabytes
of memory, and use up to petabytes of disk storage. The field is quite
internationally diverse, with top simulations having been run in China, France,
Germany, Korea, Spain, and the USA. Predictions from such simulations touch on
almost every aspect of dark matter and dark energy studies, and we give a
comprehensive overview of this connection. We also discuss the limitations of
the cold and collisionless DM-only approach, and describe in some detail
efforts to include different particle physics as well as baryonic physics in
cosmological galaxy formation simulations, including a discussion of recent
results highlighting how the distribution of dark matter in halos may be
altered. We end with an outlook for the next decade, presenting our view of how
the field can be expected to progress. (abridged)Comment: 54 pages, 4 figures, 3 tables; invited contribution to the special
issue "The next decade in Dark Matter and Dark Energy" of the new Open Access
journal "Physics of the Dark Universe". Replaced with accepted versio
Technology for the Future: In-Space Technology Experiments Program, part 2
The purpose of the Office of Aeronautics and Space Technology (OAST) In-Space Technology Experiments Program In-STEP 1988 Workshop was to identify and prioritize technologies that are critical for future national space programs and require validation in the space environment, and review current NASA (In-Reach) and industry/ university (Out-Reach) experiments. A prioritized list of the critical technology needs was developed for the following eight disciplines: structures; environmental effects; power systems and thermal management; fluid management and propulsion systems; automation and robotics; sensors and information systems; in-space systems; and humans in space. This is part two of two parts and contains the critical technology presentations for the eight theme elements and a summary listing of critical space technology needs for each theme
The growth of galaxies in cosmological simulations of structure formation
We use hydrodynamic simulations to examine how the baryonic components of
galaxies are assembled, focusing on the relative importance of mergers and
smooth accretion in the formation of ~L_* systems. In our primary simulation,
which models a (50\hmpc)^3 comoving volume of a Lambda-dominated cold dark
matter universe, the space density of objects at our (64-particle) baryon mass
resolution threshold, M_c=5.4e10 M_sun, corresponds to that of observed
galaxies with L~L_*/4. Galaxies above this threshold gain most of their mass by
accretion rather than by mergers. At the redshift of peak mass growth, z~2,
accretion dominates over merging by about 4:1. The mean accretion rate per
galaxy declines from ~40 M_sun/yr at z=2 to ~10 M_sun/yr at z=0, while the
merging rate peaks later (z~1) and declines more slowly, so by z=0 the ratio is
about 2:1. We cannot distinguish truly smooth accretion from merging with
objects below our mass resolution threshold, but extrapolating our measured
mass spectrum of merging objects, dP/dM ~ M^a with a ~ -1, implies that
sub-resolution mergers would add relatively little mass. The global star
formation history in these simulations tracks the mass accretion rate rather
than the merger rate. At low redshift, destruction of galaxies by mergers is
approximately balanced by the growth of new systems, so the comoving space
density of resolved galaxies stays nearly constant despite significant mass
evolution at the galaxy-by-galaxy level. The predicted merger rate at z<~1
agrees with recent estimates from close pairs in the CFRS and CNOC2 redshift
surveys.Comment: Submitted to ApJ, 35 pp including 15 fig
Formation of Kuiper Belt Binaries by Gravitational Collapse
A large fraction of 100-km-class low-inclination objects in the classical
Kuiper Belt (KB) are binaries with comparable mass and wide separation of
components. A favored model for their formation was capture during the
coagulation growth of bodies in the early KB. Instead, recent studies suggested
that large objects can rapidly form in the protoplanetary disks when swarms of
locally concentrated solids collapse under their own gravity. Here we examine
the possibility that KB binaries formed during gravitational collapse when the
excess of angular momentum prevented the agglomeration of available mass into a
solitary object. We find that this new mechanism provides a robust path toward
the formation of KB binaries with observed properties, and can explain wide
systems such as 2001 QW322 and multiples such as (47171) 1999 TC36. Notably,
the gravitational collapse is capable of producing 100% binary fraction for a
wide range of the swarm's initial angular momentum values. The binary
components have similar masses (80% have the secondary-over-primary radius
ratio >0.7) and their separation ranges from ~1,000 to ~100,000 km. The binary
orbits have eccentricities from e=0 to ~1, with the majority having e<0.6. The
binary orbit inclinations with respect to the initial angular momentum of the
swarm range from i=0 to ~90 deg, with most cases having i<50 deg. Our binary
formation mechanism implies that the primary and secondary components in each
binary pair should have identical bulk composition, which is consistent with
the current photometric data. We discuss the applicability of our results to
the Pluto-Charon, Orcus-Vanth, (617) Patroclus-Menoetius and (90) Antiope
binary systems.Comment: Astronomical Journal, in pres
Halo orbits in cosmological disk galaxies : tracers of information history
We analyze the orbits of stars and dark matter particles in the halo of a disk galaxy formed in a cosmological hydrodynamical simulation. The halo is oblate within the inner ∼20 kpc and triaxial beyond this radius. About 43% of orbits are short axis tubes—the rest belong to orbit families that characterize triaxial potentials (boxes, long-axis tubes and chaotic orbits), but their shapes are close to axisymmetric. We find no evidence that the self-consistent distribution function of the nearly oblate inner halo is comprised primarily of axisymmetric short-axis tube orbits. Orbits of all families and both types of particles are highly eccentric, with mean eccentricity �0.6. We find that randomly selected samples of halo stars show no substructure in “integrals of motion” space. However, individual accretion events can clearly be identified in plots of metallicity versus formation time. Dynamically young tidal debris is found primarily on a single type of orbit. However, stars associated with older satellites become chaotically mixed during the formation process (possibly due to scattering by the central bulge and disk, and baryonic processes), and appear on all four types of orbits. We find that the tidal debris in cosmological hydrodynamical simulations experiences significantly more chaotic evolution than in collisionless simulations, making it much harder to identify individual progenitors using phase space coordinates alone. However, by combining information on stellar ages and chemical abundances with the orbital properties of halo stars in the underlying self-consistent potential, the identification of progenitors is likely to be possible
A top-down scenario for the formation of massive Tidal Dwarf Galaxies
Among those objects formed out of collisional debris during galaxy mergers,
the prominent gaseous accumulations observed near the tip of some long tidal
tails are the most likely to survive long enough to form genuine recycled
galaxies. Using simple numerical models, Bournaud, Duc & Masset (2003) claimed
that tidal objects as massive as 10^9 Msun could only form, in these
simulations, within extended dark matter (DM) haloes. We present here a new set
of simulations of galaxy collisions to further investigate the structure of
tidal tails. First of all, we checked that massive objects are still produced
in full N-body codes that include feedback and a large number of particles.
Using a simpler N-body code with rigid haloes, we noticed that dissipation and
self-gravity in the tails, although important, are not the key factors.
Exploiting toy models, we found that, for truncated DM haloes, material is
stretched along the tail, while, within extended haloes, the tidal field can
efficiently carry away from the disk a large fraction of the gas, while
maintaining its surface density to a high value. This creates a density
enhancement near the tip of the tail. Only later-on, self-gravity takes over;
the gas clouds collapse and start forming stars. Thus, such objects were
fundamentally formed following a kinematical process, according to a top-down
scenario, contrary to the less massive Super Star Clusters that are also
present around mergers. This conclusion leads us to introduce a restrictive
definition for Tidal Dwarf Galaxies (TDGs) and their progenitors, considering
only the most massive ones, initially mostly made of gas, that were able to
pile up in the tidal tails. More simulations will be necessary to precisely
determine the fate of these proto--TDGs and estimate their number.Comment: 13 pages, 11 figures, accepted for publication in A&
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