147 research outputs found
Grigori Kuzmin and Stellar Dynamics
Grigori Kuzmin was a very gifted dynamicist and one of the towering figures
in the distinguished history of the Tartu Observatory. He obtained a number of
important results in relative isolation which were later rediscovered in the
West. This work laid the foundation for further advances in the theory of
stellar systems in dynamical equilibrium, thereby substantially increasing our
understanding of galaxy dynamics.Comment: 10 pages, 1 figure, to appear in Baltic Astronomy, proceedings of the
conference "Expanding the Universe" held in Tartu, Estonia, 27-29 April, 201
On the universality of the global slope -- anisotropy inequality
Recently, some intriguing results have lead to speculations whether the
central density slope -- velocity dispersion anisotropy inequality (An & Evans)
actually holds at all radii for spherical dynamical systems. We extend these
studies by providing a complete analysis of the global slope -- anisotropy
inequality for all spherical systems in which the augmented density is a
separable function of radius and potential. We prove that these systems indeed
satisfy the global inequality if their central anisotropy is .
Furthermore, we present several systems with for which the
inequality does not hold, thus demonstrating that the global density slope --
anisotropy inequality is not a universal property. This analysis is a
significant step towards an understanding of the relation for general spherical
systems.Comment: 8 pages, 1 figure, accepted for publication in Ap
Prospects and Problems of Increasing the Automotive Thermoelectric Generators Efficiency
The chapter considers the current state and trends in the field of heat recovery units for vehicles with internal combustion engines (ICE), including thermoelectric generators for cars, motorcycles, ships and railway transport. Based on the analysis of literature data, mathematical modeling and experimental studies, this chapter presents various designs of such generators. This research considers a heat exchange between exhaust gas (EG) and thermoelectric modules (TEM), as well as how their usage affects ICE operation. Furthermore, the chapter profoundly explores the challenges of installing thermoelectric generator (TEG) on vehicle system. In addition, the ways of increasing overall system efficiency, by optimizing the flow channel and reducing electrical power losses, are presented
An attractor for dark matter structures
Cosmological simulations of dark matter structures have identified a set of
universal profiles, and similar characteristics have been seen in
non-cosmological simulations. It has therefore been speculated whether these
profiles of collisionless systems relate to accretion and merger history, or if
there is an attractor for the dark matter systems. Here we identify such a
1-dimensional attractor in the 3-dimensional space spanned by the 2 radial
slopes of the density and velocity dispersion, and the velocity anisotropy.
This attractor effectively removes one degree of freedom from the Jeans
equation. It also allows us to speculate on a new fluid interpretation for the
Jeans equation, with an effective polytropic index for the dark matter
particles between 1/2 and 3/4. If this attractor solution holds for other
collisionless structures, then it may hold the key to break the mass-anisotropy
degeneracy, which presently prevents us from measuring the mass profiles in
dwarf galaxies uniquely.Comment: 7 pages, 2 figures, comments welcom
Do current WIMP direct measurements constrain light relic neutralinos?
New upper bounds on direct detection rates have recently been presented by a
number of experimental collaborations working on searches for WIMPs. In this
paper we analyze how the constraints on relic neutralinos which can be derived
from these results is affected by the uncertainties in the distribution
function of WIMPs in the halo. Various different categories of velocity
distribution functions are considered, and the ensuing implications for
supersymmetric configurations derived. We conservatively conclude that current
experimental data do not constrain neutralinos of small mass (below 50 GeV).Comment: 9 pages, 7 figures, typeset with ReVTeX4. The paper may also be found
at http://www.to.infn.it/~fornengo/papers/constraints05.ps.gz or through
http://www.astroparticle.to.infn.it/index.htm
Analytical galactic models with mild stellar cusps
In the past two decades, it has been established by high-resolution
observations of early-type galaxies that their nuclear surface brightness and
corresponding stellar mass densities are characterized by cusps. In this paper,
we present a new spherical analytical model family describing mild cuspy
centres. We study isotropic and anisotropic models of Osipkov-Merritt type. It
is shown that the associated distribution functions and intrinsic velocity
dispersions can be represented analytically in a unified way in terms of
hypergeometric series, allowing thus a straightforward comparison of these
important global quantities for galaxies having underlying mass densities which
may differ significantly in their degree of central cuspiness or radial
falloff.Comment: 9 pages, 2 figures; accepted for publication in MNRAS; different
format than previous versio
Modeling the dynamical evolution of the M87 globular cluster system
We study the dynamical evolution of the M87 globular cluster system (GCS)
with a number of numerical simulations. We explore a range of different initial
conditions for the GCS mass function (GCMF), for the GCS spatial distribution
and for the GCS velocity distribution. We confirm that an initial power-law
GCMF like that observed in young cluster systems can be readily transformed
through dynamical processes into a bell-shaped GCMF. However,only models with
initial velocity distributions characterized by a strong radial anisotropy
increasing with the galactocentric distance are able to reproduce the observed
constancy of the GCMF at all radii.We show that such strongly radial orbital
distributions are inconsistent with the observed kinematics of the M87 GCS. The
evolution of models with a bell-shaped GCMF with a turnover similar to that
currently observed in old GCS is also investigated. We show that models with
this initial GCMF can satisfy all the observational constraints currently
available on the GCS spatial distribution,the GCS velocity distribution and on
the GCMF properties.In particular these models successfully reproduce both the
lack of a radial gradient of the GCS mean mass recently found in an analysis of
HST images of M87 at multiple locations, and the observed kinematics of the M87
GCS.Our simulations also show that evolutionary processes significantly affect
the initial GCS properties by leading to the disruption of many clusters and
changing the masses of those which survive.The preferential disruption of inner
clusters flattens the initial GCS number density profile and it can explain the
rising specific frequency with radius; we show that the inner flattening
observed in the M87 GCS spatial distribution can be the result of the effects
of dynamical evolution on an initially steep density profile. (abridged)Comment: 15 pages,14 figures;accepted for publication in The Astrophysical
Journa
The galaxy halo formation in the absence of violent relaxation and a universal density profile of the halo center
While N-body simulations testify for a cuspy profile of the central region of
the dark matter haloes, observations favor a shallow, cored density profile of
the central region of, at least, some spiral galaxies and dwarf spheroidals. We
show that a central profile, very close to the observed one, inevitably forms
in the center of dark matter haloes if we make a supposition about a moderate
energy relaxation of the system during the halo formation. If we assume the
energy exchange between dark matter particles during the halo collapse to be
not too intensive, the profile is universal: it depends almost not at all on
the properties of the initial perturbation and is very akin, but not identical,
to the Einasto profile with small Einasto index . We estimate the
size of the 'central core' of the distribution, i.e., the extent of the very
central region with a respectively gentle profile, and show that the cusp
formation is unlikely, even if the dark matter is cold. The obtained profile is
in a good agreement with observational data for, at least, some types of
galaxies, but clearly disagrees with N-body simulations.Comment: 8 pages, 4 figure
The Hubble Constant from the Gravitational Lens B1608+656
We present a refined gravitational lens model of the four-image lens system
B1608+656 based on new and improved observational constraints: (i) the three
independent time-delays and flux-ratios from VLA observations, (ii) the
radio-image positions from VLBA observations, (iii) the shape of the
deconvolved Einstein Ring from optical and infrared HST images, (iv) the
extinction-corrected lens-galaxy centroids and structural parameters, and (v) a
stellar velocity dispersion, sigma_ap=247+-35 km/s, of the primary lens galaxy
(G1), obtained from an echelle spectrum taken with the Keck--II telescope. The
lens mass model consists of two elliptical mass distributions with power-law
density profiles and an external shear, totaling 22 free parameters, including
the density slopes which are the key parameters to determine the value of H_0
from lens time delays. This has required the development of a new lens code
that is highly optimized for speed. The minimum-chi^2 model reproduces all
observations very well, including the stellar velocity dispersion and the shape
of the Einstein Ring. A combined gravitational-lens and stellar dynamical
analysis leads to a value of the Hubble Constant of H_0=75(+7/-6) km/s/Mpc (68
percent CL; Omega_m=0.3, Omega_Lambda=0.7. The non-linear error analysis
includes correlations between all free parameters, in particular the density
slopes of G1 and G2, yielding an accurate determination of the random error on
H_0. The lens galaxy G1 is ~5 times more massive than the secondary lens galaxy
(G2), and has a mass density slope of gamma_G1=2.03(+0.14/-0.14) +- 0.03 (68
percent CL) for rho~r^-gamma', very close to isothermal (gamma'=2). (Abridged)Comment: 17 pages, 6 figures, 5 tables; revised version with correct fig.6 and
clarified text based on referee report; conclusions unchange
Timed-Release and Key-Insulated Public Key Encryption
In this paper we consider two security notions related to Identity Based Encryption: Key-insulated public key encryption, introduced by Dodis, Katz, Xu and Yung; and Timed-Release Public Key cryptography, introduced independently by May and Rivest, Shamir and Wagner. We first formalize the notion of secure timed-release public key encryption, and show that, despite several differences in its formulation, it is equivalent to strongly key-insulated public key encryption (with optimal threshold and random access key updates). Next, we introduce the concept of an authenticated timed-release cryptosystem, briefly consider generic constructions, and then give a construction based on a single primitive which is efficient and provably secure
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