1,431 research outputs found

### Analysis of Galaxy Formation with Hydrodynamics

We present a hydrodynamical code based on the Smooth Particle Hydrodynamics
technique implemented in an AP3M code aimed at solving the hydrodynamical and
gravitational equations in a cosmological frame. We analyze the ability of the
code to reproduce standard tests and perform numerical simulations to study the
formation of galaxies in a typical region of a CDM model. These numerical
simulations include gas and dark matter particles and take into account
physical processes such as shock waves, radiative cooling, and a simplified
model of star formation. Several observed properties of normal galaxies such as
$M_{gas}/M_{total}$ ratios, the luminosity function and the Tully-Fisher
relation are analyzed within the limits imposed by numerical resolution.Comment: 21 pages, 2 postscript tables. Submitted MNRAS 04.03.9

### YAPA: A generic tool for computing intruder knowledge

Reasoning about the knowledge of an attacker is a necessary step in many
formal analyses of security protocols. In the framework of the applied pi
calculus, as in similar languages based on equational logics, knowledge is
typically expressed by two relations: deducibility and static equivalence.
Several decision procedures have been proposed for these relations under a
variety of equational theories. However, each theory has its particular
algorithm, and none has been implemented so far. We provide a generic procedure
for deducibility and static equivalence that takes as input any convergent
rewrite system. We show that our algorithm covers most of the existing decision
procedures for convergent theories. We also provide an efficient
implementation, and compare it briefly with the tools ProVerif and KiSs

### Sharp error terms for return time statistics under mixing conditions

We describe the statistics of repetition times of a string of symbols in a
stochastic process. Denote by T(A) the time elapsed until the process spells
the finite string A and by S(A) the number of consecutive repetitions of A. We
prove that, if the length of the string grows unbondedly, (1) the distribution
of T(A), when the process starts with A, is well aproximated by a certain
mixture of the point measure at the origin and an exponential law, and (2) S(A)
is approximately geometrically distributed. We provide sharp error terms for
each of these approximations. The errors we obtain are point-wise and allow to
get also approximations for all the moments of T(A) and S(A). To obtain (1) we
assume that the process is phi-mixing while to obtain (2) we assume the
convergence of certain contidional probabilities

### Satellites of Simulated Galaxies: survival, merging, and their relation to the dark and stellar halos

We study the population of satellite galaxies formed in a suite of
N-body/gasdynamical simulations of galaxy formation in a LCDM universe. We find
little spatial or kinematic bias between the dark matter and the satellite
population. The velocity dispersion of the satellites is a good indicator of
the virial velocity of the halo: \sigma_{sat}/V_{vir}=0.9 +/- 0.2. Applied to
the Milky Way and M31 this gives V_{vir}^{MW}=109 +/- 22$ km/s and
V_{vir}^{M31} = 138 +/- 35 km/s, respectively, substantially lower than the
rotation speed of their disk components. The detailed kinematics of simulated
satellites and dark matter are also in good agreement. By contrast, the stellar
halo of the simulated galaxies is kinematically and spatially distinct from the
population of surviving satellites. This is because the survival of a satellite
depends on mass and on time of accretion; surviving satellites are biased
toward low-mass systems that have been recently accreted by the galaxy. Our
results support recent proposals for the origin of the systematic differences
between stars in the Galactic halo and in Galactic satellites: the elusive
``building blocks'' of the Milky Way stellar halo were on average more massive,
and were accreted (and disrupted) earlier than the population of dwarfs that
has survived self-bound until the present.Comment: 13 pages, 11 figures, MNRAS in press. Accepted version with minor
changes. Version with high resolution figures available at:
http://www.astro.uvic.ca/~lsales/SatPapers/SatPapers.htm

### Cosmic M\'enage \`a Trois: The Origin of Satellite Galaxies On Extreme Orbits

We examine the orbits of satellite galaxies identified in a suite of
N-body/gasdynamical simulations of the formation of $L_*$ galaxies in a LCDM
universe. Most satellites follow conventional orbits; after turning around,
they accrete into their host halo and settle on orbits whose apocentric radii
are steadily eroded by dynamical friction. However, a number of outliers are
also present, we find that ~1/3 of satellites identified at $z=0$ are on
unorthodox orbits, with apocenters that exceed their turnaround radii. This
population of satellites on extreme orbits consists typically of the faint
member of a satellite pair that has been ejected onto a highly-energetic orbit
during its first approach to the primary. Since the concurrent accretion of
multiple satellite systems is a defining feature of hierarchical models of
galaxy formation, we speculate that this three-body ejection mechanism may be
the origin of (i) some of the newly discovered high-speed satellites around M31
(such as Andromeda XIV); (ii) some of the distant fast-receding Local Group
members, such as Leo I; and (iii) the oddly isolated dwarf spheroidals Cetus
and Tucana in the outskirts of the Local Group. Our results suggest that care
must be exercised when using the orbits of the most weakly bound satellites to
place constraints on the total mass of the Local Group.Comment: 10 pages, 6 figures, MNRAS in press. Accepted version with minor
changes. Version with high resolution figures available at:
http://www.astro.uvic.ca/~lsales/SatPapers/SatPapers.htm

### The low mass end of the neutral gas mass and velocity width functions of galaxies in $\Lambda$CDM

We use the high-resolution Aquarius cosmological dark matter simulations
coupled to the semi-analytic model by Starkenburg et al. (2013) to study the HI
content and velocity width properties of field galaxies at the low mass end in
the context of $\Lambda$CDM. We compare our predictions to the observed ALFALFA
survey HI mass and velocity width functions, and find very good agreement
without fine-tuning, when considering central galaxies. Furthermore, the
properties of the dark matter halos hosting galaxies, characterised by their
peak velocity and circular velocity at 2 radial disk scalelengths overlap
perfectly with the inferred values from observations. This suggests that our
galaxies are placed in the right dark matter halos, and consequently at face
value, we do not find any discrepancy with the predictions from the
$\Lambda$CDM model. Our analysis indicates that previous tensions, apparent
when using abundance matching models, arise because this technique cannot be
straightforwardly applied for objects with masses $M_{vir} < 10^{10}
M_{\odot}$.Comment: Accepted for publication in MNRAS, 9 pages, 8 figure

### The Hierarchical Formation of the Galactic Disk

I review the results of recent cosmological simulations of galaxy formation
that highlight the importance of satellite accretion in the formation of
galactic disks. Tidal debris of disrupted satellites may contribute to the disk
component if they are compact enough to survive the decay and circularization
of the orbit as dynamical friction brings the satellite into the disk plane.
This process may add a small but non-negligible fraction of stars to the thin
and thick disks, and reconcile the presence of very old stars with the
protracted merging history expected in a hierarchically clustering universe. I
discuss various lines of evidence which suggest that this process may have been
important during the formation of the Galactic disk.Comment: paper to be read at the "Penetrating Bars through Masks of Cosmic
Dust" conference in South Afric

### Formal Verification of Security Protocol Implementations: A Survey

Automated formal verification of security protocols has been mostly focused on analyzing high-level abstract models which, however, are significantly different from real protocol implementations written in programming languages. Recently, some researchers have started investigating techniques that bring automated formal proofs closer to real implementations. This paper surveys these attempts, focusing on approaches that target the application code that implements protocol logic, rather than the libraries that implement cryptography. According to these approaches, libraries are assumed to correctly implement some models. The aim is to derive formal proofs that, under this assumption, give assurance about the application code that implements the protocol logic. The two main approaches of model extraction and code generation are presented, along with the main techniques adopted for each approac

### On the statistical distribution of first--return times of balls and cylinders in chaotic systems

We study returns in dynamical systems: when a set of points, initially
populating a prescribed region, swarms around phase space according to a
deterministic rule of motion, we say that the return of the set occurs at the
earliest moment when one of these points comes back to the original region. We
describe the statistical distribution of these "first--return times" in various
settings: when phase space is composed of sequences of symbols from a finite
alphabet (with application for instance to biological problems) and when phase
space is a one and a two-dimensional manifold. Specifically, we consider
Bernoulli shifts, expanding maps of the interval and linear automorphisms of
the two dimensional torus. We derive relations linking these statistics with
Renyi entropies and Lyapunov exponents.Comment: submitted to Int. J. Bifurcations and Chao

- â€¦