294 research outputs found
Star formation in mergers with cosmologically motivated initial conditions
We use semi-analytic models and cosmological merger trees to provide the
initial conditions for multi-merger numerical hydrodynamic simulations, and
exploit these simulations to explore the effect of galaxy interaction and
merging on star formation (SF). We compute numerical realisations of twelve
merger trees from z=1.5 to z=0. We include the effects of the large hot gaseous
halo around all galaxies, following recent obervations and predictions of
galaxy formation models. We find that including the hot gaseous halo has a
number of important effects. Firstly, as expected, the star formation rate on
long timescales is increased due to cooling of the hot halo and refuelling of
the cold gas reservoir. Secondly, we find that interactions do not always
increase the SF in the long term. This is partially due to the orbiting
galaxies transferring gravitational energy to the hot gaseous haloes and
raising their temperature. Finally we find that the relative size of the
starburst, when including the hot halo, is much smaller than previous studies
showed. Our simulations also show that the order and timing of interactions are
important for the evolution of a galaxy. When multiple galaxies interact at the
same time, the SF enhancement is less than when galaxies interact in series.
All these effects show the importance of including hot gas and cosmologically
motivated merger trees in galaxy evolution models.Comment: 19 pages, 15 figures, 6 tables. Accepted for publication in MNRA
Theory and simulation of spectral line broadening by exoplanetary atmospheric haze
Atmospheric haze is the leading candidate for the flattening of expolanetary
spectra, as it's also an important source of opacity in the atmospheres of
solar system planets, satellites, and comets. Exoplanetary transmission
spectra, which carry information about how the planetary atmospheres become
opaque to stellar light in transit, show broad featureless absorption in the
region of wavelengths corresponding to spectral lines of sodium, potassium and
water. We develop a detailed atomistic model, describing interactions of atomic
or molecular radiators with dust and atmospheric haze particulates. This model
incorporates a realistic structure of haze particulates from small nano-size
seed particles up to sub-micron irregularly shaped aggregates, accounting for
both pairwise collisions between the radiator and haze perturbers, and
quasi-static mean field shift of levels in haze environments. This formalism
can explain large flattening of absorption and emission spectra in haze
atmospheres and shows how the radiator - haze particle interaction affects the
absorption spectral shape in the wings of spectral lines and near their
centers. The theory can account for nearly all realistic structure, size and
chemical composition of haze particulates and predict their influence on
absorption and emission spectra in hazy environments. We illustrate the utility
of the method by computing shift and broadening of the emission spectra of the
sodium D line in an argon haze. The simplicity, elegance and generality of the
proposed model should make it amenable to a broad community of users in
astrophysics and chemistry.Comment: 16 pages, 4 figures, submitted to MNRA
From Discs to Bulges: effect of mergers on the morphology of galaxies
We study the effect of mergers on the morphology of galaxies by means of the
simulated merger tree approach first proposed by Moster et al. This method
combines N-body cosmological simulations and semi-analytic techniques to
extract realistic initial conditions for galaxy mergers. These are then evolved
using high resolution hydrodynamical simulations, which include dark matter,
stars, cold gas in the disc and hot gas in the halo. We show that the satellite
mass accretion is not as effective as previously thought, as there is
substantial stellar stripping before the final merger. The fraction of stellar
disc mass transferred to the bulge is quite low, even in the case of a major
merger, mainly due to the dispersion of part of the stellar disc mass into the
halo. We confirm the findings of Hopkins et al., that a gas rich disc is able
to survive major mergers more efficiently. The enhanced star formation
associated with the merger is not localised to the bulge of galaxy, but a
substantial fraction takes place in the disc too. The inclusion of the hot gas
reservoir in the galaxy model contributes to reducing the efficiency of bulge
formation. Overall, our findings suggest that mergers are not as efficient as
previously thought in transforming discs into bulges. This possibly alleviates
some of the tensions between observations of bulgeless galaxies and the
hierarchical scenario for structure formation.Comment: MNRAS Accepted, 17 pages, 11 figures, 3 Table
Thin-shell wormholes with a generalized Chaplygin gas
In this article, spherically symmetric thin-shell wormholes supported by a
generalized Chaplygin gas are constructed and their stability under
perturbations preserving the symmetry is studied. Wormholes with charge and
with a cosmological constant are analyzed and the results are compared with
those obtained for the original Chaplygin gas, which was considered in a
previous work. For some values of the parameters, one stable configuration is
also present and a new extra unstable solution is found.Comment: 14 pages, 6 figures; v2: typos corrected and minor rewordin
Mechanism of Deep-focus Earthquakes Anomalous Statistics
Analyzing the NEIC-data we have shown that the spatial deep-focus earthquake
distribution in the Earth interior over the 1993-2006 is characterized by the
clearly defined periodical fine discrete structure with period L=50 km, which
is solely generated by earthquakes with magnitude M 3.9 to 5.3 and only on the
convergent boundary of plates. To describe the formation of this structure we
used the model of complex systems by A. Volynskii and S. Bazhenov. The key
property of this model consists in the presence of a rigid coating on a soft
substratum. It is shown that in subduction processes the role of a rigid
coating plays the slab substance (lithosphere) and the upper mantle acts as a
soft substratum. Within the framework of this model we have obtained the
estimation of average values of stress in the upper mantle and Young's modulus
for the oceanic slab (lithosphere) and upper mantle.Comment: 9 pages, 7 figure
Stability of Chaplygin gas thin-shell wormholes
In this paper we construct spherical thin-shell wormholes supported by a
Chaplygin gas. For a rather general class of geometries we introduce a new
approach for the stability analysis of static solutions under perturbations
preserving the symmetry. We apply this to wormholes constructed from
Schwarzschild, Schwarzschild-de Sitter, Schwarzschild-anti de Sitter and
Reissner-Nordstrom metrics. In the last two cases, we find that there are
values of the parameters for which stable static solutions exist.Comment: 14 pages, 5 figures; v2: minor changes and new references added.
Accepted for publication in Physical Review
Isolated Gust Generation for the Investigation of Airfoil-Gust Interaction
As part of an effort to examine the impact of vortical gusts on airfoils, a simple gust
generator has been built and investigated. This consists of a heaving
at plate capable of
following a specifed transverse trajectory across a water tunnel. The relationship between
the trajectory and the properties of the gusts that are shed downstream is characterized
for non-periodic heaving motion described by Eldredge's smooth motion equation. PIV
experiments show that the circulation of the vortical gust is proportional to the heaving
speed of the plate. Tests with a downstream NACA 0018 airfoil demonstrate repeatable
forces in response to the produced gusts
On a direct approach to quasideterminant solutions of a noncommutative KP equation
A noncommutative version of the KP equation and two families of its solutions
expressed as quasideterminants are discussed. The origin of these solutions is
explained by means of Darboux and binary Darboux transformations. Additionally,
it is shown that these solutions may also be verified directly. This approach
is reminiscent of the wronskian technique used for the Hirota bilinear form of
the regular, commutative KP equation but, in the noncommutative case, no
bilinearising transformation is available.Comment: 11 page
КИНЕМАТИКА И ДИНАМИКА ШАРИКОВ ПРИ ШЛИФОВАНИИ МЕЖДУ НЕСООСНЫМИ ДИСКАМИ В ПРИВОДНОМ СЕПАРАТОРЕ
The paper contains description of tool design which is used for rough polishing of balls made of brittle materials. This tool consists of top disk, which is fi-eely set on pouring bearing of the separator shaft, and a bottom disk with elastic coating. Theoretical investigations of ball kinematics have permitted to determine an angular velocity of the balls in the machining zone, value and direction oi slipping speed with respect to the tool.it has been determined that even change of the position of bail rotation instantaneous axis relative to the separator is reached at equality of angular velocities of the bottom disk and the separator. When the top disk is freely set slipping speed is equal for all the balls of the machined lot. When the top disk is stationary slipping speed is significantly increasing however its value becomes variable and depends on ball distance to separator rotation axis.The presented equations of ball dynamics in the machining zone have made it possible to detennine a cohesive force with the bottom disk, frictional forces against the top disk and separator. Recommendations on selection of machining modes at initial and final stages of ball polishing are given in the paper.Приведено описание конструкции инструмента для грубого шлифования шариков из хрупких материалов, в состав которого входит верхний диск, свободно установленный на валу сепаратора на насыпном подшипнике, и нижний диск с эластичным покрытием. Теоретические исследования кинематики шариков позволили определить угловую скорость шариков в зоне обработки, величину и направление скорости проскальзывания относительно инструмента.Установлено, что равномерное изменение положения мгновенной оси вращения шарика относительно сепаратора достигается при равенстве угловых скоростей нижнего диска и сепаратора. При свободно установленном верхнем диске скорость проскальзывания одинакова для всех шариков в обрабатываемой партии. При неподвижном верхнем диске скорость проскальзывания значительно возрастает, однако ее величина становится переменной и зависит от расстояния шарика до оси вращения сепаратора.Приведенные уравнения динамики шарика в зоне обработки позволяют определить силу сцепления с нижним диском, силы трения о верхний диск и сепаратор. Даны рекомендации по выбору режимов обработки на начальном и завершающем этапах шлифования шариков
Power spectrum in the Chaplygin gas model: tachyonic, fluid and scalar field representations
The Chaplygin gas model, characterized by an equation of state of the type emerges naturally from the Nambu-Goto action of string
theory. This fluid representation can be recast under the form of a tachyonic
field given by a Born-Infeld type Lagrangian. At the same time, the Chaplygin
gas equation of state can be obtained from a self-interacting scalar field. We
show that, from the point of view of the supernova type Ia data, the three
representations (fluid, tachyonic, scalar field) lead to the same results.
However, concerning the matter power spectra, while the fluid and tachyonic
descriptions lead to exactly the same results, the self-interacting scalar
field representation implies different statistical estimations for the
parameters. In particular, the estimation for the dark matter density parameter
in the fluid representation favors a universe dominated almost completely by
dark matter, while in the self-interacting scalar field representation the
prediction is very closed to that obtained in the CDM model.Comment: Latex file, 10 pages, 18 figures in EPS forma
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