32 research outputs found
Galaxy merging in MOND
We present the results of N-body simulations of dissipationless galaxy
merging in Modified Newtonian Dynamics (MOND). For comparison, we also studied
Newtonian merging between galaxies embedded in dark matter halos, with internal
dynamics equivalent to the MOND systems. We found that the merging timescales
are significantly longer in MOND than in Newtonian gravity with dark matter,
suggesting that observational evidence of rapid merging could be difficult to
explain in MOND. However, when two galaxies eventually merge, the MOND merging
end-product is hardly distinguishable from the final stellar distribution of an
equivalent Newtonian merger with dark matter.Comment: 5 pages, 2 color figures. To appear in MNRAS Letters. Added
references and discussion, conclusions unchange
Vertical dynamics of disk galaxies in MOND
We investigate the possibility of discriminating between Modified Newtonian
Dynamics (MOND) and Newtonian gravity with dark matter, by studying the
vertical dynamics of disk galaxies. We consider models with the same circular
velocity in the equatorial plane (purely baryonic disks in MOND and the same
disks in Newtonian gravity embedded in spherical dark matter haloes), and we
construct their intrinsic and projected kinematical fields by solving the Jeans
equations under the assumption of a two-integral distribution function. We
found that the vertical velocity dispersion of deep-MOND disks can be much
larger than in the equivalent spherical Newtonian models. However, in the more
realistic case of high-surface density disks this effect is significantly
reduced, casting doubts on the possibility of discriminating between MOND and
Newtonian gravity with dark matter by using current observations.Comment: 8 pages, 7 figures. Accepted for publication in MNRAS. Added
referenc
Three-dimensional evolution of magnetic and velocity shear driven instabilities in a compressible magnetized jet
The problem of three-dimensional combined magnetic and velocity shear driven
instabilities of a compressible magnetized jet modeled with a plane
neutral/current double vortex sheet in the framework of the resistive
magnetohydrodynamics is addressed. The resulting dynamics given by the
stream+current sheet interaction is analyzed and the effects of a variable
geometry of the basic fields are considered. Depending on the basic asymptotic
magnetic field configuration, a selection rule of the linear instability modes
can be obtained. Hence, the system follows a two-stage path developing either
through a fully three-dimensional dynamics with a rapid evolution of kink modes
leading to a final turbulent state, or rather through a driving two-dimensional
instability pattern that develops on parallel planes on which a
reconnection+coalescence process takes place.Comment: 33 pages, 15 figures, accepted for publication in Physics of Plasma
Radial orbital anisotropy and the Fundamental Plane of elliptical galaxies
The existence of the Fundamental Plane (FP) imposes strong constraints on the
structure and dynamics of elliptical galaxies, and thus contains important
information on the processes of their formation and evolution. Here we focus on
the relations between the FP thinness and tilt and the amount of radial orbital
anisotropy. By using N-body simulations of galaxy models characterized by
observationally motivated density profiles, and also allowing for the presence
of live, massive dark matter halos, we explore the impact of radial orbital
anisotropy and instability on the FP properties. The numerical results confirm
a previous semi--analytical finding: the requirement of stability matches
almost exactly the thinness of the FP. In other words, galaxy models that are
radially anisotropic enough to be found outside the observed FP (with their
isotropic parent models lying on the FP) are unstable, and their end--products
fall back on the FP itself. We also find that a systematic increase of radial
orbit anisotropy with galaxy luminosity cannot explain by itself the whole tilt
of the FP, becoming the galaxy models unstable at moderately high luminosities:
at variance with the previous case their end--products are found well outside
the FP itself (abridged).Comment: 30 pages, 6 figures, MNRAS (accepted
Characterisation of beam driven ionisation injection in the blowout regime of Plasma Acceleration
Beam driven ionisation injection is characterised for a variety of high-Z
dopant. We discuss the region of extraction and why the position where
electrons are captured influences the final quality of the internally-injected
bunch. The beam driven ionisation injection relies on the capability to produce
a high gradient fields at the bubble closure, with magnitudes high enough to
ionise by tunnelling effect the still bounded electrons (of a high-Z dopant).
The ionised electrons are captured by the nonlinear plasma wave at the
accelerating and focusing wake phase leading to high-brightness trailing
bunches. The high transformer ratio guarantees that the ionisation only occurs
at the bubble closure. The quality of the ionisation-injected trailing bunches
strongly and non-linearly depends on the properties of the dopant gas (density
and initial ionisation state). We use the full 3D PIC code to
consider the highly three-dimensional nature of the effect. By means of a
systematic approach we have investigated the emittance and energy spread
formation and the evolution for different dopant gases and configurations
Simulation of the laser-plasma acceleration for the PLASMONX project with the PIC code ALaDyn
In this paper we will briefly introduce laser–plasma acceleration for electrons and present some numerical simulations. The simulations have been performed to find a suitable working point for one of the test experiments of the INFN–CNR PLASMONX project. FLAME (Frascati laser for acceleration and multidisciplinary experiments), a 300 TW Ti:Sa laser, is being installed and commissioned at Laboratori Nazionali di Frascati (LFN). The first pilot experiment SITE (self-injection test experiment) is planned for this year (2010). The simulations have been run using a fully self-consistent particle-in-cell code AlaDyn (Acceleration by LAser and DYNamics of charged particles) developed and maintained at the Department of Physics at the University of Bologna within the PLAMSONX project
Rise time of proton cut-off energy in 2D and 3D PIC simulations
The Target Normal Sheath Acceleration (TNSA) regime for proton acceleration
by laser pulses is experimentally consolidated and fairly well understood.
However, uncertainties remain in the analysis of particle-in-cell (PIC)
simulation results.
The energy spectrum is exponential with a cut-off, but the maximum energy
depends on the simulation time, following different laws in two and three
dimensional (2D, 3D) PIC simulations, so that the determination of an
asymptotic value has some arbitrariness.
We propose two empirical laws for rise time of the cut-off energy in 2D and
3D PIC simulations, suggested by a model in which the proton acceleration is
due to a surface charge distribution on the target rear side. The kinetic
energy of the protons that we obtain follows two distinct laws, which appear to
be nicely satisfied by PIC simulations. The laws depend on two parameters: the
scaling time, at which the energy starts to rise, and the asymptotic cut-off
energy.
The values of the cut-off energy, obtained by fitting the 2D and 3D
simulations for the same target and laser pulse, are comparable. This suggests
that parametric scans can be performed with 2D simulations, since 3D ones are
computationally very expensive. In this paper, the simulations are carried out
for with the PIC code ALaDyn by changing the target thickness and
the incidence angle . A monotonic dependence, on for normal
incidence and on for fixed , is found, as in the experimental
results for high temporal contrast pulses