62 research outputs found
Galactic fountains and gas accretion
Star-forming disc galaxies such as the Milky Way need to accrete \gsim 1
of gas each year to sustain their star formation. This gas
accretion is likely to come from the cooling of the hot corona, however it is
still not clear how this process can take place. We present simulations
supporting the idea that this cooling and the subsequent accretion are caused
by the passage of cold galactic-fountain clouds through the hot corona. The
Kelvin-Helmholtz instability strips gas from these clouds and the stripped gas
causes coronal gas to condense in the cloud's wake. For likely parameters of
the Galactic corona and of typical fountain clouds we obtain a global accretion
rate of the order of that required to feed the star formation.Comment: 2 pages, 1 figure, to appear in "Hunting for the Dark: The Hidden
Side of Galaxy Formation", Malta, 19-23 Oct. 2009, eds. V.P. Debattista &
C.C. Popescu, AIP Conf. Se
Fountain-driven gas accretion by the Milky Way
Accretion of fresh gas at a rate of ~ 1 M_{sun} yr^{-1} is necessary in
star-forming disc galaxies, such as the Milky Way, in order to sustain their
star-formation rates. In this work we present the results of a new hydrodynamic
simulation supporting the scenario in which the gas required for star formation
is drawn from the hot corona that surrounds the star-forming disc. In
particular, the cooling of this hot gas and its accretion on to the disc are
caused by the passage of cold galactic fountain clouds through the corona.Comment: 2 pages, 1 figure. To appear in the proceedings of the conference
"Assembling the Puzzle of the Milky Way", Le Grand-Bornand 17-22 April 2011,
European Physical Journal, editors C. Reyl\'e, A. Robin and M. Schulthei
Self-gravity at the scale of the polar cell
We present the exact calculus of the gravitational potential and acceleration
along the symmetry axis of a plane, homogeneous, polar cell as a function of
mean radius a, radial extension e, and opening angle f. Accurate approximations
are derived in the limit of high numerical resolution at the geometrical mean
of the inner and outer radii (a key-position in current FFT-based Poisson
solvers). Our results are the full extension of the approximate formula given
in the textbook of Binney & Tremaine to all resolutions. We also clarify
definitely the question about the existence (or not) of self-forces in polar
cells. We find that there is always a self-force at radius except if the
shape factor a.f/e reaches ~ 3.531, asymptotically. Such cells are therefore
well suited to build a polar mesh for high resolution simulations of
self-gravitating media in two dimensions. A by-product of this study is a newly
discovered indefinite integral involving complete elliptic integral of the
first kind over modulus.Comment: 4 pages, 4 figures, A&A accepte
Diffusion of energetic particles in turbulent MHD plasmas
In this paper we investigate the transport of energetic particles in
turbulent plasmas. A numerical approach is used to simulate the effect of the
background plasma on the motion of energetic protons. The background plasma is
in a dynamically turbulent state found from numerical MHD simulations, where we
use parameters typical for the heliosphere. The implications for the transport
parameters (i.e. pitch-angle diffusion coefficients and mean free path) are
calculated and deviations from the quasi-linear theory are discussed.Comment: Accepted for publication in Ap
Lopsidedness of cluster galaxies in modified gravity
We point out an interesting theoretical prediction for elliptical galaxies
residing inside galaxy clusters in the framework of modified Newtonian dynamics
(MOND), that could be used to test this paradigm. Apart from the central
brightest cluster galaxy, other galaxies close enough to the centre experience
a strong gravitational influence from the other galaxies of the cluster. This
influence manifests itself only as tides in standard Newtonian gravity, meaning
that the systematic acceleration of the centre of mass of the galaxy has no
consequence. However, in the context of MOND, a consequence of the breaking of
the strong equivalence principle is that the systematic acceleration changes
the own self-gravity of the galaxy. We show here that, in this framework,
initially axisymmetric elliptical galaxies become lopsided along the external
field's direction, and that the centroid of the galaxy, defined by the outer
density contours, is shifted by a few hundreds parsecs with respect to the
densest point.Comment: accepted for publication in JCA
Constrained-Transport Magnetohydrodynamics with Adaptive-Mesh-Refinement in CHARM
We present the implementation of a three-dimensional, second order accurate
Godunov-type algorithm for magneto-hydrodynamic (MHD), in the
adaptive-mesh-refinement (AMR) cosmological code {\tt CHARM}. The algorithm is
based on the full 12-solve spatially unsplit Corner-Transport-Upwind (CTU)
scheme. The fluid quantities are cell-centered and are updated using the
Piecewise-Parabolic-Method (PPM), while the magnetic field variables are
face-centered and are evolved through application of the Stokes theorem on cell
edges via a Constrained-Transport (CT) method. The multidimensional MHD source
terms required in the predictor step for high-order accuracy are applied in a
simplified form which reduces their complexity in three dimensions without loss
of accuracy or robustness. The algorithm is implemented on an AMR framework
which requires specific synchronization steps across refinement levels. These
include face-centered restriction and prolongation operations and a {\it
reflux-curl} operation, which maintains a solenoidal magnetic field across
refinement boundaries. The code is tested against a large suite of test
problems, including convergence tests in smooth flows, shock-tube tests,
classical two- and three-dimensional MHD tests, a three-dimensional shock-cloud
interaction problem and the formation of a cluster of galaxies in a fully
cosmological context. The magnetic field divergence is shown to remain
negligible throughout.Comment: 53 pages, 17 figs, under review by ApJ
Multi-GeV Electron Spectrometer
The advance in laser plasma acceleration techniques pushes the regime of the
resulting accelerated particles to higher energies and intensities. In
particular the upcoming experiments with the FLAME laser at LNF will enter the
GeV regime with almost 1pC of electrons. From the current status of
understanding of the acceleration mechanism, relatively large angular and
energy spreads are expected. There is therefore the need to develop a device
capable to measure the energy of electrons over three orders of magnitude (few
MeV to few GeV) under still unknown angular divergences. Within the PlasmonX
experiment at LNF a spectrometer is being constructed to perform these
measurements. It is made of an electro-magnet and a screen made of
scintillating fibers for the measurement of the trajectories of the particles.
The large range of operation, the huge number of particles and the need to
focus the divergence present unprecedented challenges in the design and
construction of such a device. We will present the design considerations for
this spectrometer and the first results from a prototype.Comment: 7 pages, 6 figures, submitted to NIM
A divergence-cleaning scheme for cosmological SPMHD simulations
In magnetohydrodynamics (MHD), the magnetic field is evolved by the induction
equation and coupled to the gas dynamics by the Lorentz force. We perform
numerical smoothed particle magnetohydrodynamics (Spmhd) simulations and study
the influence of a numerical magnetic divergence. For instabilities arising
from divergence B related errors, we find the hyperbolic/parabolic cleaning
scheme suggested by Dedner et al. 2002 to give good results and prevent
numerical artifacts from growing. Additionally, we demonstrate that certain
current Spmhd implementations of magnetic field regularizations give rise to
unphysical instabilities in long-time simulations. We also find this effect
when employing Euler potentials (divergenceless by definition), which are not
able to follow the winding-up process of magnetic field lines properly.
Furthermore, we present cosmological simulations of galaxy cluster formation at
extremely high resolution including the evolution of magnetic fields. We show
synthetic Faraday rotation maps and derive structure functions to compare them
with observations. Comparing all the simulations with and without divergence
cleaning, we are able to confirm the results of previous simulations performed
with the standard implementation of MHD in Spmhd at normal resolution. However,
at extremely high resolution, a cleaning scheme is needed to prevent the growth
of numerical errors at small scales.Comment: 15 pages, 19 figures, submitted to MNRA
Enzyme variability and neonatal jaundice. The role of adenosine deaminase and acid phosphatase
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