690 research outputs found
Stability of disk galaxies in the modified dynamics
General analytic arguments lead us to expect that in the modified dynamics
(MOND) self-gravitating disks are more stable than their like in Newtonian
dynamics. We study this question numerically, using a particle-mesh code based
on a multi-grid solver for the (nonlinear) MOND field equation. We start with
equilibrium distribution functions for MOND disk models having a smoothly
truncated, exponential surface-density profiles and a constant Toomre
parameter. We find that, indeed, disks of a given ``temperature'' are locally
more stable in MOND than in Newtonian dynamics. As regards global instability
to bar formation, we find that as the mean acceleration in the disk is lowered,
the stability of the disk is increased as we cross from the Newtonian to the
MOND regime. The degree of stability levels off deep in the MOND regime, as
expected from scaling laws in MOND. For the disk model we use, this maximum
degree of stability is similar to the one imparted to a Newtonian disk by a
halo three times as massive at five disk scale lengths.Comment: 20 pages, Latex, 8 embedded figures, version to be published in The
Astrophys.
Snake states and their symmetries in graphene
Snake states are open trajectories for charged particles propagating in two
dimensions under the influence of a spatially varying perpendicular magnetic
field. In the quantum limit they are protected edge modes that separate
topologically inequivalent ground states and can also occur when the particle
density rather than the field is made nonuniform. We examine the correspondence
of snake trajectories in single-layer graphene in the quantum limit for two
families of domain walls: (a) a uniform doped carrier density in an
antisymmetric field profile and (b) antisymmetric carrier distribution in a
uniform field. These families support different internal symmetries but the
same pattern of boundary and interface currents. We demonstrate that these
physically different situations are gauge equivalent when rewritten in a Nambu
doubled formulation of the two limiting problems. Using gauge transformations
in particle-hole space to connect these problems, we map the protected
interfacial modes to the Bogoliubov quasiparticles of an interfacial
one-dimensional p-wave paired state. A variational model is introduced to
interpret the interfacial solutions of both domain wall problems
The shape of "dark matter" halos in disc galaxies according to the modified dynamics
Analyses of halo shapes for disc galaxies are said to give incongruous
results. I point out that the modified dynamics (MOND) predicts for disc
galaxies a distribution of fictitious dark matter that comprises two
components: a pure disc and a rounder halo. The former dominates the true disc
in regions of small accelerations, where it controls the z-dynamics in the disc
(disc flaring etc.); it has a finite total mass. It also dominates the round
component near the centre where the geometry is nearly planar. The second
component controls motions far from the plane, has a total enclosed mass that
diverges linearly with radius, and determines the rotation curve at large
radii. Its ellipticity may be appreciable at small radii but vanishes
asymptotically. This prediction of MOND differs from what one expects from
galaxy-formation scenarios with dark matter.
Analyses to date, which, as they do, assume one component--usually with a
constant ellipticity, perforce give conflicting results for the best-value
ellipticity, depending on whether they probe the disc or the sphere, small
radii or large ones.Comment: 8 page
Contracts in the Wild: A Study of Java Programs (Artifact)
This artefact contains a dataset of open-source programs obtained from the Maven Central Repository and scripts that first extract contracts from these programs and then perform several analyses on the contracts extracted. The extraction and analysis is fully automated and directly produces the tables presented in the accompanying paper.
The results show how contracts are used in real-world program, and how their usage changes between versions and within inheritance hierarchies
High-grade malignant glioma: ESMO Clinical Practice Guidelines for diagnosis, treatment and follow-up
A QUMOND galactic N-body code I: Poisson solver and rotation curve fitting
Here we present a new particle-mesh galactic N-body code that uses the full
multigrid algorithm for solving the modified Poisson equation of the Quasi
Linear formulation of Modified Newtonian Dynamics (QUMOND). A novel approach
for handling the boundary conditions using a refinement strategy is implemented
and the accuracy of the code is compared with analytical solutions of Kuzmin
disks. We then employ the code to compute the predicted rotation curves for a
sample of five spiral galaxies from the THINGS sample. We generated static
N-body realisations of the galaxies according to their stellar and gaseous
surface densities and allowed their distances, mass-to-light ratios (M/L) and
both the stellar and gas scale-heights to vary in order to estimate the best
fit parameters. We found that NGC 3621, NGC 3521 and DDO 154 are well fit by
MOND using expected values of the distance and M/L. NGC 2403 required a
moderately larger than expected and NGC 2903 required a substantially
larger value. The surprising result was that the scale-height of the dominant
baryonic component was well constrained by the rotation curves: the gas
scale-height for DDO 154 and the stellar scale-height for the others. In fact,
if the suggested stellar scale-height (one-fifth the stellar scale-length) was
used in the case of NGC 3621 and NGC 3521 it would not be possible to produce a
good fit to the inner rotation curve. For each of the four stellar dominated
galaxies, we calculated the vertical velocity dispersions which we found to be,
on the whole, quite typical compared with observed stellar vertical velocity
dispersions of face on spirals. We conclude that modelling the gas
scale-heights of the gas rich dwarf spiral galaxies will be vital in order to
make precise conclusions about MOND.Comment: 14 pages, 11 figures, MNRAS in pres
Milky Way Mass Models and MOND
Using the Tuorla-Heidelberg model for the mass distribution of the Milky Way,
I determine the rotation curve predicted by MOND. The result is in good
agreement with the observed terminal velocities interior to the solar radius
and with estimates of the Galaxy's rotation curve exterior thereto. There are
no fit parameters: given the mass distribution, MOND provides a good match to
the rotation curve. The Tuorla-Heidelberg model does allow for a variety of
exponential scale lengths; MOND prefers short scale lengths in the range 2.0 to
2.5 kpc. The favored value of scale length depends somewhat on the choice of
interpolation function. There is some preference for the `simple' interpolation
function as found by Famaey & Binney. I introduce an interpolation function
that shares the advantages of the simple function on galaxy scales while having
a much smaller impact in the solar system. I also solve the inverse problem,
inferring the surface mass density distribution of the Milky Way from the
terminal velocities. The result is a Galaxy with `bumps and wiggles' in both
its luminosity profile and rotation curve that are reminiscent of those
frequently observed in external galaxies.Comment: Accepted for publication in the Astrophysical Journal. 31 pages
including 8 figures and 3 table
- …