6,695 research outputs found
Internal kinematics of isolated modelled disk galaxies
We present a systematic investigation of rotation curves (RCs) of fully
hydrodynamically simulated galaxies, including cooling, star formation with
associated feedback and galactic winds. Applying two commonly used fitting
formulae to characterize the RCs, we investigate systematic effects on the
shape of RCs both by observational constraints and internal properties of the
galaxies. We mainly focus on effects that occur in measurements of intermediate
and high redshift galaxies. We find that RC parameters are affected by the
observational setup, like slit misalignment or the spatial resolution and also
depend on the evolution of a galaxy. Therefore, a direct comparison of
quantities derived from measured RCs with predictions of semi-analytic models
is difficult. The virial velocity V_c, which is usually calculated and used by
semi-analytic models can differ significantly from fit parameters like V_max or
V_opt inferred from RCs. We find that V_c is usually lower than typical
characteristic velocities derived from RCs. V_max alone is in general not a
robust estimator for the virial mass.Comment: 9 pages, 15 figures, accepted for publication in A&
Target space duality and moduli stabilization in String Gas Cosmology
Motivated by string gas cosmology, we investigate the stability of moduli
fields coming from compactifications of string gas on torus with background
flux. It was previously claimed that moduli are stabilized only at a single
fixed point in moduli space, a self-dual point of T-duality with vanishing
flux. Here, we show that there exist other stable fixed points on moduli space
with non-vanishing flux. We also discuss the more general target space
dualities associated with these fixed points.Comment: 12 pages, 1 figur
Internal kinematics of modelled interacting disc galaxies
We present an investigation of galaxy-galaxy interactions and their effects
on the velocity fields of disc galaxies in combined N-body/hydrodynamic
simulations, which include cooling, star formation with feedback, and galactic
winds. Rotation curves (RCs) of the gas are extracted from these simulations in
a way that follows the procedure applied to observations of distant, small, and
faint galaxies as closely as possible. We show that galaxy-galaxy mergers and
fly-bys disturb the velocity fields significantly and hence the RCs of the
interacting galaxies, leading to asymmetries and distortions in the RCs.
Typical features of disturbed kinematics are significantly rising or falling
profiles in the direction of the companion galaxy and pronounced bumps in the
RCs. In addition, tidal tails can leave strong imprints on the rotation curve.
All these features are observable for intermediate redshift galaxies, on which
we focus our investigations. We use a quantitative measure for the asymmetry of
rotation curves to show that the appearance of these distortions strongly
depends on the viewing angle. We also find in this way that the velocity fields
settle back into relatively undisturbed equilibrium states after unequal mass
mergers and fly-bys. About 1 Gyr after the first encounter, the RCs show no
severe distortions anymore. These results are consistent with previous
theoretical and observational studies. As an illustration of our results, we
compare our simulated velocity fields and direct images with rotation curves
from VLT/FORS spectroscopy and ACS images of a cluster at z=0.53 and find
remarkable similarities.Comment: 13 pages, 14 figures, accepted for publication in A&A, some
improvements and changes, main conclusions are unaffecte
Spin dynamics in copper metaborate studied by muon spin relaxation
Copper metaborate CuBO was studied by muon spin relaxation
measurements in order to clarify its static and dynamic magnetic properties.
The time spectra of muon spin depolarization suggest that the local fields at
the muon site contain both static and fluctuating components in all ordered
phases down to 0.3 K. In the weak ferromagnetic phase (20 K~~9.3 K), the
static component is dominant. On the other hand, upon cooling the fluctuating
component becomes dominant in the incommensurate helix phase (9.3K > T > 1.4K).
The dynamical fluctuations of the local fields persist down to 0.3K, where a
new incommensurate phase (T < 1.4K) is expected to appear. This result suggests
that spins fluctuate even at T \to 0. We propose two possible origins of the
remnant dynamical spin fluctuations: frustration of the exchange interactions
and the dynamic behavior of the soliton lattice
Historical roots of Agile methods: where did “Agile thinking” come from?
The appearance of Agile methods has been the most noticeable change to software process thinking in the last fifteen years [16], but in fact many of the “Agile ideas” have been around since 70’s or even before. Many studies and reviews have been conducted about Agile methods which ascribe their emergence as a reaction against traditional methods. In this paper, we argue that although Agile methods are new as a whole, they have strong roots in the history of software engineering. In addition to the iterative and incremental approaches that have been in use since 1957 [21], people who criticised the traditional methods suggested alternative approaches which were actually Agile ideas such as the response to change, customer involvement, and working software over documentation. The authors of this paper believe that education about the history of Agile thinking will help to develop better understanding as well as promoting the use of Agile methods. We therefore present and discuss the reasons behind the development and introduction of Agile methods, as a reaction to traditional methods, as a result of people's experience, and in particular focusing on reusing ideas from histor
Anisotropic superexchange of a 90 degree Cu-O-Cu bond
The magnetic anisotropy af a rectangular Cu-O-Cu bond is investigated in
second order of the spin-orbit interaction. Such a bond is characteristic for
cuprates having edge sharing CuO_2 chains, and exists also in the Cu_3O_4 plane
or in ladder compounds. For a ferromagnetic coupling between the copper spins
an easy axis is found perpendicular to the copper oxygen plaquettes in
agreement with the experimental spin structure of Li_2CuO_2. In addition, a
pseudo-dipolar interaction is derived. Its estimation in the case of the
Cu_3O_4 plane (which is present for instance in Ba_2Cu_3O_4Cl_2 or
Sr_2Cu_3O_4Cl_2) gives a value which is however two orders of magnitude smaller
than the usual dipole-dipole interaction.Comment: 6 pages, 2 figures, improved referenc
Dynamics of the two-dimensional S=1/2 dimer system (C5H6N2F)2CuCl4
Inelastic neutron scattering was used to study a quantum S=1/2
antiferromagnetic Heisenberg system-Bis(2-amino-5-fluoropyridinium)
Tetrachlorocuprate(II). The magnetic excitation spectrum was shown to be
dominated by long-lived excitations with an energy gap as 1.07(3) meV. The
measured dispersion relation is consistent with a simple two-dimensional square
lattice of weakly-coupled spin dimers. Comparing the data to a random phase
approximation treatment of this model gives the intra-dimer and inter-dimer
exchange constants J=1.45(2) meV and J'=0.31(3) meV, respectively.Comment: 4 pages, 4 figure
Vibrating Winding Branes, Wrapping Democracy and Stabilization of Extra Dimensions in Dilaton Gravity
We show that, in the context of dilaton gravity, a recently proposed
democratic principle for intersection possibilities of branes winding around
extra dimensions yield stabilization, even with the inclusion of momentum modes
of the wrapped branes on top of the winding modes. The constraints for
stabilization massaged by string theory inputs forces the number of observed
dimensions to be three. We also discuss consequences of adding ordinary matter
living in the observed dimensions.Comment: Added a section discussing the linear and non-linear stability of the
equilibrium point of the scale factors of the extra dimensions. Corrected a
typo in the original field equations and other typos. Added and changed
references. Final version appeared in JHE
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