367 research outputs found
The Fast Multipole Method and Point Dipole Moment Polarizable Force Fields
We present an implementation of the fast multipole method for computing
coulombic electrostatic and polarization forces from polarizable force-fields
based on induced point dipole moments. We demonstrate the expected
scaling of that approach by performing single energy point calculations on
hexamer protein subunits of the mature HIV-1 capsid. We also show the long time
energy conservation in molecular dynamics at the nanosecond scale by performing
simulations of a protein complex embedded in a coarse-grained solvent using a
standard integrator and a multiple time step integrator. Our tests show the
applicability of FMM combined with state-of-the-art chemical models in
molecular dynamical systems.Comment: 11 pages, 8 figures, accepted by J. Chem. Phy
A New Estimate of the Hubble Time with Improved Modeling of Gravitational Lenses
This paper examines free-form modeling of gravitational lenses using Bayesian
ensembles of pixelated mass maps. The priors and algorithms from previous work
are clarified and significant technical improvements are made. Lens
reconstruction and Hubble Time recovery are tested using mock data from simple
analytic models and recent galaxy-formation simulations. Finally, using
published data, the Hubble Time is inferred through the simultaneous
reconstruction of eleven time-delay lenses. The result is
H_0^{-1}=13.7^{+1.8}_{-1.0} Gyr.Comment: 24 pages, 9 figures. Accepted to Ap
An Optimizing Symbolic Algebra Approach for Generating Fast Multipole Method Operators
We have developed a symbolic algebra approach to automatically produce,
verify, and optimize computer code for the Fast Multipole Method (FMM)
operators. This approach allows for flexibility in choosing a basis set and
kernel, and can generate computer code for any expansion order in multiple
languages. The procedure is implemented in the publicly available Python
program Mosaic. Optimizations performed at the symbolic level through algebraic
manipulations significantly reduce the number of mathematical operations
compared with a straightforward implementation of the equations. We find that
the optimizer is able to eliminate 20-80% of the floating-point operations and
for the expansion orders it changes the observed scaling properties.
We present our approach using three variants of the operators with the
Cartesian basis set for the harmonic potential kernel , including the use
of totally symmetric and traceless multipole tensors.Comment: Updated to final version submitted to Computer Physics
Communications. Accepted on 20 November 201
Gravitational lens recovery with glass: measuring the mass profile and shape of a lens
We use a new non-parametric gravitational modelling tool - glass - to determine what quality of data (strong lensing, stellar kinematics, and/or stellar masses) are required to measure the circularly averaged mass profile of a lens and its shape. glass uses an underconstrained adaptive grid of mass pixels to model the lens, searching through thousands of models to marginalize over model uncertainties. Our key findings are as follows: (i) for pure lens data, multiple sources with wide redshift separation give the strongest constraints as this breaks the well-known mass-sheet or steepness degeneracy; (ii) a single quad with time delays also performs well, giving a good recovery of both the mass profile and its shape; (iii) stellar masses - for lenses where the stars dominate the central potential - can also break the steepness degeneracy, giving a recovery for doubles almost as good as having a quad with time-delay data, or multiple source redshifts; (iv) stellar kinematics provide a robust measure of the mass at the half-light radius of the stars r1/2 that can also break the steepness degeneracy if the Einstein radius rE ≠r1/2; and (v) if rE∼r1/2, then stellar kinematic data can be used to probe the stellar velocity anisotropy β - an interesting quantity in its own right. Where information on the mass distribution from lensing and/or other probes becomes redundant, this opens up the possibility of using strong lensing to constrain cosmological model
Lessons from a blind study of simulated lenses: image reconstructions do not always reproduce true convergence
In the coming years, strong gravitational lens discoveries are expected to
increase in frequency by two orders of magnitude. Lens-modelling techniques are
being developed to prepare for the coming massive influx of new lens data, and
blind tests of lens reconstruction with simulated data are needed for
validation. In this paper we present a systematic blind study of a sample of 15
simulated strong gravitational lenses from the EAGLE suite of hydrodynamic
simulations. We model these lenses with a free-form technique and evaluate
reconstructed mass distributions using criteria based on shape, orientation,
and lensed image reconstruction. Especially useful is a lensing analogue of the
Roche potential in binary star systems, which we call the . This we introduce in order to factor out the well-known
problem of steepness or mass-sheet degeneracy. Einstein radii are on average
well recovered with a relative error of for quads and
for doubles; the position angle of ellipticity is on average also reproduced
well up to , but the reconstructed mass maps tend to be too
round and too shallow. It is also easy to reproduce the lensed images, but
optimising on this criterion does not guarantee better reconstruction of the
mass distribution.Comment: 20 pages, 12 figures. Published in MNRAS. Agrees with published
versio
Light versus dark in strong-lens galaxies: Dark matter haloes that are rounder than their stars
We measure the projected density profile, shape and alignment of the stellar
and dark matter mass distribution in 11 strong-lens galaxies. We find that the
projected dark matter density profile - under the assumption of a Chabrier
stellar initial mass function - shows significant variation from galaxy to
galaxy. Those with an outermost image beyond kpc are very well fit by
a projected NFW profile; those with images within 10 kpc appear to be more
concentrated than NFW, as expected if their dark haloes contract due to
baryonic cooling. We find that over several half-light radii, the dark matter
haloes of these lenses are rounder than their stellar mass distributions. While
the haloes are never more elliptical than , their stars can
extend to . Galaxies with high dark matter ellipticity and weak
external shear show strong alignment between light and dark; those with strong
shear () can be highly misaligned. This is reassuring since
isolated misaligned galaxies are expected to be unstable. Our results provide a
new constraint on galaxy formation models. For a given cosmology, these must
explain the origin of both very round dark matter haloes and misaligned
strong-lens systems.Comment: 16 pages, 7 figures, 4 tables. Accepted for publication by MNRA
The SATIN project I: Turbulent multi-phase ISM in Milky Way simulations with SNe feedback from stellar clusters
We introduce the star formation and Supernova (SN) feedback model of the
SATIN (Simulating AGNs Through ISM with Non-Equilibrium Effects) project to
simulate the evolution of the star forming multi-phase interstellar medium
(ISM) of entire disk galaxies. This galaxy-wide implementation of a successful
ISM feedback model naturally covers an order of magnitude in gas surface
density, shear and radial motions. It is implemented in the adaptive mesh
refinement code RAMSES at a peak resolution of 9 pc. New stars are represented
by star cluster (sink) particles with individual SN delay times for massive
stars. With SN feedback, cooling and gravity, the galactic ISM develops a
realistic three-phase structure. The star formation rates naturally follow
observed scaling relations for the local Milky Way gas surface density. SNe
drive additional turbulence in the warm (300 K < < 10 K) gas and
increase the kinetic energy of the cold gas, cooling out of the warm phase. The
majority of the gas leaving the galactic ISM is warm and hot with mass loading
factors of . While the hot gas is leaving the system, the
warm and cold gas falls back onto the disc in a galactic fountain flow.Comment: Submitted to MNRA
- …