91,170 research outputs found
Galaxy density profiles and shapes -- II. selection biases in strong lensing surveys
[Abridged] Many current and future astronomical surveys will rely on samples
of strong gravitational lens systems to draw conclusions about galaxy mass
distributions. We use a new strong lensing pipeline (presented in Paper I of
this series) to explore selection biases that may cause the population of
strong lensing systems to differ from the general galaxy population. Our focus
is on point-source lensing by early-type galaxies with two mass components
(stellar and dark matter) that have a variety of density profiles and shapes
motivated by observational and theoretical studies of galaxy properties. We
seek not only to quantify but also to understand the physics behind selection
biases related to: galaxy mass, orientation and shape; dark matter profile
parameters such as inner slope and concentration; and adiabatic contraction. We
study how all of these properties affect the lensing Einstein radius, total
cross-section, quad/double ratio, and image separation distribution. We find
significant (factors of several) selection biases with mass; orientation, for a
given galaxy shape at fixed mass; cusped dark matter profile inner slope and
concentration; concentration of the stellar and dark matter deprojected Sersic
models. Interestingly, the intrinsic shape of a galaxy does not strongly
influence its lensing cross-section when we average over viewing angles. Our
results are an important first step towards understanding how strong lens
systems relate to the general galaxy population.Comment: 26 pages, 15 figures; paper I at arXiv:0808.2493; accepted for
publication in MNRAS (minor revisions); PDF file with full resolution figures
at http://www.sns.ias.edu/~rmandelb/paper2.pd
View Direction, Surface Orientation and Texture Orientation for Perception of Surface Shape
Textures are commonly used to enhance the representation of shape in non-photorealistic rendering applications such as medical drawings. Textures that have elongated linear elements appear to be superior to random textures in that they can, by the way they conform to the surface, reveal the surface shape. We observe that shape following hache marks commonly used in cartography and copper-plate illustration are locally similar to the effect of the lines that can be generated by the intersection of a set of parallel planes with a surface. We use this as a basis for investigating the relationships between view direction, texture orientation and surface orientation in affording surface shape perception. We report two experiments using parallel plane textures. The results show that textures constructed from planes more nearly orthogonal to the line of sight tend to be better at revealing surface shape. Also, viewing surfaces from an oblique view is much better for revealing surface shape than viewing them from directly above
Strong lensing in the MareNostrum Universe: biases in the cluster lens population
Strong lensing is one of the most direct probes of the mass distribution in
the inner regions of galaxy clusters. It can be used to constrain the density
profiles and to measure the mass of the lenses. Moreover, the abundance of
strong lensing events can be used to constrain the structure formation and the
cosmological parameters through the so-called "arc-statistics" approach.
However, several issues related to the usage of strong lensing clusters in
cosmological applications are still controversial, leading to the suspect that
several biases may affect this very peculiar class of objects. With this study
we aim at better understanding the properties of galaxy clusters which can
potentially act as strong lenses. We do so by investigating the properties of a
large sample of galaxy clusters extracted from the N-body/hydrodynamical
simulation MareNostrum Universe. We explore the correlation between the cross
section for lensing and many properties of clusters, like the mass, the
three-dimensional and projected shapes, their concentrations, the X-ray
luminosity and the dynamical activity. We find that the probability of strong
alignments between the major axes of the lenses and the line of sight is a
growing function of the lensing cross section. In projection, the strong lenses
appear rounder within R200, but we find that their cores tend to be more
elliptical as the lensing cross section increases. We also find that the
cluster concentrations estimated from the projected density profiles tend to be
biased high. The X-ray luminosity of strong lensing clusters is higher than
that of normal lenses of similar mass and redshift. This is particular
significant for the least massive lenses. Finally, we find that the strongest
lenses generally exhibit an excess of kinetic energy within the virial radius,
indicating that they are more dynamically active than usual clusters.Comment: 22 pages, 18 figures, accepted for publication on A&
A Comparison of Simple Mass Estimators for Galaxy Clusters
High-resolution N-body simulations are used to investigate systematic trends
in the mass profiles and total masses of clusters as derived from 3 simple
estimators: (1) the weak gravitational lensing shear field under the assumption
of an isothermal cluster potential, (2) the dynamical mass obtained from the
measured velocity dispersion under the assumption of an isothermal cluster
potential, and (3) the classical virial estimator. The clusters consist of
order 2.5e+05 particles of mass m_p \simeq 10^{10} \Msun, have triaxial mass
distributions, and significant substructure exists within their virial radii.
Not surprisingly, the level of agreement between the mass profiles obtained
from the various estimators and the actual mass profiles is found to be
scale-dependent.
The virial estimator yields a good measurement of the total cluster mass,
though it is systematically underestimated by of order 10%. This result
suggests that, at least in the limit of ideal data, the virial estimator is
quite robust to deviations from pure spherical symmetry and the presence of
substructure. The dynamical mass estimate based upon a measurement of the
cluster velocity dispersion and an assumption of an isothermal potential yields
a poor measurement of the total mass. The weak lensing estimate yields a very
good measurement of the total mass, provided the mean shear used to determine
the equivalent cluster velocity dispersion is computed from an average of the
lensing signal over the entire cluster (i.e. the mean shear is computed
interior to the virial radius). [abridged]Comment: Accepted for publication in The Astrophysical Journal. Complete
paper, including 3 large colour figures can also be obtained from
http://bu-ast.bu.edu/~brainerd/preprints
ColDICE: a parallel Vlasov-Poisson solver using moving adaptive simplicial tessellation
Resolving numerically Vlasov-Poisson equations for initially cold systems can
be reduced to following the evolution of a three-dimensional sheet evolving in
six-dimensional phase-space. We describe a public parallel numerical algorithm
consisting in representing the phase-space sheet with a conforming,
self-adaptive simplicial tessellation of which the vertices follow the
Lagrangian equations of motion. The algorithm is implemented both in six- and
four-dimensional phase-space. Refinement of the tessellation mesh is performed
using the bisection method and a local representation of the phase-space sheet
at second order relying on additional tracers created when needed at runtime.
In order to preserve in the best way the Hamiltonian nature of the system,
refinement is anisotropic and constrained by measurements of local Poincar\'e
invariants. Resolution of Poisson equation is performed using the fast Fourier
method on a regular rectangular grid, similarly to particle in cells codes. To
compute the density projected onto this grid, the intersection of the
tessellation and the grid is calculated using the method of Franklin and
Kankanhalli (1993) generalised to linear order. As preliminary tests of the
code, we study in four dimensional phase-space the evolution of an initially
small patch in a chaotic potential and the cosmological collapse of a
fluctuation composed of two sinusoidal waves. We also perform a "warm" dark
matter simulation in six-dimensional phase-space that we use to check the
parallel scaling of the code.Comment: Code and illustration movies available at:
http://www.vlasix.org/index.php?n=Main.ColDICE - Article submitted to Journal
of Computational Physic
Morphological evolution of a 3D CME cloud reconstructed from three viewpoints
The propagation properties of coronal mass ejections (CMEs) are crucial to
predict its geomagnetic effect. A newly developed three dimensional (3D) mask
fitting reconstruction method using coronagraph images from three viewpoints
has been described and applied to the CME ejected on August 7, 2010. The CME's
3D localisation, real shape and morphological evolution are presented. Due to
its interaction with the ambient solar wind, the morphology of this CME changed
significantly in the early phase of evolution. Two hours after its initiation,
it was expanding almost self-similarly. CME's 3D localisation is quite helpful
to link remote sensing observations to in situ measurements. The investigated
CME was propagating to Venus with its flank just touching STEREO B. Its
corresponding ICME in the interplanetary space shows a possible signature of a
magnetic cloud with a preceding shock in VEX observations, while from STEREO B
only a shock is observed. We have calculated three principle axes for the
reconstructed 3D CME cloud. The orientation of the major axis is in general
consistent with the orientation of a filament (polarity inversion line)
observed by SDO/AIA and SDO/HMI. The flux rope axis derived by the MVA analysis
from VEX indicates a radial-directed axis orientation. It might be that locally
only the leg of the flux rope passed through VEX. The height and speed profiles
from the Sun to Venus are obtained. We find that the CME speed possibly had
been adjusted to the speed of the ambient solar wind flow after leaving COR2
field of view and before arriving Venus. A southward deflection of the CME from
the source region is found from the trajectory of the CME geometric center. We
attribute it to the influence of the coronal hole where the fast solar wind
emanated from.Comment: ApJ, accepte
3D Imaging of a Phase Object from a Single Sample Orientation Using an Optical Laser
Ankylography is a new 3D imaging technique, which, under certain
circumstances, enables reconstruction of a 3D object from a single sample
orientation. Here, we provide a matrix rank analysis to explain the principle
of ankylography. We then present an ankylography experiment on a microscale
phase object using an optical laser. Coherent diffraction patterns are acquired
from the phase object using a planar CCD detector and are projected onto a
spherical shell. The 3D structure of the object is directly reconstructed from
the spherical diffraction pattern. This work may potentially open the door to a
new method for 3D imaging of phase objects in the visible light region.
Finally, the extension of ankylography to more complicated and larger objects
is suggested.Comment: 22 pages 5 figure
Electron tomography at 2.4 {\AA} resolution
Transmission electron microscopy (TEM) is a powerful imaging tool that has
found broad application in materials science, nanoscience and biology(1-3).
With the introduction of aberration-corrected electron lenses, both the spatial
resolution and image quality in TEM have been significantly improved(4,5) and
resolution below 0.5 {\AA} has been demonstrated(6). To reveal the 3D structure
of thin samples, electron tomography is the method of choice(7-11), with
resolutions of ~1 nm^3 currently achievable(10,11). Recently, discrete
tomography has been used to generate a 3D atomic reconstruction of a silver
nanoparticle 2-3 nm in diameter(12), but this statistical method assumes prior
knowledge of the particle's lattice structure and requires that the atoms fit
rigidly on that lattice. Here we report the experimental demonstration of a
general electron tomography method that achieves atomic scale resolution
without initial assumptions about the sample structure. By combining a novel
projection alignment and tomographic reconstruction method with scanning
transmission electron microscopy, we have determined the 3D structure of a ~10
nm gold nanoparticle at 2.4 {\AA} resolution. While we cannot definitively
locate all of the atoms inside the nanoparticle, individual atoms are observed
in some regions of the particle and several grains are identified at three
dimensions. The 3D surface morphology and internal lattice structure revealed
are consistent with a distorted icosahedral multiply-twinned particle. We
anticipate that this general method can be applied not only to determine the 3D
structure of nanomaterials at atomic scale resolution(13-15), but also to
improve the spatial resolution and image quality in other tomography
fields(7,9,16-20).Comment: 27 pages, 17 figure
Multi-method-modeling of interacting galaxies. I. A unique scenario for NGC 4449?
(abridged) We combined several N-body methods in order to investigate the
interaction scenario between NGC 4449 and DDO 125, a close companion in
projected space. In a first step fast restricted N-body models are used to
confine a region in parameter space reproducing the main observational
features. In a second step a genetic algorithm is applied for a uniqueness test
of our preferred parameter set. We show that our genetic algorithm reliably
recovers orbital parameters, provided that the data are sufficiently accurate,
i.e. all the key features are included.
In the third step the results of the restricted N-body models are compared
with self-consistent N-body simulations. In the case of NGC 4449, the
applicability of the simple restricted N-body calculations is demonstrated.
Additionally, it is shown that the HI gas can be modeled here by a purely
stellar dynamical approach.
In a series of simulations, we demonstrate that the observed features of the
extended HI disc can be explained by a gravitational interaction between NGC
4449 and DDO 125. According to these calculations the closest approach between
both galaxies happened yr ago at a minimum distance of
kpc on a parabolic or slightly elliptic orbit. In the case of an
encounter scenario, the dynamical mass of DDO 125 should not be smaller than
10% of NGC 4449's mass. Before the encounter, the observed HI gas was arranged
in a disc with a radius of 35-40 kpc around the center of NGC 4449. It had the
same orientation as the central ellipsoidal HI structure. The origin of this
disc is still unclear, but it might have been caused by a previous interaction.Comment: 19 pages with 19 figures, accepted for publication in Astron. &
Astrophys., a full PostScript version is available at
http://www.astrophysik.uni-kiel.de/pershome/theis/pub.htm
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