34,721 research outputs found
The dynamical distance and intrinsic structure of the globular cluster omega Centauri
We determine the dynamical distance D, inclination i, mass-to-light ratio M/L
and the intrinsic orbital structure of the globular cluster omega Cen, by
fitting axisymmetric dynamical models to the ground-based proper motions of van
Leeuwen et al. and line-of-sight velocities from four independent data-sets. We
correct the observed velocities for perspective rotation caused by the space
motion of the cluster, and show that the residual solid-body rotation component
in the proper motions can be taken out without any modelling other than
assuming axisymmetry. This also provides a tight constraint on D tan i.
Application of our axisymmetric implementation of Schwarzschild's orbit
superposition method to omega Cen reveals no dynamical evidence for a
significant radial dependence of M/L. The best-fit dynamical model has a
stellar V-band mass-to-light ratio M/L_V = 2.5 +/- 0.1 M_sun/L_sun and an
inclination i = 50 +/- 4 degrees, which corresponds to an average intrinsic
axial ratio of 0.78 +/- 0.03. The best-fit dynamical distance D = 4.8 +/- 0.3
kpc (distance modulus 13.75 +/- 0.13 mag) is significantly larger than obtained
by means of simple spherical or constant-anisotropy axisymmetric dynamical
models, and is consistent with the canonical value 5.0 +/- 0.2 kpc obtained by
photometric methods. The total mass of the cluster is (2.5 +/- 0.3) x 10^6
M_sun. The best-fit model is close to isotropic inside a radius of about 10
arcmin and becomes increasingly tangentially anisotropic in the outer region,
which displays significant mean rotation. This phase-space structure may well
be caused by the effects of the tidal field of the Milky Way. The cluster
contains a separate disk-like component in the radial range between 1 and 3
arcmin, contributing about 4% to the total mass.Comment: 37 pages (23 figures), accepted for publication in A&A, abstract
abridged, for PS and PDF file with full resolution figures, see
http://www.strw.leidenuniv.nl/~vdven/oc
Study of the neutron quantum states in the gravity field
We have studied neutron quantum states in the potential well formed by the
earth's gravitational field and a horizontal mirror. The estimated
characteristic sizes of the neutron wave functions in the two lowest quantum
states correspond to expectations with an experimental accuracy. A
position-sensitive neutron detector with an extra-high spatial resolution of ~2
microns was developed and tested for this particular experiment, to be used to
measure the spatial density distribution in a standing neutron wave above a
mirror for a set of some of the lowest quantum states. The present experiment
can be used to set an upper limit for an additional short-range fundamental
force. We studied methodological uncertainties as well as the feasibility of
improving further the accuracy of this experiment
How do neural networks see depth in single images?
Deep neural networks have lead to a breakthrough in depth estimation from
single images. Recent work often focuses on the accuracy of the depth map,
where an evaluation on a publicly available test set such as the KITTI vision
benchmark is often the main result of the article. While such an evaluation
shows how well neural networks can estimate depth, it does not show how they do
this. To the best of our knowledge, no work currently exists that analyzes what
these networks have learned.
In this work we take the MonoDepth network by Godard et al. and investigate
what visual cues it exploits for depth estimation. We find that the network
ignores the apparent size of known obstacles in favor of their vertical
position in the image. Using the vertical position requires the camera pose to
be known; however we find that MonoDepth only partially corrects for changes in
camera pitch and roll and that these influence the estimated depth towards
obstacles. We further show that MonoDepth's use of the vertical image position
allows it to estimate the distance towards arbitrary obstacles, even those not
appearing in the training set, but that it requires a strong edge at the ground
contact point of the object to do so. In future work we will investigate
whether these observations also apply to other neural networks for monocular
depth estimation.Comment: Submitte
Velocity Structure Diagnostics of Simulated Galaxy Clusters
Gas motions in the hot intracluster medium of galaxy clusters have an
important effect on the mass determination of the clusters through X-ray
observations. The corresponding dynamical pressure has to be accounted for in
addition to the hydrostatic pressure support to achieve a precise mass
measurement. An analysis of the velocity structure of the ICM for simulated
cluster-size haloes, especially focusing on rotational patterns, has been
performed, demonstrating them to be an intermittent phenomenon, strongly
related to the internal dynamics of substructures. We find that the expected
build-up of rotation due to mass assembly gets easily destroyed by passages of
gas-rich substructures close to the central region. Though, if a typical
rotation pattern is established, the corresponding mass contribution is
estimated to be up to ~17% of the total mass in the innermost region, and one
has to account for it. Extending the analysis to a larger sample of simulated
haloes we statistically observe that (i) the distribution of the rotational
component of the gas velocity in the innermost region has typical values of
~200-300 km/s; (ii) except for few outliers, there is no monotonic increase of
the rotational velocity with decreasing redshift, as we would expect from
approaching a relaxed configuration. Therefore, the hypothesis that the
build-up of rotation is strongly influenced by internal dynamics is confirmed,
and minor events like gas-rich substructures passing close to the equatorial
plane can easily destroy any ordered rotational pattern.Comment: 13 pages, 10 figures; Accepted for publication in MNRA
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