93,996 research outputs found
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
Image registration with sparse approximations in parametric dictionaries
We examine in this paper the problem of image registration from the new
perspective where images are given by sparse approximations in parametric
dictionaries of geometric functions. We propose a registration algorithm that
looks for an estimate of the global transformation between sparse images by
examining the set of relative geometrical transformations between the
respective features. We propose a theoretical analysis of our registration
algorithm and we derive performance guarantees based on two novel important
properties of redundant dictionaries, namely the robust linear independence and
the transformation inconsistency. We propose several illustrations and insights
about the importance of these dictionary properties and show that common
properties such as coherence or restricted isometry property fail to provide
sufficient information in registration problems. We finally show with
illustrative experiments on simple visual objects and handwritten digits images
that our algorithm outperforms baseline competitor methods in terms of
transformation-invariant distance computation and classification
Bounding right-arm rotation distances
Rotation distance measures the difference in shape between binary trees of
the same size by counting the minimum number of rotations needed to transform
one tree to the other. We describe several types of rotation distance where
restrictions are put on the locations where rotations are permitted, and
provide upper bounds on distances between trees with a fixed number of nodes
with respect to several families of these restrictions. These bounds are sharp
in a certain asymptotic sense and are obtained by relating each restricted
rotation distance to the word length of elements of Thompson's group F with
respect to different generating sets, including both finite and infinite
generating sets.Comment: 30 pages, 11 figures. This revised version corrects some typos and
has some clearer proofs of the results for the lower bounds and better
figure
Protein Docking by the Underestimation of Free Energy Funnels in the Space of Encounter Complexes
Similarly to protein folding, the association of two proteins is driven
by a free energy funnel, determined by favorable interactions in some neighborhood of the
native state. We describe a docking method based on stochastic global minimization of
funnel-shaped energy functions in the space of rigid body motions (SE(3)) while accounting
for flexibility of the interface side chains. The method, called semi-definite
programming-based underestimation (SDU), employs a general quadratic function to
underestimate a set of local energy minima and uses the resulting underestimator to bias
further sampling. While SDU effectively minimizes functions with funnel-shaped basins, its
application to docking in the rotational and translational space SE(3) is not
straightforward due to the geometry of that space. We introduce a strategy that uses
separate independent variables for side-chain optimization, center-to-center distance of the
two proteins, and five angular descriptors of the relative orientations of the molecules.
The removal of the center-to-center distance turns out to vastly improve the efficiency of
the search, because the five-dimensional space now exhibits a well-behaved energy surface
suitable for underestimation. This algorithm explores the free energy surface spanned by
encounter complexes that correspond to local free energy minima and shows similarity to the
model of macromolecular association that proceeds through a series of collisions. Results
for standard protein docking benchmarks establish that in this space the free energy
landscape is a funnel in a reasonably broad neighborhood of the native state and that the
SDU strategy can generate docking predictions with less than 5 � ligand interface Ca
root-mean-square deviation while achieving an approximately 20-fold efficiency gain compared
to Monte Carlo methods
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