411 research outputs found
Ring Star Formation Rates in Barred and Nonbarred Galaxies
Nonbarred ringed galaxies are relatively normal galaxies showing bright rings
of star formation in spite of lacking a strong bar. This morphology is
interesting because it is generally accepted that a typical ring forms when
material collects near a resonance, set up by the pattern speed of a bar or
bar-like perturbation. Our goal in this paper is to examine whether the ring
star formation properties are related to the non-axisymmetric gravity potential
in general. For this purpose, we obtained H{\alpha} emission line images and
calculated the line fluxes and star formation rates (SFRs) for 16 nonbarred SA
galaxies and four weakly barred SAB galaxies with rings. For comparison, we
combine our observations with a re-analysis of previously published data on
five SA, seven SAB, and 15 SB galaxies with rings, three of which are
duplicates from our sample. With these data, we examine what role a bar may
play in the star formation process in rings. Compared to barred ringed
galaxies, we find that the inner ring SFRs and H{\alpha}+[N ii] equivalent
widths in nonbarred ringed galaxies show a similar range and trend with
absolute blue magnitude, revised Hubble type, and other parameters. On the
whole, the star formation properties of inner rings, excluding the distribution
of H ii regions, are independent of the ring shapes and the bar strength in our
small samples. We confirm that the deprojected axis ratios of inner rings
correlate with maximum relative gravitational force Q_g; however, if we
consider all rings, a better correlation is found when local bar forcing at the
radius of the ring, Q_r, is used. Individual cases are described and other
correlations are discussed. By studying the physical properties of these
galaxies, we hope to gain a better understanding of their placement in the
scheme of the Hubble sequence and how they formed rings without the driving
force of a bar.Comment: 55 pages; 21 figures and 9 tables. Article has been accepted for
publication in the Astronomical Journa
The 4-Body Problem in a (1+1)-Dimensional Self-Gravitating System
We report on the results of a study of the motion of a four particle
non-relativistic one-dimensional self-gravitating system. We show that the
system can be visualized in terms of a single particle moving within a
potential whose equipotential surfaces are shaped like a box of pyramid-shaped
sides. As such this is the largest -body system that can be visualized in
this way. We describe how to classify possible states of motion in terms of
Braid Group operators, generalizing this to bodies. We find that the
structure of the phase\textcolor{black}{{} space of each of these systems
yields a large variety of interesting dynamics, containing regions of
quasiperiodicity and chaos. Lyapunov exponents are calculated for many
trajectories to measure stochasticity and previously unseen phenomena in the
Lyapunov graphs are observed.Comment: 40 pages, 23 figures, to appear in Journal of Mathematical Physic
Dirichlet fundamental domains and complex-projective varieties
We prove that for every finitely-presented group G there exists a
2-dimensional irreducible complex-projective variety W with the fundamental
group G, so that all singularities of W are normal crossings and Whitney
umbrellas.Comment: 1 figur
Deconstructing Approximate Offsets
We consider the offset-deconstruction problem: Given a polygonal shape Q with
n vertices, can it be expressed, up to a tolerance \eps in Hausdorff distance,
as the Minkowski sum of another polygonal shape P with a disk of fixed radius?
If it does, we also seek a preferably simple-looking solution P; then, P's
offset constitutes an accurate, vertex-reduced, and smoothened approximation of
Q. We give an O(n log n)-time exact decision algorithm that handles any
polygonal shape, assuming the real-RAM model of computation. A variant of the
algorithm, which we have implemented using CGAL, is based on rational
arithmetic and answers the same deconstruction problem up to an uncertainty
parameter \delta; its running time additionally depends on \delta. If the input
shape is found to be approximable, this algorithm also computes an approximate
solution for the problem. It also allows us to solve parameter-optimization
problems induced by the offset-deconstruction problem. For convex shapes, the
complexity of the exact decision algorithm drops to O(n), which is also the
time required to compute a solution P with at most one more vertex than a
vertex-minimal one.Comment: 18 pages, 11 figures, previous version accepted at SoCG 2011,
submitted to DC
Scaling Limits for Minimal and Random Spanning Trees in Two Dimensions
A general formulation is presented for continuum scaling limits of stochastic
spanning trees. A spanning tree is expressed in this limit through a consistent
collection of subtrees, which includes a tree for every finite set of endpoints
in . Tightness of the distribution, as , is established for
the following two-dimensional examples: the uniformly random spanning tree on
, the minimal spanning tree on (with random edge
lengths), and the Euclidean minimal spanning tree on a Poisson process of
points in with density . In each case, sample trees are
proven to have the following properties, with probability one with respect to
any of the limiting measures: i) there is a single route to infinity (as was
known for ), ii) the tree branches are given by curves which are
regular in the sense of H\"older continuity, iii) the branches are also rough,
in the sense that their Hausdorff dimension exceeds one, iv) there is a random
dense subset of , of dimension strictly between one and two, on the
complement of which (and only there) the spanning subtrees are unique with
continuous dependence on the endpoints, v) branching occurs at countably many
points in , and vi) the branching numbers are uniformly bounded. The
results include tightness for the loop erased random walk (LERW) in two
dimensions. The proofs proceed through the derivation of scale-invariant power
bounds on the probabilities of repeated crossings of annuli.Comment: Revised; 54 pages, 6 figures (LaTex
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