254 research outputs found
Hexagonal Tilings: Tutte Uniqueness
We develop the necessary machinery in order to prove that hexagonal tilings
are uniquely determined by their Tutte polynomial, showing as an example how to
apply this technique to the toroidal hexagonal tiling.Comment: 12 figure
Hexagonal Tilings and Locally C6 Graphs
We give a complete classification of hexagonal tilings and locally C6 graphs,
by showing that each of them has a natural embedding in the torus or in the
Klein bottle. We also show that locally grid graphs are minors of hexagonal
tilings (and by duality of locally C6 graphs) by contraction of a perfect
matching and deletion of the resulting parallel edges, in a form suitable for
the study of their Tutte uniqueness.Comment: 14 figure
Serendipitous discovery of a cluster of galaxies with a peculiar central galaxy
We report the serendipitous discovery of a cluster of galaxies at z=0.369.
Thirty-eight candidate members were identified based on rough broad-band
photometric redshifts, and three members were confirmed spectroscopically. The
brightest cluster galaxy (BCG) is exceptionally blue, with B-V=0.12 and
V-I=1.02. The surface-brightness profile of the BCG follows an r^(1/4)-law
profile out to 3" in all three bands. The effective radius is significantly
smaller in bluer bandpasses, resulting in a blue core and a color gradient
opposite to the metallicity-induced color gradient observed in typical
elliptical galaxies. Beyond 3" an extended envelope of emission in excess of
the r^(1/4)-law profile is observed, the position angle of which coincides with
the major axis of the galaxy cluster. The spectrum of the BCG contains strong
Balmer absorption, a minimal 4000 A break, and a broad Mg II emission line,
suggesting that the galaxy has undergone recent star formation and may harbor a
central AGN. The presence of numerous nearby bright stars makes this cluster an
interesting target for next-generation adaptive optics using natural guide
stars.Comment: 21 pages, 8 figures, accepted for publication in the Astronomical
Journa
The Origin of the Brightest Cluster Galaxies
Most clusters and groups of galaxies contain a giant elliptical galaxy in
their centres which far outshines and outweighs normal ellipticals. The origin
of these brightest cluster galaxies is intimately related to the collapse and
formation of the cluster. Using an N-body simulation of a cluster of galaxies
in a hierarchical cosmological model, we show that galaxy merging naturally
produces a massive, central galaxy with surface brightness and velocity
dispersion profiles similar to observed BCG's. To enhance the resolution of the
simulation, 100 dark halos at are replaced with self-consistent
disk+bulge+halo galaxy models following a Tully-Fisher relation using 100000
particles for the 20 largest galaxies and 10000 particles for the remaining
ones. This technique allows us to analyze the stellar and dark matter
components independently. The central galaxy forms through the merger of
several massive galaxies along a filament early in the cluster's history.
Galactic cannibalism of smaller galaxies through dynamical friction over a
Hubble time only accounts for a small fraction of the accreted mass. The galaxy
is a flattened, triaxial object whose long axis aligns with the primordial
filament and the long axis of the cluster galaxy distribution agreeing with
observed trends for galaxy-cluster alignment.Comment: Revised and accepted in ApJ, 25 pages, 10 figures, online version
available at http://www.cita.utoronto.ca/~dubinski/bcg
An Incremental Learning Method to Support the Annotation of Workflows with Data-to-Data Relations
Workflow formalisations are often focused on the representation of a process with the primary objective to support execution. However, there are scenarios where what needs to be represented is the effect of the process on the data artefacts involved, for example when reasoning over the corresponding data policies. This can be achieved by annotating the workflow with the semantic relations that occur between these data artefacts. However, manually producing such annotations is difficult and time consuming. In this paper we introduce a method based on recommendations to support users in this task. Our approach is centred on an incremental rule association mining technique that allows to compensate the cold start problem due to the lack of a training set of annotated workflows. We discuss the implementation of a tool relying on this approach and how its application on an existing repository of workflows effectively enable the generation of such annotations
Globular Clusters in Dense Clusters of Galaxies
Deep imaging data from the Keck II telescope are employed to study the
globular cluster (GC) populations in the cores of six rich Abell clusters. The
sample includes A754, A1644, A2124, A2147, A2151, and A2152, and spans the
redshift range z = 0.035-0.066. The clusters also range in morphology from
spiral-rich, irregular systems to centrally concentrated cD clusters rich in
early-type galaxies. Globular cluster specific frequencies S_N and luminosity
function dispersions are measured for a total of 9 galaxies in six central
fields. The measured values of S_N for the six brightest cluster galaxies
(BCGs) are all higher than typical values for giant ellipticals, in accord with
the known S_N-density correlations. The three non-BCGs analyzed also have
elevated values of S_N, confirming that central location is a primary factor.
The number of GCs per unit mass for these fields are consistent with those
found in an earlier sample, giving further evidence that GC number scales with
mass and that the S_N variations are due to a deficit of halo light, i.e., S_N
reflects mass-to-light ratio.
The discussion builds on an earlier suggestion that the GCs (both metal rich
and metal poor) around the central cluster galaxies were assembled at early
times, and that star formation halted prematurely in the central galaxies at
the epoch of cluster collapse. This is consistent with recent simulations of
BCG/cluster formation. The subsequent addition of luminous material through
cluster dynamical evolution can cause S_N to decrease, and we may be seeing the
first evidence of this. Finally, the GC luminosity function measurements are
used to constrain the relative distances of the three clusters that make up the
Hercules supercluster.Comment: Uses emulateapj.sty (included); 17 pages with 9 included PostScript
figures. Figures 1-6 are separate GIF images (so 15 figures total) available
from http://astro.caltech.edu/~jpb/clusters -- the full PostScript version of
the paper (20 pages; 2.2 Mb compressed) incorporating Figures 1-6 can also be
grabbed from this URL. Accepted for publication in A
The Dependence of Galaxy Shape on Luminosity and Surface Brightness Profile
For a sample of 96,951 galaxies from the Sloan Digital Sky Survey Data
Release 3, we study the distribution of apparent axis ratios as a function of
r-band absolute magnitude and surface brightness profile type. We use the
parameter fracDeV to quantify the profile type (fracDeV = 1 for a de
Vaucouleurs profile; fracDeV = 0 for an exponential profile). When the apparent
axis ratio q_{am} is estimated from the moments of the light distribution, the
roundest galaxies are very bright (M_r \sim -23) de Vaucouleurs galaxies and
the flattest are modestly bright (M_r \sim -18) exponential galaxies. When the
apparent axis ratio q_{25} is estimated from the axis ratio of the 25
mag/arcsec^2 isophote, we find that de Vaucouleurs galaxies are flatter than
exponential galaxies of the same absolute magnitude. For a given surface
brightness profile type, very bright galaxies are rounder, on average, than
fainter galaxies. We deconvolve the distributions of apparent axis ratios to
find the distribution of the intrinsic short-to-long axis ratio gamma, assuming
constant triaxiality T. For all profile types and luminosities, the
distribution of apparent axis ratios is inconsistent with a population of
oblate spheroids, but is usually consistent with a population of prolate
spheroids. Bright galaxies with a de Vaucouleurs profile (M_r < -21.84, fracDeV
> 0.9) have a distribution of q_{am} that is consistent with triaxiality in the
range 0.4 < T < 0.8, with mean intrinsic axis ratio 0.66 < gamma < 0.69. The
fainter de Vaucouleurs galaxies are best fit with prolate spheroids (T = 1)
with mean axis ratio gamma = 0.51.Comment: 32 pages, 12 figures, to appear in Ap
Effectiveness of the Lapidus plate system in foot surgery: A PRISMA compliant systematic review
Lapidus arthrodesis is an established standard procedure to correct various foot surgery problems and hallux limitus, hypermobility of the first ray, hallux abductus valgus, and symptomatic lesser metatarsal. After decades, many fixation methods in the orthopaedic surgery industry have been developed for decreasing complications and improving this technique. The aim of this PRISMA compliant systematic review is to analyse the effectiveness of several lapidus plate systems in foot surgery. We have carried out the first systematic review of the relevant published literature so as to systematically evaluate the scientific knowledge available now on this association, assigning predefined eligibility criteria. Fourteen studies were selected which had an overall of 738 cases. The first tarsal metatarsal joint and hallux valgus were treated by the application of different types of Lapidus plate system. The optimal level of the fixation in these procedures is related with the type and system insertion place of the plate with or without screw in each specific foot disease. There is an insufficient number of studies about the effectiveness of the different types of Lapidus plate system in foot surgery, and there is a need to increase outcomes knowledge on the level of the fixation, sort of the system, and insert place in foot surgery
Singular perturbations in the quadratic family with multiple poles
Agraïments: The first author is partially supported by the European Community through the project 035651-1-2-CODY.We consider the quadratic family of complex maps given by qc(z) = z2 + c where c is the center of a hyperbolic component in the Mandelbrot set. Then, we introduce a singular perturbation on the corresponding bounded superattracting cycle by adding one pole to each point in the cycle. When c = −1 the Julia set of q−1 is the well known basilica and the perturbed map is given by fλ(z) = z2 − 1 + λ/(z d0 (z + 1)d1) where d0, d1 ≥ 1 are integers, and λ is a complex parameter such that (...) is very small. We focus on the topological characteristics of the Julia and Fatou sets of fλ that arise when the parameter λ becomes nonzero. We give sufficient conditions on the order of the poles so that for small λ the Julia sets consist of the union of homeomorphic copies of the unperturbed Julia set, countably many Cantor sets of concentric closed curves, and Cantor sets of point components that accumulate on them
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