1,108 research outputs found
On the nature of the barlens component in barred galaxies: what do boxy/peanut bulges look like when viewed face-on?
Barred galaxies have interesting morphological features whose presence and
properties set constraints on galactic evolution. Here we examine barlenses,
i.e. lens-like components whose extent along the bar major axis is shorter than
that of the bar and whose outline is oval or circular. We identify and analyse
barlenses in -body plus SPH simulations, compare them extensively with those
from the NIRS0S (Near-IR S0 galaxy survey) and the SG samples (Spitzer
Survey of Stellar Structure in Galaxies) and find very good agreement. We
observe barlenses in our simulations from different viewing angles. This
reveals that barlenses are the vertically thick part of the bar seen face-on,
i.e. a barlens seen edge-on is a boxy/peanut/X bulge. In morphological studies,
and in the absence of kinematics or photometry, a barlens, or part of it, may
be mistaken for a classical bulge. Thus the true importance of classical
bulges, both in numbers and mass, is smaller than currently assumed, which has
implications for galaxy formation studies. Finally, using the shape of the
isodensity curves, we propose a rule of thumb for measuring the barlens extent
along the bar major axis of moderately inclined galaxies, thus providing an
estimate of which part of the bar is thicker.Comment: 21 pages, 11 figures, revised version as published in MNRA
Mathematical Morphology for Quantification in Biological & Medical Image Analysis
Mathematical morphology is an established field of image processing first introduced as an application of set and lattice theories. Originally used to characterise particle distributions, mathematical morphology has gone on to be a core tool required for such important analysis methods as skeletonisation and the watershed transform. In this thesis, I introduce a selection of new image analysis techniques based on mathematical morphology.
Utilising assumptions of shape, I propose a new approach for the enhancement of vessel-like objects in images: the bowler-hat transform. Built upon morphological operations, this approach is successful at challenges such as junctions and robust against noise. The bowler-hat transform is shown to give better results than competitor methods on challenging data such as retinal/fundus imagery.
Building further on morphological operations, I introduce two novel methods for particle and blob detection. The first of which is developed in the context of colocalisation, a standard biological assay, and the second, which is based on Hilbert-Edge Detection And Ranging (HEDAR), with regard to nuclei detection and counting in fluorescent microscopy. These methods are shown to produce accurate and informative results for sub-pixel and supra-pixel object counting in complex and noisy biological scenarios.
I propose a new approach for the automated extraction and measurement of object thickness for intricate and complicated vessels, such as brain vascular in medical images. This pipeline depends on two key technologies: semi-automated segmentation by advanced level-set methods and automatic thickness calculation based on morphological operations. This approach is validated and results demonstrating the broad range of challenges posed by these images and the possible limitations of this pipeline are shown.
This thesis represents a significant contribution to the field of image processing using mathematical morphology and the methods within are transferable to a range of complex challenges present across biomedical image analysis
Detecting a disk bending wave in a barred-spiral galaxy at redshift 4.4
The recent discovery of barred spiral galaxies in the early universe ()
poses questions of how these structures form and how they influence galaxy
properties in the early universe. In this study, we investigate the morphology
and kinematics of the far infrared (FIR) continuum and [CII] emission in
BRI1335-0417 at from ALMA observations. The variations in
position angle and ellipticity of the isophotes show the characteristic
signature of a barred galaxy. The bar, kpc long in radius
and bridging the previously identified two-armed spiral, is evident in both
[CII] and FIR images, driving the galaxy's rapid evolution by channelling gas
towards the nucleus. Fourier analysis of the [CII] velocity field reveals an
unambiguous mode with a line-of-sight velocity amplitude of up to
km s; the plausible explanation is the disk's vertical
bending mode triggered by external perturbation, which presumably induced the
high star formation rate and the bar/spiral structure. The bar identified in
[CII] and FIR images of the gas-rich disk galaxy (\% of the total
mass within radius disk scale lengths) suggests a new
perspective of early bar formation -- a gravitationally unstable gas-rich disk
creating a star-forming gaseous bar, rather than a stellar bar emerging from a
pre-existing stellar disk.Comment: Submitted to MNRAS. We welcome comments
Using machine learning to optimise chameleon fifth force experiments
The chameleon is a theorised scalar field that couples to matter and possess
a screening mechanism, which weakens observational constraints from experiments
performed in regions of higher matter density. One consequence of this
screening mechanism is that the force induced by the field is dependent on the
shape of the source mass (a property that distinguishes it from gravity).
Therefore an optimal shape must exist for which the chameleon force is
maximised. Such a shape would allow experiments to improve their sensitivity by
simply changing the shape of the source mass. In this work we use a combination
of genetic algorithms and the chameleon solving software SELCIE to find shapes
that optimise the force at a single point in an idealised experimental
environment. We note that the method we used is easily customised, and so could
be used to optimise a more realistic experiment involving particle trajectories
or the force acting on an extended body. We find the shapes outputted by the
genetic algorithm possess common characteristics, such as a preference for
smaller source masses, and that the largest fifth forces are produced by small
`umbrella'-like shapes with a thickness such that the source is unscreened but
the field reaches its minimum inside the source. This remains the optimal shape
even as we change the chameleon potential, and the distance from the source,
and across a wide range of chameleon parameters. We find that by optimising the
shape in this way the fifth force can be increased by times when
compared to a sphere, centred at the origin, of the same volume and mass.Comment: 28 pages, 17 figures, The SELCIE code is available at:
https://github.com/C-Briddon/SELCI
Bar Diagnostics in Edge-On Spiral Galaxies. I. The Periodic Orbits Approach
We develop diagnostics to detect the presence and orientation of a bar in an
edge-on disk, using its kinematical signature in the position-velocity diagram
(PVD) of a spiral galaxy observed edge-on. Using a well-studied barred spiral
galaxy mass model, we briefly review the orbital properties of two-dimensional
non-axisymmetric disks and identify the main families of periodic orbits. We
use those families as building blocks to model real galaxies and calculate the
PVDs obtained for various realistic combinations of periodic orbit families and
for a number of viewing angles with respect to the bar. We show that the global
structure of the PVD is a reliable bar diagnostic in edge-on disks.
Specifically, the presence of a gap between the signatures of the families of
periodic orbits in the PVD follows directly from the non-homogeneous
distribution of the orbits in a barred galaxy. Similarly, material in the two
so-called forbidden quadrants of the PVD results from the elongated shape of
the orbits. We show how the shape of the signatures of the dominant x1 and x2
families of periodic orbits in the PVD can be used efficiently to determine the
viewing angle with respect to the bar and, to a lesser extent, to constrain the
mass distribution of an observed galaxy. We also address the limitations of the
models when interpreting observational data.Comment: 22 pages, 9 figures (AASTeX, aaspp4.sty). Accepted for publication in
The Astrophysical Journa
Numerical Simulation and Characterisation of the Packing of Granular Materials
The scientific problems related to granular matter are ubiquitous. It is currently an
active area of research for physicists and earth scientists, with a wide range of applications
within the industrial community. Simple analogue experiments exhibit behaviour that is
neither predicted nor described by any current theory. The work presented here consists
of modelling granular media using a two-dimensional combined Finite-Discrete Element
Method (FEM-DEM). While computationally expensive, as well as modelling accurately
the dynamic interactions between independent and arbitrarily shaped grains, this method
allows for a complete description of the stress state within individual grains during their
transient motion.
After a detailed description of FEM-DEM principles, this computational approach is
used to investigate the packing of elliptical particles. The work is aimed at understanding
the influence of the particle shape (the ellipse aspect ratio) on the emergent properties of
the granular matrix such as the particle coordination number and the packing density. The
diff erences in microstructure of the resultant packing are analysed using pair correlation
functions, particle orientations and pore size distributions. A comparison between frictional
and frictionless systems is carried out. It shows great diff erences not only in the calculated
porosity and coordination number, but also in terms of structural arrangement and stress
distribution. The results suggest that the particle's shape a ffects the structural order of the
particle assemblage, which itself controls the stress distribution between the pseudo-static
grains.
The study then focuses on describing the stress patterns or \force chains" naturally
generated in a frictional system. An algorithm based on the analysis of the contact
force network is proposed and applied to various packs in order to identify the force
chains. A statistical analysis of the force chains looking at their orientation, length and
proportion of the particles that support the loads is then performed. It is observed
that force chains propagate less efficiently and more heterogeneously through granular
systems made of elliptical particles than through systems of discs and it is proposed
that structural diff erences due to the particle shape lead to a signifi cant reduction in the length of the stress path that propagates across connected particles. Finally, the e ffect
of compression on the granular packing, the emergent properties and the contact force
distribution is examined. Results show that the force network evolves towards a more
randomly distributed system (from an exponential to a Gaussian distribution), and it
confi rms the observations made from simulations using discs.
To conclude, the combined finite-discrete element method applied to the study of
granular systems provides an attractive modelling strategy to improve the knowledge of
granular matter. This is due to the wide range of static and dynamic problems that can be
treated with a rigorous physical basis. The applicability of the method was demonstrated
through to a variety of problems that involve di fferent physical processes modelled with
the FEM-DEM (internal deformations, fracture, and complex geometry). With the rapid
extension of the practical limits of computational models, this work emphasizes the
opportunity to move towards a modern generation of computer software to understand
the complexity of the phenomena associated with discontinua
Population Genetic Structure is Unrelated to Shell Shape, Thickness and Organic Content in European Populations of the Soft-Shell Clam Mya Arenaria.
The soft-shell clam Mya arenaria is one of the most ancient invaders of European coasts and is present in many coastal ecosystems, yet little is known about its genetic structure in Europe. We collected 266 samples spanning a latitudinal cline from the Mediterranean to the North Sea and genotyped them at 12 microsatellite loci. In parallel, geometric morphometric analysis of shell outlines was used to test for associations between shell shape, latitude and genotype, and for a selection of shells we measured the thickness and organic content of the granular prismatic (PR), the crossed-lamellar (CL) and the complex crossed-lamellar (CCL) layers. Strong population structure was detected, with Bayesian cluster analysis identifying four groups located in the Mediterranean, Celtic Sea, along the continental coast of the North Sea and in Scotland. Multivariate analysis of shell shape uncovered a significant effect of collection site but no associations with any other variables. Shell thickness did not vary significantly with either latitude or genotype, although PR thickness and calcification were positively associated with latitude, while CCL thickness showed a negative association. Our study provides new insights into the population structure of this species and sheds light on factors influencing shell shape, thickness and microstructure
The evolution of dwarf galaxy satellites with different dark matter density profiles in the ErisMod simulations. I. The early infalls
We present the first simulations of tidal stirring of dwarf galaxies in the
Local Group carried out in a cosmological context. We use the ErisDARK
simulation of a MW-sized galaxy to identify some of the most massive subhalos
() that fall into the main host before .
Subhalos are replaced before infall with high-resolution models of dwarf
galaxies comprising a faint stellar disk embedded in a dark matter halo. The
set of models contains cuspy halos as well as halos with "cored" profiles (with
asymptotic inner slope ). The simulations are then run to
with as many as 54 million particles and resolution as small as pc
using the N-Body code ChaNGa. The stellar components of all satellites are
significantly affected by tidal stirring, losing stellar mass and undergoing a
morphological transformation towards a pressure supported spheroidal system.
However, while some remnants with cuspy halos maintain significant rotational
flattening and disk-like features, all the shallow halo models achieve
and round shapes typical of dSph satellites of the MW and M31.
Mass loss is also enhanced in the latter, and remnants can reach luminosities
and velocity dispersions as low as those of Ultra Faint Dwarfs (UFDs). We argue
that cuspy progenitors must be the exception rather than the rule among
satellites of the MW since all the MW and M31 satellites in the luminosity
range of our remnants are dSphs, a result matched only in the simulation with
"cored" models
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