2,796 research outputs found
How clumpy is my image? Evaluating crowdsourced annotation tasks
13th UK Workshop on Computational Intelligence (UKCI), Guildford, UK, 9-11 September 2013This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.The use of citizen science to obtain annotations from multiple annotators has been shown to be an effective method for annotating datasets in which computational methods alone are not feasible. The way in which the annotations are obtained is an important consideration which affects the quality of the resulting consensus estimates. In this paper, we examine three separate approaches to obtaining scores for instances rather than merely classifications. To obtain a consensus score annotators were asked to make annotations in one of three paradigms: classification, scoring and ranking. A web-based citizen science experiment is described which implements the three approaches as crowdsourced annotation tasks. The tasks are evaluated in relation to the accuracy and agreement among the participants using both simulated and real-world data from the experiment. The results show a clear difference in performance between the three tasks, with the ranking task obtaining the highest accuracy and agreement among the participants. We show how a simple evolutionary optimiser may be used to improve the performance by reweighting the importance of annotators
How clumpy is my image?: Scoring in crowdsourced annotation tasks
The use of citizen science to obtain annotations from multiple annotators has been shown to be an effective method for annotating datasets in which computational methods alone are not feasible. The way in which the annotations are obtained is an important consideration which affects the quality of the resulting consensus annotation. In this paper, we examine three separate approaches to obtaining consensus scores for instances rather than merely binary classifications. To obtain a consensus score, annotators were asked to make annotations in one of three paradigms: classification, scoring and ranking. A web-based citizen science experiment is described which implements the three approaches as crowdsourced annotation tasks. The tasks are evaluated in relation to the accuracy and agreement among the participants using both simulated and real-world data from the experiment. The results show a clear difference in performance between the three tasks, with the ranking task obtaining the highest accuracy and agreement among the participants. We show how a simple evolutionary optimiser may be used to improve the performance by reweighting the importance of annotators
Formation of stars and clusters over cosmological time
The concept that stars form in the modern era began some 60 years ago with
the key observation of expanding OB associations. Now we see that these
associations are an intermediate scale in a cascade of hierarchical structures
that begins on the ambient Jeans length close to a kiloparsec in size and
continues down to the interiors of clusters, perhaps even to binary and
multiple stellar systems. The origin of this structure lies with the dynamical
nature of cloud and star formation, driven by supersonic turbulence and
interstellar gravity. Dynamical star formation is relatively fast compared to
the timescale for cosmic accretion, and then the star formation rate keeps up
with the accretion rate, leading to a sequence of near-equilibrium states
during galaxy formation and evolution. Dynamical star formation also helps to
explain the formation of bound clusters, which require a local efficiency that
exceeds the average by more than an order of magnitude. Efficiency increases
with density in a hierarchically structured gas. Cluster formation should vary
with environment as the relative degree of cloud self-binding varies, and this
depends on the ratio of the interstellar velocity dispersion to the galaxy
rotation speed. As this ratio increases, galaxies become more clumpy, thicker,
and have more tightly bound star-forming regions. The formation of old globular
clusters is understood in this context, with the metal-rich and metal-poor
globulars forming in high-mass and low-mass galaxies, respectively, because of
the galactic mass-metallicity relation. Metal-rich globulars remain in the
disks and bulge regions where they formed, while metal-poor globulars get
captured as parts of satellite galaxies and end up in today's spiral galaxy
halos. Blue globulars in the disk could have formed very early when the whole
Milky Way had a low mass.Comment: 14 pages, 1 figure, in conference "Lessons from the Local Group," ed.
K. Freeman et al., Springer, 201
Large and small-scale structures and the dust energy balance problem in spiral galaxies
The interstellar dust content in galaxies can be traced in extinction at
optical wavelengths, or in emission in the far-infrared. Several studies have
found that radiative transfer models that successfully explain the optical
extinction in edge-on spiral galaxies generally underestimate the observed
FIR/submm fluxes by a factor of about three. In order to investigate this
so-called dust energy balance problem, we use two Milky Way-like galaxies
produced by high-resolution hydrodynamical simulations. We create mock optical
edge-on views of these simulated galaxies (using the radiative transfer code
SKIRT), and we then fit the parameters of a basic spiral galaxy model to these
images (using the fitting code FitSKIRT). The basic model includes smooth
axisymmetric distributions along a S\'ersic bulge and exponential disc for the
stars, and a second exponential disc for the dust. We find that the dust mass
recovered by the fitted models is about three times smaller than the known dust
mass of the hydrodynamical input models. This factor is in agreement with
previous energy balance studies of real edge-on spiral galaxies. On the other
hand, fitting the same basic model to less complex input models (e.g. a smooth
exponential disc with a spiral perturbation or with random clumps), does
recover the dust mass of the input model almost perfectly. Thus it seems that
the complex asymmetries and the inhomogeneous structure of real and
hydrodynamically simulated galaxies are a lot more efficient at hiding dust
than the rather contrived geometries in typical quasi-analytical models. This
effect may help explain the discrepancy between the dust emission predicted by
radiative transfer models and the observed emission in energy balance studies
for edge-on spiral galaxies.Comment: 9 pages, 5 figures, accepted for publication in A&
The Dawes Review 1: Kinematic studies of star-forming galaxies across cosmic time
The last seven years have seen an explosion in the number of Integral Field
galaxy surveys, obtaining resolved 2D spectroscopy, especially at
high-redshift. These have taken advantage of the mature capabilities of 8-10 m
class telescopes and the development of associated technology such as AO.
Surveys have leveraged both high spectroscopic resolution enabling internal
velocity measurements and high spatial resolution from AO techniques and sites
with excellent natural seeing. For the first time, we have been able to glimpse
the kinematic state of matter in young, assembling star-forming galaxies and
learn detailed astrophysical information about the physical processes and
compare their kinematic scaling relations with those in the local Universe.
Observers have measured disc galaxy rotation, merger signatures, and
turbulence-enhanced velocity dispersions of gas-rich discs. Theorists have
interpreted kinematic signatures of galaxies in a variety of ways (rotation,
merging, outflows, and feedback) and attempted to discuss evolution vs.
theoretical models and relate it to the evolution in galaxy morphology. A key
point that has emerged from this activity is that substantial fractions of
high-redshift galaxies have regular kinematic morphologies despite irregular
photometric morphologies and this is likely due to the presence of a large
number of highly gas-rich discs. There has not yet been a review of this
burgeoning topic. In this first Dawes review, I will discuss the extensive
kinematic surveys that have been done and the physical models that have arisen
for young galaxies at high-redshift.Comment: 51 pages, 34,000 words, 16 figures. A few minor corrections have been
made to the journal version. High-resolution PDF and iPad optimised ePUB
versions available from http://astronomy.swin.edu.au/karl/dawe
Unveiling the nature of bright z ~ 7 galaxies with the Hubble Space Telescope
We present new Hubble Space Telescope/Wide Field Camera 3 imaging of 25
extremely luminous (-23.2 < M_ UV < -21.2) Lyman-break galaxies (LBGs) at z ~
7. The sample was initially selected from 1.65 deg^2 of ground-based imaging in
the UltraVISTA/COSMOS and UDS/SXDS fields, and includes the extreme Lyman-alpha
emitters, `Himiko' and `CR7'. A deconfusion analysis of the deep Spitzer
photometry available suggests that these galaxies exhibit strong rest-frame
optical nebular emission lines (EW_0(H_beta + [OIII]) > 600A). We find that
irregular, multiple-component morphologies suggestive of clumpy or merging
systems are common (f_multi > 0.4) in bright z ~ 7 galaxies, and ubiquitous at
the very bright end (M_UV < -22.5). The galaxies have half-light radii in the
range r_1/2 ~ 0.5-3 kpc. The size measurements provide the first determination
of the size-luminosity relation at z ~ 7 that extends to M_UV ~ -23. We find
the relation to be steep with r_1/2 ~ L^1/2. Excluding clumpy, multi-component
galaxies however, we find a shallower relation that implies an increased
star-formation rate surface density in bright LBGs. Using the new, independent,
HST/WFC3 data we confirm that the rest-frame UV luminosity function at z ~ 7
favours a power-law decline at the bright-end, compared to an exponential
Schechter function drop-off. Finally, these results have important implications
for the Euclid mission, which we predict will detect > 1000 similarly bright
galaxies at z ~ 7. Our new HST imaging suggests that the vast majority of these
galaxies will be spatially resolved by Euclid, mitigating concerns over dwarf
star contamination.Comment: 26 pages, 11 figures and 5 tables. Updated to match MNRAS accepted
versio
SKIRT: the design of a suite of input models for Monte Carlo radiative transfer simulations
The Monte Carlo method is the most popular technique to perform radiative
transfer simulations in a general 3D geometry. The algorithms behind and
acceleration techniques for Monte Carlo radiative transfer are discussed
extensively in the literature, and many different Monte Carlo codes are
publicly available. On the contrary, the design of a suite of components that
can be used for the distribution of sources and sinks in radiative transfer
codes has received very little attention. The availability of such models, with
different degrees of complexity, has many benefits. For example, they can serve
as toy models to test new physical ingredients, or as parameterised models for
inverse radiative transfer fitting. For 3D Monte Carlo codes, this requires
algorithms to efficiently generate random positions from 3D density
distributions. We describe the design of a flexible suite of components for the
Monte Carlo radiative transfer code SKIRT. The design is based on a combination
of basic building blocks (which can be either analytical toy models or
numerical models defined on grids or a set of particles) and the extensive use
of decorators that combine and alter these building blocks to more complex
structures. For a number of decorators, e.g. those that add spiral structure or
clumpiness, we provide a detailed description of the algorithms that can be
used to generate random positions. Advantages of this decorator-based design
include code transparency, the avoidance of code duplication, and an increase
in code maintainability. Moreover, since decorators can be chained without
problems, very complex models can easily be constructed out of simple building
blocks. Finally, based on a number of test simulations, we demonstrate that our
design using customised random position generators is superior to a simpler
design based on a generic black-box random position generator.Comment: 15 pages, 4 figures, accepted for publication in Astronomy and
Computin
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