5,659 research outputs found
Topology of Neutral Hydrogen Within the Small Magellanic Cloud
In this paper, genus statistics have been applied to an HI column density map
of the Small Magellanic Cloud in order to study its topology. To learn how
topology changes with the scale of the system, we provide the study of topology
for column density maps at varying resolution. To evaluate the statistical
error of the genus we randomly reassign the phases of the Fourier modes while
keeping the amplitudes. We find, that at the smallest scales studied () the genus shift is in all regions negative,
implying a clump topology. At the larger scales () the topology shift is detected to be negative in 4 cases and positive
(``swiss cheese'' topology) in 2 cases. In 4 regions there is no statistically
significant topology shift at large scales
Bias and Hierarchical Clustering
It is now well established that galaxies are biased tracers of the
distribution of matter, although it is still not known what form this bias
takes. In local bias models the propensity for a galaxy to form at a point
depends only on the overall density of matter at that point. Hierarchical
scaling arguments allow one to build a fully-specified model of the underlying
distribution of matter and to explore the effects of local bias in the regime
of strong clustering. Using a generating-function method developed by
Bernardeau & Schaeffer (1992), we show that hierarchical models lead one
directly to the conclusion that a local bias does not alter the shape of the
galaxy correlation function relative to the matter correlation function on
large scales. This provides an elegant extension of a result first obtained by
Coles (1993) for Gaussian underlying fields and confirms the conclusions of
Scherrer & Weinberg (1998) obtained using a different approach. We also argue
that particularly dense regions in a hierarchical density field display a form
of bias that is different from that obtained by selecting such peaks in
Gaussian fields: they are themselves hierarchically distributed with scaling
parameters . This kind of bias is also factorizable, thus in
principle furnishing a simple test of this class of models.Comment: Latex, accepted for publication in ApJL; moderate revision
Theory of Parabolic Arcs in Interstellar Scintillation Spectra
Our theory relates the secondary spectrum, the 2D power spectrum of the radio
dynamic spectrum, to the scattered pulsar image in a thin scattering screen
geometry. Recently discovered parabolic arcs in secondary spectra are generic
features for media that scatter radiation at angles much larger than the rms
scattering angle. Each point in the secondary spectrum maps particular values
of differential arrival-time delay and fringe rate (or differential Doppler
frequency) between pairs of components in the scattered image. Arcs correspond
to a parabolic relation between these quantities through their common
dependence on the angle of arrival of scattered components. Arcs appear even
without consideration of the dispersive nature of the plasma. Arcs are more
prominent in media with negligible inner scale and with shallow wavenumber
spectra, such as the Kolmogorov spectrum, and when the scattered image is
elongated along the velocity direction. The arc phenomenon can be used,
therefore, to constrain the inner scale and the anisotropy of scattering
irregularities for directions to nearby pulsars. Arcs are truncated by finite
source size and thus provide sub micro arc sec resolution for probing emission
regions in pulsars and compact active galactic nuclei. Multiple arcs sometimes
seen signify two or more discrete scattering screens along the propagation
path, and small arclets oriented oppositely to the main arc persisting for long
durations indicate the occurrence of long-term multiple images from the
scattering screen.Comment: 22 pages, 11 figures, submitted to the Astrophysical Journa
Reconfiguring experimental archaeology using 3D reconstruction
Experimental archaeology has long yielded valuable insights into the tools and techniques that featured in past peoples’ relationship with the material world around them. We can determine, for example, how many trees would need to be felled to construct a large round-house of the southern British Iron Age (over one hundred), infer the exact angle needed to strike a flint core in order to knap an arrowhead in the manner of a Neolithic hunter-gatherer, or recreate the precise environmental conditions needed to store grain in underground silos over the winter months, with only the technologies and materials available to Romano-Briton villagers (see Coles 1973; Reynolds 1993). However, experimental archaeology has, hitherto, confined itself to rather rigid, empirical and quantitative questions such as those posed in these examples. This is quite understandable, and in line with good scientific practice, which stipulates that any ‘experiment’ must be based on replicable data, and be reproducible. Despite their potential in this area however, it is notable that digital reconstruction technologies have yet to play a significant role in experimental archaeology. Whilst many excellent examples of digital 3D reconstruction of heritage sites exist (for example the Digital Roman Forum project: http://dlib.etc.ucla.edu/projects/Forum) most, if not all, of these are characterized by a drive to establish a photorealistic re-creation of physical features. This paper will discuss possibilities that lie beyond straightforward positivist re-creation of heritage sites, in the experimental reconstruction of intangible heritage. Between 2010 and 2012, the authors led the Motion in Place Platform project (MiPP: http://www.motioninplace.org/), a capital grant under the AHRC's DEDEFI scheme developing motion capture and analysis tools for exploring how people move through spaces. In the course of MiPP, a series of experiments were conducted using motion capture hardware and software at the Silchester Roman town archaeological excavation in Hampshire, and at the Butser Ancient Farm facility, where Romano-British and Iron Age dwellings have been constructed according to the best experimental practice. As well as reconstructing such Roman and early British dwellings in 3D, the authors were able to use motion capture to reconstruct the kind of activities that – according to the material evidence – are likely to have been carried out by the occupants who used them. Bespoke motion capture suits developed for the project were employed, and the traces captured and rendered with a combination of Autodesk and Unity3D software. This sheds new light on how the reconstructed spaces - and, by inference, their ancient counterparts - were most likely to have been used. In particular the exercises allowed the evaluation and visualisation of changes in behaviour which occur as a result of familiarity with an environment and the acquisition of expertise over time; and to assess how interaction between different actors affects how everyday tasks are carried out
Electrochromic properties of a poly(dithienylfuran) derivative featuring a redox-active dithiin unit
A teraryl monomer containing a 1,4-dithiin-furan central unit has been synthesised and characterised by single crystal X-ray crystallography. The di(thienyl)furan monomer 11 was successfully polymerised electrochemically and shown to possess a lower electrochemical band gap than its terthiophene analogue (1.97 eV cf. 2.11 eV). The electrochromic properties of this polymer proved to be superior to PEDOT, with fast switching and reversible colour transformation at high colour contrast (CE = 212 cm(2) C-1 cf. 183 cm(2) C-1 for PEDOT at 95% optical switch)
Analytical modeling of large-angle CMBR anisotropies from textures
We propose an analytic method for predicting the large angle CMBR temperature
fluctuations induced by model textures. The model makes use of only a small
number of phenomenological parameters which ought to be measured from simple
simulations. We derive semi-analytically the -spectrum for together with its associated non-Gaussian cosmic variance error bars. A
slightly tilted spectrum with an extra suppression at low is found, and we
investigate the dependence of the tilt on the parameters of the model. We also
produce a prediction for the two point correlation function. We find a high
level of cosmic confusion between texture scenarios and standard inflationary
theories in any of these quantities. However, we discover that a distinctive
non-Gaussian signal ought to be expected at low , reflecting the prominent
effect of the last texture in these multipoles
Optical vortex generation from molecular chromophore arrays
The generation of light endowed with orbital angular momentum, frequently termed optical vortex light, is commonly achieved by passing a conventional beam through suitably constructed optical elements. This Letter shows that the necessary phase structure for vortex propagation can be directly produced through the creation of twisted light from the vacuum. The mechanism is based on optical emission from a family of chromophore nanoarrays that satisfy specific geometric and symmetry constraints. Each such array can support pairs of electronically delocalized doubly degenerate excitons whose azimuthal phase progression is responsible for the helical wave front of the emitted radiation. The exciton symmetry dictates the maximum magnitude of topological charge; detailed analysis secures the conditions necessary to deliver optical vortices of arbitrary order
An integrated information retrieval and document management system
This paper describes the requirements and prototype development for an intelligent document management and information retrieval system that will be capable of handling millions of pages of text or other data. Technologies for scanning, Optical Character Recognition (OCR), magneto-optical storage, and multiplatform retrieval using a Standard Query Language (SQL) will be discussed. The semantic ambiguity inherent in the English language is somewhat compensated-for through the use of coefficients or weighting factors for partial synonyms. Such coefficients are used both for defining structured query trees for routine queries and for establishing long-term interest profiles that can be used on a regular basis to alert individual users to the presence of relevant documents that may have just arrived from an external source, such as a news wire service. Although this attempt at evidential reasoning is limited in comparison with the latest developments in AI Expert Systems technology, it has the advantage of being commercially available
On-chip electrically controlled routing of photons from a single quantum dot
Electrical control of on-chip routing of photons emitted by a single InAs/GaAs self-assembled quantum dot (SAQD) is demonstrated in a photonic crystal cavity-waveguide system. The SAQD is located inside an H1 cavity, which is coupled to two photonic crystal waveguides. The SAQD emission wavelength is electrically tunable by the quantum-confined Stark effect. When the SAQD emission is brought into resonance with one of two H1 cavity modes, it is preferentially routed to the waveguide to which that mode is selectively coupled. This proof of concept provides the basis for scalable, low-power, high-speed operation of single-photon routers for use in integrated quantum photonic circuits
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