36,668 research outputs found
Testing the Cosmological Principle in the radio sky
The Cosmological Principle states that the Universe is statistically
isotropic and homogeneous on large scales. In particular, this implies
statistical isotropy in the galaxy distribution, after removal of a dipole
anisotropy due to the observer's motion. We test this hypothesis with number
count maps from the NVSS radio catalogue. We use a local variance estimator
based on patches of different angular radii across the sky and compare the
source count variance between and within these patches. In order to assess the
statistical significance of our results, we simulate radio maps with the NVSS
specifications and mask. We conclude that the NVSS data is consistent with
statistical isotropy.Comment: 7 pages, 3 figures. To appear in JCA
Non-invasive vibrational mode spectroscopy of ion Coulomb crystals through resonant collective coupling to an optical cavity field
We report on a novel non-invasive method to determine the normal mode
frequencies of ion Coulomb crystals in traps based on the resonance enhanced
collective coupling between the electronic states of the ions and an optical
cavity field at the single photon level. Excitations of the normal modes are
observed through a Doppler broadening of the resonance. An excellent agreement
with the predictions of a zero-temperature uniformly charged liquid plasma
model is found. The technique opens up for investigations of the heating and
damping of cold plasma modes, as well as the coupling between them.Comment: 4 pages, 4 figure
Greenhouse gas emissions from soils under organic management
This report was presented at the UK Organic Research 2002 Conference.
Land emissions of N2O, CO2 and NH3 have been subject to little study under organic systems, yet form important aspects of sustainability of such systems. We describe innovative methods developed at SAC to assess trace gas emission using both automatic closed chamber systems (intensive, short term monitoring) and manually-operated closed chamber systems (occasional, long term monitoring). Long-term data were collected from organic ley-arable rotation trials in North-east of Scotland. Short term data were collected to show the effect of timing and depth of ploughing-out of the ley phase on gas emissions. Ploughing gave a shortterm stimulation of CO2 and, more markedly, of N2O emission. Emissions of N2O from organic grass-clover leys were considerably lower than from conventional grass. However, some N2O emissions from organic arable are higher than from conventional systems, particularly in the first year after ploughing out ley. Ammonia emissions after spreading manure on grass were significant in the summer, though only short-lived
Negative refractive index in coaxial plasmon waveguides
We theoretically show that coaxial waveguides composed of a metallic core, surrounded by a dielectric cylinder and clad by a metal outer layer exhibit negative refractive index modes over a broad spectral range in the visible. For narrow dielectric gaps (10 nm GaP embedded in Ag) a figure-of-merit of 18 can be achieved at λ_0 = 460 nm. For larger dielectric gaps the negative index spectral range extends well below the surface plasmon resonance frequency. By fine-tuning the coaxial geometry the special case of n = −1 at a figure-of-merit of 5, or n = 0 for a decay length of 500 nm can be achieved
Bose-Einstein condensation in complex networks
The evolution of many complex systems, including the world wide web, business
and citation networks is encoded in the dynamic web describing the interactions
between the system's constituents. Despite their irreversible and
non-equilibrium nature these networks follow Bose statistics and can undergo
Bose-Einstein condensation. Addressing the dynamical properties of these
non-equilibrium systems within the framework of equilibrium quantum gases
predicts that the 'first-mover-advantage', 'fit-get-rich' and
'winner-takes-all' phenomena observed in competitive systems are
thermodynamically distinct phases of the underlying evolving networks
Plasmon Dispersion in Coaxial Waveguides from Single-Cavity Optical Transmission Measurements
We determine the plasmon dispersion relation in coaxial waveguides composed of a circular channel separating a metallic core and cladding. Optical transmission measurements are performed on isolated coaxial nanoapertures fabricated on a Ag film using focused ion-beam lithography. The dispersion depends strongly on the dielectric material and layer thickness. Our experimental results agree well with an analytical model for plasmon dispersion in coaxial waveguides. We observe large phase shifts at reflection from the end facets of the coaxial cavity, which strongly affect the waveguide resonances and can be tuned by changing the coax geometry, composition, and surrounding dielectric index, enabling coaxial cavities with ultrasmall mode volumes
On the Wilf-Stanley limit of 4231-avoiding permutations and a conjecture of Arratia
We construct a sequence of finite automata that accept subclasses of the
class of 4231-avoiding permutations. We thereby show that the Wilf-Stanley
limit for the class of 4231-avoiding permutations is bounded below by 9.35.
This bound shows that this class has the largest such limit among all classes
of permutations avoiding a single permutation of length 4 and refutes the
conjecture that the Wilf-Stanley limit of a class of permutations avoiding a
single permutation of length k cannot exceed (k-1)^2.Comment: Submitted to Advances in Applied Mathematic
Topology of evolving networks: local events and universality
Networks grow and evolve by local events, such as the addition of new nodes
and links, or rewiring of links from one node to another. We show that
depending on the frequency of these processes two topologically different
networks can emerge, the connectivity distribution following either a
generalized power-law or an exponential. We propose a continuum theory that
predicts these two regimes as well as the scaling function and the exponents,
in good agreement with the numerical results. Finally, we use the obtained
predictions to fit the connectivity distribution of the network describing the
professional links between movie actors.Comment: 13 pages, 3 figure
The Stellar Populations of NGC 3109: Another Dwarf Irregular Galaxy with a Population II Stellar Halo
We have obtained V and I-band photometry for about 17500 stars in the field
of the dwarf irregular galaxy NGC3109, located in the outskirts of the Local
Group. The photometry allows us to study the stellar populations present inside
and outside the disk of this galaxy. From the VI color-magnitude diagram we
infer metallicities and ages for the stellar populations in the main body and
in the halo of NGC3109. The stars in the disk of this galaxy have a wide
variety of ages, including very young stars with approximately 10^7 yr. Our
main result is to establish the presence of a halo consisting of population II
stars, extending out to about 4.5 arcmin (or 1.8 kpc) above and below the plane
of this galaxy. For these old stars we derive an age of > 10 Gyr and a
metallicity of [Fe/H] = -1.8 +/- 0.2. We construct a deep luminosity function,
obtaining an accurate distance modulus (m-M)_0 = 25.62 +/- 0.1 for this galaxy
based on the I-magnitude of the red giant branch (RGB) tip and adopting E(V-I)
= 0.05.Comment: Accepted for publication in the Astronomical Journal 23 pages, latex,
12 Figures (Fig 1 not available in electronic format
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