23,754 research outputs found
Spitzer IRS observations of k plus a galaxies: a link between polycyclic aromatic hydrocarbon emission properties and active galactic nucleus feedback?
We have performed Spitzer InfraRed Spectrograph (IRS) low-resolution 5-12 mu m spectroscopy on a sample of galaxies selected to be at three distinct poststarburst evolutionary stages based on their optical spectral indices. The resulting IRS spectra show distinctive polycyclic aromatic hydrocarbon (PAH) emission line structures at 6.2, 7.7, 8.6, and 11.3 mu m and little silicate absorption, indicative of ongoing star formation. However, the PAH interline ratios, in particular the 11.3/6.2 mu m and 7.7/6.2 mu m ratio, show large variations. These variations are found to correlate with both time since the most recent starburst and active galactic nucleus (AGN) activity. We speculate that the evolution observed in these PAH ratios is related to an increase in AGN activity with time since starburst
Bayesian methods of astronomical source extraction
We present two new source extraction methods, based on Bayesian model
selection and using the Bayesian Information Criterion (BIC). The first is a
source detection filter, able to simultaneously detect point sources and
estimate the image background. The second is an advanced photometry technique,
which measures the flux, position (to sub-pixel accuracy), local background and
point spread function. We apply the source detection filter to simulated
Herschel-SPIRE data and show the filter's ability to both detect point sources
and also simultaneously estimate the image background. We use the photometry
method to analyse a simple simulated image containing a source of unknown flux,
position and point spread function; we not only accurately measure these
parameters, but also determine their uncertainties (using Markov-Chain Monte
Carlo sampling). The method also characterises the nature of the source
(distinguishing between a point source and extended source). We demonstrate the
effect of including additional prior knowledge. Prior knowledge of the point
spread function increase the precision of the flux measurement, while prior
knowledge of the background has onlya small impact. In the presence of higher
noise levels, we show that prior positional knowledge (such as might arise from
a strong detection in another waveband) allows us to accurately measure the
source flux even when the source is too faint to be detected directly. These
methods are incorporated in SUSSEXtractor, the source extraction pipeline for
the forthcoming Akari FIS far-infrared all-sky survey. They are also
implemented in a stand-alone, beta-version public tool that can be obtained at
http://astronomy.sussex.ac.uk/rss23/sourceMiner\_v0.1.2.0.tar.gzComment: Accepted for publication by ApJ (this version compiled used
emulateapj.cls
Probabilistic soil moisture projections to assess Great Britain's future clay-related subsidence hazard
Clay-related subsidence is Great Britainâs (GB) most damaging soil-related geohazard, costing the economy up to ÂŁ500 million per annum. Soil-related geohazard models based on mineralogy and potential soil moisture deficit (PSMD) derived from historic weather data have been used in risk management since the 1990s. United Kingdom Climate Projections (UKCP09) suggest that regions of GB will experience hotter, drier summers and warmer, wetter winters through to 2050. As a result, PSMD fluctuations are expected to increase, exacerbating the shrinkage and swelling of clay soils. A forward-looking approach is now required to mitigate the impacts of future climate on GBâs built environment. We present a framework for incorporating probabilistic projections of PSMD, derived from a version of the UKCP09 stochastic weather generator, into a clay subsidence model. This provides a novel, national-scale thematic model of the likelihood of clay-related subsidence, related to the top 1-1.5m soil layer, for three time periods; baseline (1961-1990), 2030 (2020-2049) and 2050 (2040-2069). Results indicate that much of GB, with the exception of upland areas, will witness significantly higher PSMDs through to the 2050âs. As a result, areas with swelling clay soils will be subject to proportionately increased subsidence hazard. South-east England will likely incur the highest hazard exposure to clay-related subsidence through to 2050. Potential impacts include increased incidence of property foundation subsidence, alongside deterioration and increased failure rates of GBâs infrastructure networks. Future clay-subsidence hazard scenarios provide benefit to many sectors, including: finance, central and local government, residential property markets, utilities and infrastructure operators.EPSR
Soil geohazard mapping for improved asset management of UK local roads
Unclassified roads comprise 60% of the road network
in the United Kingdom (UK). The resilience of this locally
important network is declining. It is considered by the
Institution of Civil Engineers to be âat riskâ and is ranked
26th in the world. Many factors contribute to the degradation
and ultimate failure of particular road sections. However,
several UK local authorities have identified that in drought
conditions, road sections founded upon shrinkâswell susceptible
clay soils undergo significant deterioration compared
with sections on non-susceptible soils. This arises from the
local road network having little, if any, structural foundations.
Consequently, droughts in East Anglia have resulted
in millions of pounds of damage, leading authorities to seek
emergency governmental funding.
This paper assesses the use of soil-related geohazard assessments
in providing soil-informed maintenance strategies
for the asset management of the locally important road network
of the UK. A case study draws upon the UK administrative
county of Lincolnshire, where road assessment data have
been analysed against mapped clay-subsidence risk. This reveals
a statistically significant relationship between road condition
and susceptible clay soils. Furthermore, incorporation
of UKCP09 future climate projections within the geohazard
models has highlighted roads likely to be at future risk of
clay-related subsidence
Enhanced visualization of the flat landscape of the Cambridgeshire Fenlands
The Fenlands of East Anglia, England, represent a subtle landscape, where topographic highs rarely exceed 30 m above sea level. However, the fens represent an almost full sequence of Quaternary deposits which, together with islands of Cretaceous and Jurassic outcrops, make the area of geological importance. This feature discusses the advantages of using 3D visualization coupled with high-resolution topographical data, over traditional 2D techniques, when undertaking an analysis of the landscape. Conclusions suggest that the use of 3D visualization will result in a higher level of engagement, particularly when communicating geological information to a wider public
Extragalactic Source Counts in the Spitzer 24-micron Band: What Do We Expect From ISOCAM 15-micron Data and Models?
The comparison between the new Spitzer data at 24 micron and the previous
ISOCAM data at 15 micron is a key tool to understand galaxy properties and
evolution in the infrared and to interpret the observed number counts, since
the combination of Spitzer with the ISO cosmological surveys provides for the
first time the direct view of the Universe in the Infrared up to z~2. We
present the prediction in the Spitzer 24-micron band of a phenomenological
model for galaxy evolution derived from the 15-micron data. Without any ``a
posteriori'' update, the model predictions seem to agree well with the recently
published 24-micron extragalactic source counts, suggesting that the peak in
the 24-micron counts is dominated by ``starburst'' galaxies like those detected
by ISOCAM at 15 micron, but at higher redshifts (1 < z < 2 instead of 0.5 < z <
1.5).Comment: 8 pages: 4 pages of main text + 5 postscript figures, use aastex.
Accepted for publication in ApJL. Replaced with the proof version (added
missing references and corrected a few sentences
Design considerations for a monolithic, GaAs, dual-mode, QPSK/QASK, high-throughput rate transceiver
A monolithic, GaAs, dual mode, quadrature amplitude shift keying and quadrature phase shift keying transceiver with one and two billion bits per second data rate is being considered to achieve a low power, small and ultra high speed communication system for satellite as well as terrestrial purposes. Recent GaAs integrated circuit achievements are surveyed and their constituent device types are evaluated. Design considerations, on an elemental level, of the entire modem are further included for monolithic realization with practical fabrication techniques. Numerous device types, with practical monolithic compatability, are used in the design of functional blocks with sufficient performances for realization of the transceiver
Highly entangled photons from hybrid piezoelectric-semiconductor quantum dot devices
Entanglement resources are key ingredients of future quantum technologies. If
they could be efficiently integrated into a semiconductor platform a new
generation of devices could be envisioned, whose quantum-mechanical
functionalities are controlled via the mature semiconductor technology.
Epitaxial quantum dots (QDs) embedded in diodes would embody such ideal quantum
devices, but QD structural asymmetries lower dramatically the degree of
entanglement of the sources and hamper severely their real exploitation in the
foreseen applications. In this work, we overcome this hurdle using
strain-tunable optoelectronic devices, where any QD can be tuned for the
emission of highly polarization-entangled photons. The electrically-controlled
sources violate Bell inequalities without the need of spectral or temporal
filtering and they feature the highest degree of entanglement ever reported for
QDs, with concurrence as high as 0.75(2). These quantum-devices are at present
the most promising candidates for the direct implementation of QD-based
entanglement-resources in quantum information science and technology
New tests of the pp-wave correspondence.
The pp-wave/SYM correspondence is an equivalence relation, H string = Î-J , between the hamiltonian H string of string field theory in the pp-wave background and the dilatation operator Î in = 4 Super Yang-Mills in the double scaling limit. We calculate matrix elements of these operators in string field theory and in gauge theory. In the string theory Hilbert space we use the natural string basis, and in the gauge theory we use the basis which is isomorphic to it. States in this basis are specific linear combinations of the original BMN operators, and were constructed previously for the case of two scalar impurities. We extend this construction to incorporate BMN operators with vector and mixed impurities. This enables us to verify from the gauge theory perspective two key properties of the three-string interaction vertex of Spradlin and Volovich: (1) the vanishing of the three-string amplitude for string states with one vector and one scalar impurity; and (2) the relative minus sign in the string amplitude involving states with two vector impurities compared to that with two scalar impurities. This implies a spontaneous breaking of the 2 symmetry of the string field theory in the pp-wave background. Furthermore, we calculate the gauge theory matrix elements of Î-J for states with an arbitrary number of scalar impurities. In all cases we find perfect agreement with the corresponding string amplitudes derived from the three-string vertex
Galaxy formation in the Planck cosmology - II. Star-formation histories and post-processing magnitude reconstruction
We adapt the L-Galaxies semi-analytic model to follow the star-formation
histories (SFH) of galaxies -- by which we mean a record of the formation time
and metallicities of the stars that are present in each galaxy at a given time.
We use these to construct stellar spectra in post-processing, which offers
large efficiency savings and allows user-defined spectral bands and dust models
to be applied to data stored in the Millennium data repository.
We contrast model SFHs from the Millennium Simulation with observed ones from
the VESPA algorithm as applied to the SDSS-7 catalogue. The overall agreement
is good, with both simulated and SDSS galaxies showing a steeper SFH with
increased stellar mass. The SFHs of blue and red galaxies, however, show poor
agreement between data and simulations, which may indicate that the termination
of star formation is too abrupt in the models.
The mean star-formation rate (SFR) of model galaxies is well-defined and is
accurately modelled by a double power law at all redshifts: SFR proportional to
, where Gyr, is the age of the
stars and is the loopback time to the onset of galaxy formation; above a
redshift of unity, this is well approximated by a gamma function: SFR
proportional to , where Gyr. Individual
galaxies, however, show a wide dispersion about this mean. When split by mass,
the SFR peaks earlier for high-mass galaxies than for lower-mass ones, and we
interpret this downsizing as a mass-dependence in the evolution of the quenched
fraction: the SFHs of star-forming galaxies show only a weak mass dependence.Comment: Accepted version of the paper, to appear in MNRAS. Compared to the
original version, contains more detail on the post-processing of magnitudes,
including a table of rms magnitude errors. SFHs available on Millennium
database http://gavo.mpa-garching.mpg.de/MyMillennium
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