2,619 research outputs found
Analytical solution of a model for complex food webs
We investigate numerically and analytically a recently proposed model for
food webs [Nature {\bf 404}, 180 (2000)] in the limit of large web sizes and
sparse interaction matrices. We obtain analytical expressions for several
quantities with ecological interest, in particular the probability
distributions for the number of prey and the number of predators. We find that
these distributions have fast-decaying exponential and Gaussian tails,
respectively. We also find that our analytical expressions are robust to
changes in the details of the model.Comment: 4 pages (RevTeX). Final versio
A new era of wide-field submillimetre imaging: on-sky performance of SCUBA-2
SCUBA-2 is the largest submillimetre wide-field bolometric camera ever built.
This 43 square arc-minute field-of-view instrument operates at two wavelengths
(850 and 450 microns) and has been installed on the James Clerk Maxwell
Telescope on Mauna Kea, Hawaii. SCUBA-2 has been successfully commissioned and
operational for general science since October 2011. This paper presents an
overview of the on-sky performance of the instrument during and since
commissioning in mid-2011. The on-sky noise characteristics and NEPs of the 450
and 850 micron arrays, with average yields of approximately 3400 bolometers at
each wavelength, will be shown. The observing modes of the instrument and the
on-sky calibration techniques are described. The culmination of these efforts
has resulted in a scientifically powerful mapping camera with sensitivities
that allow a square degree of sky to be mapped to 10 mJy/beam rms at 850 micron
in 2 hours and 60 mJy/beam rms at 450 micron in 5 hours in the best weather.Comment: 18 pages, 15 figures.SPIE Conference series 8452, Millimetre,
Submillimetre and Far-infrared Detectors and Instrumentation for Astronomy VI
201
Serendipitous detection of an overdensity of Herschel-SPIRE 250 micron sources south of MRC1138-26
We report the serendipitous detection of a significant overdensity of
Herschel-SPIRE 250 micron sources in the vicinity of MRC1138-26. We use an
adaptive kernel density estimate to quantify the significance, including a
comparison with other fields. The overdensity has a size of ~3.5-4' and stands
out at ~5sigma with respect to the background estimate. No features with
similar significance were found in four extragalactic control fields:
GOODS-North, Lockman, COSMOS and UDS. The chance of having a similar
overdensity in a field with the same number but randomly distributed sources is
less than 2%. The clump is also visible as a low surface brightness feature in
the Planck 857 GHz map. We detect 76 sources at 250 micron (with a
signal-to-noise ratio greater than 3), in a region of 4' radius; 43 of those
are above a flux density limit of 20 mJy. This is a factor of 3.6 in excess
over the average in the four control fields, considering only the sources above
20 mJy. We also find an excess in the number counts of sources with 250 micron
flux densities between 30 and 40 mJy, compared to deep extragalactic
blank-field number counts. Assuming a fixed dust temperature (30 K) and
emissivity (beta=1.5) a crude, blackbody-derived redshift distribution, zBB, of
the detected sources is significantly different from the distributions in the
control fields and exhibits a significant peak at zBB ~ 1.5, although the
actual peak redshift is highly degenerate with the temperature. We tentatively
suggest, based on zBB and the similar S250/S350 colours of the sources within
the peak, that a significant fraction of the sources in the clump may be at a
similar redshift. Since the overdensity lies ~7' south of the z=2.16 Spiderweb
protocluster MRC1138-26, an intriguing possibility (that is presently
unverifiable given the data in hand) is that it lies within the same
large-scale structure.(abridged)Comment: 10 pages, 8 figures, accepted for publication in MNRA
SANEPIC: A Map-Making Method for Timestream Data From Large Arrays
We describe a map-making method which we have developed for the Balloon-borne
Large Aperture Submillimeter Telescope (BLAST) experiment, but which should
have general application to data from other submillimeter arrays. Our method
uses a Maximum Likelihood based approach, with several approximations, which
allows images to be constructed using large amounts of data with fairly modest
computer memory and processing requirements. This new approach, Signal And
Noise Estimation Procedure Including Correlations (SANEPIC), builds upon
several previous methods, but focuses specifically on the regime where there is
a large number of detectors sampling the same map of the sky, and explicitly
allowing for the the possibility of strong correlations between the detector
timestreams. We provide real and simulated examples of how well this method
performs compared with more simplistic map-makers based on filtering. We
discuss two separate implementations of SANEPIC: a brute-force approach, in
which the inverse pixel-pixel covariance matrix is computed; and an iterative
approach, which is much more efficient for large maps. SANEPIC has been
successfully used to produce maps using data from the 2005 BLAST flight.Comment: 27 Pages, 15 figures; Submitted to the Astrophysical Journal; related
results available at http://blastexperiment.info/ [the BLAST Webpage
The source counts of submillimetre galaxies detected at 1.1 mm
The source counts of galaxies discovered at sub-millimetre and millimetre
wavelengths provide important information on the evolution of infrared-bright
galaxies. We combine the data from six blank-field surveys carried out at 1.1
mm with AzTEC, totalling 1.6 square degrees in area with root-mean-square
depths ranging from 0.4 to 1.7 mJy, and derive the strongest constraints to
date on the 1.1 mm source counts at flux densities S(1100) = 1-12 mJy. Using
additional data from the AzTEC Cluster Environment Survey to extend the counts
to S(1100) ~ 20 mJy, we see tentative evidence for an enhancement relative to
the exponential drop in the counts at S(1100) ~ 13 mJy and a smooth connection
to the bright source counts at >20 mJy measured by the South Pole Telescope;
this excess may be due to strong lensing effects. We compare these counts to
predictions from several semi-analytical and phenomenological models and find
that for most the agreement is quite good at flux densities > 4 mJy; however,
we find significant discrepancies (>3sigma) between the models and the observed
1.1 mm counts at lower flux densities, and none of them are consistent with the
observed turnover in the Euclidean-normalised counts at S(1100) < 2 mJy. Our
new results therefore may require modifications to existing evolutionary models
for low luminosity galaxies. Alternatively, the discrepancy between the
measured counts at the faint end and predictions from phenomenological models
could arise from limited knowledge of the spectral energy distributions of
faint galaxies in the local Universe.Comment: 16 pages, 3 figures, 4 tables; accepted for publication in MNRA
APOE ɛ4 is associated with postictal confusion in patients with medically refractory temporal lobe epilepsy
This study examined the relationship between the APOE ɛ4 allele and postictal confusion in patients with medically intractable temporal lobe epilepsy (TLE). Patients with at least one ɛ4 allele (n = 22) were three times more likely to exhibit postictal confusion (68%) than the 63 patients without ɛ4 (43%). These preliminary results demonstrate that APOE ɛ4 is associated with an increased risk of postictal confusion in patients with medically intractable TLE, suggesting possible dysfunction in neuronal recovery mechanisms
Arctic system on trajectory to new state
The Arctic system is moving toward a new state that falls outside the envelope of glacial-interglacial fluctuations that prevailed during recent Earth history. This future Arctic is likely to have dramatically less permanent ice than exists at present. At the present rate of change, a summer ice-free Arctic Ocean within a century is a real possibility, a state not witnessed for at least a million years. The change appears to be driven largely by feedback-enhanced global climate warming, and there seem to be few, if any processes or feedbacks within the Arctic system that are capable of altering the trajectory toward this “super interglacial” state
Diffractive Phenomena and Shadowing in Deep-Inelastic Scattering
Shadowing effects in deep-inelastic lepton-nucleus scattering probe the mass
spectrum of diffractive leptoproduction from individual nucleons. We explore
this relationship using current experimental information on both processes. In
recent data from the NMC and E665 collaboration, taken at small x << 0.1 and
Q^2 < 1 GeV^2, shadowing is dominated by the diffractive excitation and
coherent interaction of low mass vector mesons. If shadowing is explored at
small x > 1 GeV^2 as discussed at HERA, the situation is
different. Here dominant contributions come from the coherent interaction of
diffractively produced heavy mass states. Furthermore we observe that the
energy dependence of shadowing is directly related to the mass dependence of
the diffractive production cross section for free nucleon targets.Comment: 12 pages Latex, 8 figure
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