6,953 research outputs found
Distal occurrence of mid-Holocene Whakatane Tephra on the Chatham Islands, New Zealand, and potential for cryptotephra studies
The Whakatane Tephra, a rhyolitic tephra erupted ca. 5500 cal. BP from Okataina Volcanic Centre, central North Island, has been identified on the Chatham Islands which lie ˜900 km east of Christchurch, New Zealand. The visible tephra layer, ˜5 mm in thickness and preserved within peat on Pitt Island, was identified using both radiocarbon dating and analysis of glass shards by electron microprobe. Whakatane Tephra is the first Holocene tephra to be identified on the Chatham Islands, and it is the most distal Holocene tephra yet recorded in the New Zealand region, being ˜850 km from source. The Pitt Island occurrence extends the tephra's dispersal area markedly, by an order of magnitude, possibly to ˜300,000 km2. An estimated dispersal index (D) of approximately 105 km2 indicates that the eruption generated a very high plinian column, possibly exceeding ˜30 km in height, with strong winds blowing the ash plume southeastwards. This new discovery of distal Whakatane Tephra as a thin but visible layer strongly implies that cryptotephras are likely to be preserved on the Chatham Islands and within adjacent ocean floor sediments. Therefore the potential exists to develop enhanced cryptotephrostratigraphic records from these distal areas, which in turn would help facilitate precise correlation via tephrochronology of palaeoenvironmental records (such as NZ-INTIMATE) from mainland New Zealand, the southwest Pacific Ocean, and the Chatham Islands
Probing Dark Matter Substructure in Lens Galaxies
We investigate the effects of numerous dark matter subhalos in a galaxy-sized
halo on the events of strong lensing, to assess their presence as expected from
the cold dark matter scenario. Lens galaxies are represented by a smooth
ellipsoid in an external shear field and additional cold dark matter subhalos
taken from Monte Carlo realizations which accord with recent N-body results. We
also consider other possible perturbers, globular clusters and luminous dwarf
satellites, for comparison. We then apply the models to the particular lens
systems with four images, B1422+231 and PG1115+080, for which smooth lens
models are unable to reproduce both the positions of the images and their radio
flux ratios or dust-free optical flux ratios simultaneously. We show that the
perturbations by both globular clusters and dwarf satellites are too small to
change the flux ratios, whereas cold dark matter subhalos are most likely
perturbers to reproduce the observed flux ratios in a statistically significant
manner. This result suggests us the presence of numerous subhalos in lens
galaxies, which is consistent with the results of cosmological N-body
simulations.Comment: 19 pages, including 5 figures, ApJ in pres
Testing use of mitochondrial COI sequences for the identification and phylogenetic analysis of New Zealand caddisflies (Trichoptera)
We tested the hypothesis that cytochrome c oxidase subunit 1 (COI) sequences would successfully discriminate recognised species of New Zealand caddisflies. We further examined whether phylogenetic analyses, based on the COI locus, could recover currently recognised superfamilies and suborders. COI sequences were obtained from 105 individuals representing 61 species and all 16 families of Trichoptera known from New Zealand. No sequence sharing was observed between members of different species, and congeneric species showed from 2.3 to 19.5% divergence. Sequence divergence among members of a species was typically low (mean = 0.7%; range 0.0–8.5%), but two species showed intraspecific divergences in excess of 2%. Phylogenetic reconstructions based on COI were largely congruent with previous conclusions based on morphology, although the sequence data did not support placement of the purse-cased caddisflies (Hydroptilidae) within the uncased caddisflies, and, in particular, the Rhyacophiloidea. We conclude that sequence variation in the COI gene locus is an effective tool for the identification of New Zealand caddisfly species, and can provide preliminary phylogenetic inferences. Further research is needed to ascertain the significance of the few instances of high intra-specific divergence and to determine if any instances of sequence sharing will be detected with larger sample sizes
Socially intelligent reasoning for autonomous agents
Socially intelligent agents are autonomous problem solvers that have to achieve their objectives by interacting with other similarly autonomous entities. A major concern, therefore, is with the design of the decision-making mechanism that such agents employ in order to determine which actions to take to achieve their goals. An attractive and much sought after property of this mechanism is that it produces decisions that are rational from the perspective of the individual agent. However, some agents are also inherently social. Moreover, individual and social concerns often conflict, leading to the possibility of inefficient performance of the individual and the system. To address these problems we propose a framework for making socially acceptable decisions, based on social welfare functions, that combines social and individual perspectives in a unified and flexible manner. The framework is realized in an exemplar computational setting and an empirical analysis is made of the relative performance of varying sociable decision-making functions in a range of environments. This analysis is then used to design an agent that adapts its decision-making to reflect the resource constraints that it faces at any given time. A further round of empirical evaluation shows how adding such a metalevel mechanism enhances the performance of the agent by directing reasoning to adopt different strategies in different contexts. Finally, the possibility and efficacy of making the metalevel mechanism adaptive, so that experience of past encounters can be factored into the decision-making, is demonstrated
Does collimation affect patient dose in antero-posterior thoraco-lumbar spine?
BACKGROUND: The purpose of this study is to determine the effect of collimation on the lifetime attributable risk (LAR) of cancer incidence in all body organs (effective risk) in patients undergoing antero-posterior (AP) examinations of the spine. This is of particular importance for patients suffering from scoliosis as in their case regular repeat examinations are required and also because such patients are usually young and more susceptible to the effects of ionising radiation than are older patients.
METHOD: High sensitivity thermo-luminescent dosimeters (TLDs) were used to measure radiation dose to all organs of an adult male dosimetry phantom, positioned for an AP projection of the thoraco-lumbar spine. Exposures were made, first applying tight collimation and then subsequently with loose collimation, using the same acquisition factors. In each case, the individual TLDs were measured to determine the local absorbed dose and those representing each organ averaged to calculate organ dose.
This information was then used to calculate the effective risk of cancer incidence for each decade of life from 20 to 80, and to compare the likelihood of cancer incidence when using tight and loose collimation.
RESULTS: The calculated figures for effective risk of cancer incidence suggest that the risk when using loose collimation compared to the use of tight collimation is over three times as high and this is the case across all age decades from 20 to 80.
CONCLUSION: Tight collimation can greatly reduce radiation dose and risk of cancer incidence. However collimation in scoliotic patients can be necessarily limited
The oxygen-II luminosity density of the Universe
Equivalent widths of [OII] 3727 A lines are measured in 375 faint galaxy
spectra taken as part of the Caltech Faint Galaxy Redshift Survey centered on
the Hubble Deep Field. The sensitivity of the survey spectra to the [OII] line
is computed as a function of magnitude, color and redshift. The luminosity
function of galaxies in the [OII] line and the integrated luminosity density of
the Universe in the [OII] line are computed as a function of redshift. It is
found that the luminosity density in the [OII] line was a factor of ~10 higher
at redshifts z~1 than it is at the present day. The simplest interpretation is
that the star formation rate density of the Universe has declined dramatically
since z~1.Comment: accepted for publication in Ap
Early-type galaxies in the SDSS. II. Correlations between observables
A magnitude limited sample of nearly 9000 early-type galaxies, in the
redshift range 0.01 < z < 0.3, was selected from the Sloan Digital Sky Survey
using morphological and spectral criteria. The sample was used to study how
early-type galaxy observables, including luminosity L, effective radius R_o,
surface brightness I_o, color, and velocity dispersion sigma, are correlated
with one another. Measurement biases are understood with mock catalogs which
reproduce all of the observed scaling relations and their dependences on
fitting technique. At any given redshift, the intrinsic distribution of
luminosities, sizes and velocity dispersions in our sample are all
approximately Gaussian. A maximum likelihood analysis shows that sigma ~
L^{0.25\pm 0.012}, R_o ~ L^{0.63\pm 0.025}, and R_o ~ I^{-0.75\pm 0.02} in the
r* band. In addition, the mass-to-light ratio within the effective radius
scales as M_o/L ~ L^{0.14\pm 0.02} or M_o/L ~ M_o^{0.22\pm 0.05}, and galaxies
with larger effective masses have smaller effective densities: Delta_o ~
M_o^{-0.52\pm 0.03}. These relations are approximately the same in the g*, i*
and z* bands. Relative to the population at the median redshift in the sample,
galaxies at lower and higher redshifts have evolved only little, with more
evolution in the bluer bands. The luminosity function is consistent with weak
passive luminosity evolution and a formation time of about 9 Gyrs ago.Comment: 29 pages, 11 figures. Accepted by AJ (scheduled for April 2003). This
paper is part II of a revised version of astro-ph/011034
Cosmic homogeneity demonstrated with luminous red galaxies
We test the homogeneity of the Universe at with the Luminous Red
Galaxy (LRG) spectroscopic sample of the Sloan Digital Sky Survey. First, the
mean number of LRGs within completely surveyed LRG-centered spheres of
comoving radius is shown to be proportional to at radii greater than
. The test has the virtue that it does not rely
on the assumption that the LRG sample has a finite mean density; its results
show, however, that there \emph{is} such a mean density. Secondly, the survey
sky area is divided into 10 disjoint solid angular regions and the fractional
rms density variations of the LRG sample in the redshift range
among these () regions is found to be 7
percent of the mean density. This variance is consistent with typical biased
\lcdm models and puts very strong constraints on the quality of SDSS
photometric calibration.Comment: submitted to Ap
New analysis in the field of open cluster Collinder 223
The present study of the open cluster Collinder 223 (Cr 223) has been mainly
depended on the photoelectric data of Claria & Lapasset (1991; hereafter CL91).
This data of CL91 has been used with the cluster's image of AAO-DSS in order to
re-investigate and improve the main parameters of Cr 223. Stellar count has
been achieved to determine the stellar density, the cluster's center and the
cluster's diameter. In addition, the luminosity function, mass function, and
the total mass of the cluster have been estimated.Comment: 12 pages, 8 figure
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