7,400 research outputs found
The Arecibo Legacy Fast ALFA Survey: VI. Second HI Source Catalog of the Virgo Cluster Region
We present the third installment of HI sources extracted from the Arecibo
Legacy Fast ALFA extragalactic survey. This dataset continues the work of the
Virgo ALFALFA catalog. The catalogs and spectra published here consist of data
obtained during the 2005 and 2006 observing sessions of the survey. The catalog
consists of 578 HI detections within the range 11h 36m < R.A.(J2000) < 13h 52m
and +08 deg < Dec.(J2000) < +12 deg, and cz_sun < 18000 km/s. The catalog
entries are identified with optical counterparts where possible through the
examination of digitized optical images. The catalog detections can be
classified into three categories: (a) detections of high reliability with S/N >
6.5; (b) high velocity clouds in the Milky Way or its periphery; and (c)
signals of lower S/N which coincide spatially with an optical object and known
redshift. 75% of the sources are newly published HI detections. Of particular
note is a complex of HI clouds projected between M87 and M49 that do not
coincide with any optical counterparts. Candidate objects without optical
counterparts are few. The median redshift for this sample is 6500 km/s and the
cz distribution exhibits the local large scale structure consisting of Virgo
and the background void and the A1367-Coma supercluster regime at cz_sun ~7000
km/s. Position corrections for telescope pointing errors are applied to the
dataset by comparing ALFALFA continuum centroid with those cataloged in the
NRAO VLA Sky Survey. The uncorrected positional accuracy averages
27 arcsec ~(21 arcsec ~median) for all sources with S/N > 6.5 and is of order
~21 arcsec ~(16 arcsec ~median) for signals with S/N > 12. Uncertainties in
distances toward the Virgo cluster can affect the calculated HI mass
distribution.Comment: 25 pages, 1 Table, 8 figures, Accepted by the Astronomical Journa
Historical collaborative geocoding
The latest developments in digital have provided large data sets that can
increasingly easily be accessed and used. These data sets often contain
indirect localisation information, such as historical addresses. Historical
geocoding is the process of transforming the indirect localisation information
to direct localisation that can be placed on a map, which enables spatial
analysis and cross-referencing. Many efficient geocoders exist for current
addresses, but they do not deal with the temporal aspect and are based on a
strict hierarchy (..., city, street, house number) that is hard or impossible
to use with historical data. Indeed historical data are full of uncertainties
(temporal aspect, semantic aspect, spatial precision, confidence in historical
source, ...) that can not be resolved, as there is no way to go back in time to
check. We propose an open source, open data, extensible solution for geocoding
that is based on the building of gazetteers composed of geohistorical objects
extracted from historical topographical maps. Once the gazetteers are
available, geocoding an historical address is a matter of finding the
geohistorical object in the gazetteers that is the best match to the historical
address. The matching criteriae are customisable and include several dimensions
(fuzzy semantic, fuzzy temporal, scale, spatial precision ...). As the goal is
to facilitate historical work, we also propose web-based user interfaces that
help geocode (one address or batch mode) and display over current or historical
topographical maps, so that they can be checked and collaboratively edited. The
system is tested on Paris city for the 19-20th centuries, shows high returns
rate and is fast enough to be used interactively.Comment: WORKING PAPE
A search for hyperluminous X-ray sources in the XMM-Newton source catalog
We present a new method to identify luminous off-nuclear X-ray sources in the
outskirts of galaxies from large public redshift surveys, distinguishing them
from foreground and background interlopers. Using the 3XMM-DR5 catalog of X-ray
sources and the SDSS DR12 spectroscopic sample of galaxies, with the help of
this off-nuclear cross-matching technique, we selected 98 sources with inferred
X-ray luminosities in the range , compatible with hyperluminous X-ray objects (HLX). To validate
the method, we verify that it allowed us to recover known HLX candidates such
as ESO 24349 HLX1 and M82 X1. From a statistical study, we
conservatively estimate that up to of these sources may be fore- or
background sources, statistically leaving at least 16 that are likely to be
HLXs, thus providing support for the existence of the HLX population. We
identify two good HLX candidates and using other publicly available datasets,
in particular the VLA FIRST in radio, UKIDSS in the near-infrared, GALEX in the
ultra-violet and CFHT Megacam archive in the optical, we present evidence that
these objects are unlikely to be foreground or background X-ray objects of
conventional types, e.g. active galactic nuclei, BL Lac objects, Galactic X-ray
binaries or nearby stars. However, additional dedicated X-ray and optical
observations are needed to confirm their association with the assumed host
galaxies and thus secure their HLX classification.Comment: 20 pages, 6 figures, accepted to Ap
Probing discs around massive young stellar objects with CO first overtone emission
We present high resolution (R~50,000) spectroastrometry over the CO 1st
overtone bandhead of a sample of seven intermediate/massive young stellar
objects. These are primarily drawn from the red MSX source (RMS) survey, a
systematic search for young massive stars which has returned a large, well
selected sample of such objects. The mean luminosity of the sample is
approximately 5 times 10^4 L_\odot, indicating the objects typically have a
mass of ~15 solar masses. We fit the observed bandhead profiles with a model of
a circumstellar disc, and find good agreement between the models and
observations for all but one object. We compare the high angular precision
(0.2-0.8 mas) spectroastrometric data to the spatial distribution of the
emitting material in the best-fitting models. No spatial signatures of discs
are detected, which is entirely consistent with the properties of the
best-fitting models. Therefore, the observations suggest that the CO bandhead
emission of massive young stellar objects originates in small-scale disks, in
agreement with previous work. This provides further evidence that massive stars
form via disc accretion, as suggested by recent simulations.Comment: Accepted for publication in MNRA
Analytical modelling of positional and thematic uncertainties in the integration of remote sensing and geographical information systems
This paper describes three aspects of uncertainty in geographical information systems (GIS) and remote sensing. First, the positional uncertainty of an area object in a GIS is discussed as a function of positional uncertainties of line segments and boundary line features. Second, the thematic uncertainty of a classified remote sensing image is described using the probability vectors from a maximum likelihood classification. Third, the 'S-band' model is used to quantify uncertainties after combining GIS and remote sensing data.Department of Land Surveying and Geo-Informatic
A Survey on Array Storage, Query Languages, and Systems
Since scientific investigation is one of the most important providers of
massive amounts of ordered data, there is a renewed interest in array data
processing in the context of Big Data. To the best of our knowledge, a unified
resource that summarizes and analyzes array processing research over its long
existence is currently missing. In this survey, we provide a guide for past,
present, and future research in array processing. The survey is organized along
three main topics. Array storage discusses all the aspects related to array
partitioning into chunks. The identification of a reduced set of array
operators to form the foundation for an array query language is analyzed across
multiple such proposals. Lastly, we survey real systems for array processing.
The result is a thorough survey on array data storage and processing that
should be consulted by anyone interested in this research topic, independent of
experience level. The survey is not complete though. We greatly appreciate
pointers towards any work we might have forgotten to mention.Comment: 44 page
From Accessibility and Exposure to Engagement: A Multi-scalar Approach to Measuring Environmental Determinants of Children’s Health Using Geographic Information Systems
A growing body of research suggests that increasing the accessibility to health-related environmental features and increasing exposure to and engagement in outdoor environments leads to positive benefits for the overall health and well-being of children. Additionally, research over the last twenty-five years has documented a decline in the time children spend outdoors. Outdoor activity in children is associated with increased levels of physical fitness, and cognitive well-being. Despite acknowledging this connection, problems occur for researchers when attempting to identify the child’s location and to measure whether a child has made use of an accessible health-related facility, or where, when and for how long a child spends time outdoors.
The purpose of this thesis is to measure children’s accessibility to, exposure to, and engagement with health-promoting features of their environment. The research on the environment-health link aims to meet two objectives: 1) to quantify the magnitude of positional discrepancies and accessibility misclassification that result from using several commonly-used address proxies; and 2) to examine how individual-level, household-level, and neighbourhood-level factors are associated with the quantity of time children spend outdoors. This will be achieved by employing the use of GPS tracking to objectively quantify the time spent outdoors using a novel machine learning algorithm, and by applying a hexagonal grid to extract built environment measures.
This study aims to identify the impact of positional discrepancies when measuring accessibility by examining misclassification of address proxies to several health-related facilities throughout the City of London and Middlesex County, Ontario, Canada. Positional errors are quantified by multiple neighbourhood types. Findings indicate that the shorter the threshold distance used to measure accessibility between subject population and health-related facility, the higher the proportion of misclassified addresses. Using address proxies based on large aggregated units, such as centroids of census tracts or dissemination areas, can result in vast positional discrepancies, and therefore should be avoided in spatial epidemiologic research.
To reduce the misclassification, and positional errors, the use of individual portable passive GPS receivers were employed to objectively track the spatial patterns, and quantify the time spent outdoors of children (aged 7 to 13 years) in London, Ontario across multiple neighbourhood types. On the whole, children spent most of their outdoor time during school hours (recess time) and the non-school time outdoors in areas immediately surrounding their home.
From these findings, policymakers, educators, and parents can support children’s health by making greater efforts to promote outdoor activities for improved health and quality of life in children. This thesis aims to advance our understanding of the environment and health-link and suggests practical steps for more well-informed decision making by combining novel classification and mapping techniques
HST/WFPC2 observations of the LMC pulsar PSR B0540-69
The study of the younger, and brighter, pulsars is important to understand
the optical emission properties of isolated neutron stars. PSRB0540-69, the
second brightest (V~22) optical pulsar, is obviously a very interesting target
for these investigations. The aim of this work is threefold: constraining the
pulsar proper motion and its velocity on the plane of the sky through optical
astrometry, obtaining a more precise characterisation of the pulsar optical
spectral energy distribution (SED) through a consistent set of multi-band,
high-resolution, imaging photometry observations, measuring the pulsar optical
phase-averaged linear polarisation, for which only a preliminary and uncertain
measurement was obtained so far from ground-based observations. We performed
high-resolution observations of PSRB0540-69 with the WFPC2 aboard the HST, in
both direct imaging and polarimetry modes. From multi-epoch astrometry we set a
3sigma upper limit of 1 mas/yr on the pulsar proper motion, implying a
transverse velocity <250 km/s at the 50 kpc LMC distance. Moreover, we
determined the pulsar absolute position with an unprecedented accuracy of 70
mas. From multi-band photometry we characterised the pulsar power-law spectrum
and we derived the most accurate measurement of the spectral index
(0.70+/-0.07) which indicates a spectral turnover between the optical and X-ray
bands. Finally, from polarimetry we obtained a new measurement of the pulsar
phase-averaged polarisation degree (16+/-4%),consistent with magnetosphere
models depending on the actual intrinsic polarisation degree and depolarisation
factor, and we found that the polarisation vector (22+/-12deg position angle)
is possibly aligned with the semi-major axis of the pulsar-wind nebula and with
the apparent proper motion direction of its bright emission knot.Comment: 14 pages, 12 figures, accepted for publication in Astronomy &
Astrophysic
Techniques for the Visualization of Positional Geospatial Uncertainty
Geospatial data almost always contains some amount of uncertainty due to inaccuracies in its acquisition and transformation. While the data is commonly visualized (e.g. on digital maps), there are unanswered needs for visualizing uncertainty along with it. Most research on effectively doing this addresses uncertainty in data values at geospatial positions, e.g. water depth, human population, or land-cover classification. Uncertainty in the data’s geospatial positions themselves (positional uncertainty) has not been previously focused on in this regard. In this thesis, techniques were created for visualizing positional uncertainty using World Vector Shoreline as an example dataset. The techniques consist of a shoreline buffer zone to which visual effects such as gradients, transparency, and randomized dots were applied. They are viewed interactively via Web Map Service (WMS). In clutter testing with human subjects, a transparency-gradient technique performed the best, followed by a solid-fill technique, with a dots-density-gradient technique performing worst
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