1,029 research outputs found
Using negative detections to estimate source finder reliability
We describe a simple method to determine the reliability of source finders
based on the detection of sources with both positive and negative total flux.
Under the assumption that the noise is symmetric and that real sources have
positive total flux, negative detections can be used to assign to each positive
detection a probability of being real. We discuss this method in the context of
upcoming, interferometric HI surveys.Comment: Accepted for publication on the 2012 PASA source finding special
issu
BUDHIES I: characterizing the environments in and around two clusters at z~0.2
We present the optical spectroscopy for the Blind Ultra Deep HI Environmental
Survey (BUDHIES). With the Westerbork Synthesis Radio Telescope, BUDHIES has
detected HI in over 150 galaxies in and around two Abell clusters at z~0.2.
With the aim of characterizing the environments of the HI-detected galaxies, we
obtained multi-fiber spectroscopy with the William Herschel Telescope. In this
paper, we describe the spectroscopic observations, report redshifts and EW[OII]
measurements for ~600 galaxies, and perform an environmental analysis. In
particular, we present cluster velocity dispersion measurements for 5 clusters
and groups in the BUDHIES volume, as well as a detailed substructure analysis.Comment: v2: Typos and small corrections after proofs added. 14 pages (plus
small appendix), 12 figures. Accepted for publication in MNRAS. Adobe Acrobat
Reader is required to correctly display the (3D) animated figures (Fig. 9).
Full data tables and supporting videos are also available at the BUDHIES
project website: http://www.astro.rug.nl/budhies
Asymmetric Drift and the Stellar Velocity Ellipsoid
We present the decomposition of the stellar velocity ellipsoid using stellar
velocity dispersions within a 40 deg wedge about the major-axis (sigma_maj),
the epicycle approximation, and the asymmetric drift equation. Thus, we employ
no fitted forms for sigma_maj and escape interpolation errors resulting from
comparisons of the major and minor axes. We apply the theoretical construction
of the method to integral field data taken for NGC 3949 and NGC 3982. We derive
the vertical-to-radial velocity dispersion ratio (sigma_z / sigma_R) and find
(1) our decomposition method is accurate and reasonable, (2) NGC 3982 appears
to be rather typical of an Sb type galaxy with sigma_z / sigma_R = 0.73
(+0.13/-0.11) despite its high surface brightness and small size, and (3) NGC
3949 has a hot disk with sigma_z / sigma_R = 1.18 (+0.36/-0.28).Comment: 4 pages including 3 figures, to appear in "Island Universes:
Structure and Evolution of Disk Galaxies", Terschelling, Netherlands, July
3-8, 200
From gas to galaxies
The unsurpassed sensitivity and resolution of the Square Kilometer Array
(SKA) will make it possible for the first time to probe the continuum emission
of normal star forming galaxies out to the edges of the universe. This opens
the possibility for routinely using the radio continuum emission from galaxies
for cosmological research as it offers an independent probe of the evolution of
the star formation density in the universe. In addition it offers the
possibility to detect the first star forming objects and massive black holes.
In deep surveys SKA will be able to detect HI in emission out to redshifts of
and hence be able to trace the conversion of gas into stars
over an era where considerable evolution is taking place. Such surveys will be
able to uniquely determine the respective importance of merging and accreting
gas flows for galaxy formation over this redshift range (i.e. out to when the
universe was only one third its present age). It is obvious that only SKA will
able to see literally where and how gas is turned into stars.
These and other aspects of SKA imaging of galaxies will be discussed.Comment: To be published in New Astronomy Reviews, Elsevier, Amsterdam as part
of "Science with the Square Kilometre Array", eds. C. Carilli and S.
Rawlings. 18 pages + 13 figures; high resolution version and other chapters
of "Science with the Square Kilometre Array" available at
http://www.skatelescope.org/pages/science_gen.ht
Nilpotent normal form for divergence-free vector fields and volume-preserving maps
We study the normal forms for incompressible flows and maps in the
neighborhood of an equilibrium or fixed point with a triple eigenvalue. We
prove that when a divergence free vector field in has nilpotent
linearization with maximal Jordan block then, to arbitrary degree, coordinates
can be chosen so that the nonlinear terms occur as a single function of two
variables in the third component. The analogue for volume-preserving
diffeomorphisms gives an optimal normal form in which the truncation of the
normal form at any degree gives an exactly volume-preserving map whose inverse
is also polynomial inverse with the same degree.Comment: laTeX, 20 pages, 1 figur
Electrowetting on superhydrophobic SU-8 patterned surfaces
Electrowetting on micro-patterned layers of SU-8 photoresist with an amorphous Teflon (R) coating has been observed. The cosine of the contact angle is shown to be proportional to the square of the applied voltage for increasing bias. However, this does not apply below 40 V and we suggest that this may be explained in terms of penetration of fluid into the pattern of the surface. Assuming that the initial application of a bias voltage converts the drop from Cassie-Baxter to Wenzel regime, we have used this as a technique to estimate the roughness factor of the surface
A2626 and Friends:Large- And Small-scale Structure
New MMT/Hectospec spectroscopy centered on the galaxy cluster A2626 and
covering a area out to more than
doubles the number of galaxy redshifts in this region. The spectra confirm four
clusters previously identified photometrically. A2625, which was previously
thought to be a close neighbor of A2626, is in fact much more distant. The new
data show six substructures associated with A2626 and five more associated with
A2637. There is also a highly collimated collection of galaxies and galaxy
groups between A2626 and A2637 having at least three and probably four
substructures. At larger scales, the A2626--A2637 complex is not connected to
the Pegasus--Perseus filament.Comment: 18 pages, 13 figures, accepted for publication in the Astronomical
Journa
Incidence and survival rate of women with cervical cancer in the Greater Amsterdam area
To evaluate the effect of population-based cervical cancer screening on the occurrence of cervical cancer in The Netherlands, we investigated the incidence and survival of cervical cancer registered by a cancer registry in the Greater Amsterdam area. The incidence rate of squamous cell carcinoma decreased significantly from 9.2/100,000 women in 1988 to 5.9/100,000 in 2000 (P<0.001). The incidence rate of adenocarcinomas remained stable. After adjustment for age, stage and lymph node involvement, the relative risk of death was 1.6 times higher for patients with adenocarcinomas than for patients with squamous cell carcinoma (95% CI 1.2-2.1). The decreased survival was related to histological type, as the effect remained significant after correction for confounding factors. Over time, the prognosis of women with squamous cell carcinoma improved significantly. No significant change was observed for women diagnosed with adenocarcinoma. These results suggest that the screening programme in The Netherlands as executed in the Greater Amsterdam area is associated with a decreased incidence and increased survival of patients with squamous cell carcinoma, but fails to detect (pre)malignant lesions of adenocarcinoma. Since more than 92% of adenocarcinomas and its precursors contain high-risk HPV, adding HPV testing to cytologic screening might improve the present screening programme in detecting adenocarcinoma and its precursor lesions
Cosmological surveys with the Australian Square Kilometre Array Pathfinder
This is a design study into the capabilities of the Australian Square
Kilometre Array Pathfinder in performing a full-sky low redshift neutral
hydrogen survey, termed WALLABY, and the potential cosmological constraints one
can attain from measurement of the galaxy power spectrum. We find that the full
sky survey will likely attain 0.6 million redshifts which, when combined with
expected Planck CMB data, will constrain the Dark Energy equation of state to
20%, representing a coming of age for radio observations in creating
cosmological constraints.Comment: 10 pages, 6 figures, accepted in PASA, updated to match published
versio
Generating crop calendars with Web search data
This paper demonstrates the potential of using Web search volumes for generating crop specific planting and harvesting dates in the USA integrating climatic, social and technological factors affecting crop calendars. Using Google Insights for Search, clear peaks in volume occur at times of planting and harvest at the national level, which were used to derive corn specific planting and harvesting dates at a weekly resolution. Disaggregated to state level, search volumes for corn planting generally are an agreement with planting dates from a global crop calendar dataset. However, harvest dates were less discriminatory at the state level, indicating that peaks in search volume may be blurred by broader searches on harvest as a time of cultural events. The timing of other agricultural activities such as purchase of seed and response to weed and pest infestation was also investigated. These results highlight the future potential of using Web search data to derive planting dates in countries where the data are sparse or unreliable, once sufficient search volumes are realized, as well as the potential for monitoring in real time the response of farmers to climate change over the coming decades. Other potential applications of search volume data of relevance to agronomy are also discussed
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