778 research outputs found
The ACS Virgo Cluster Survey IV: Data Reduction Procedures for Surface Brightness Fluctuation Measurements with the Advanced Camera for Surveys
The Advanced Camera for Surveys (ACS) Virgo Cluster Survey is a large program
to image 100 early-type Virgo galaxies using the F475W and F850LP bandpasses of
the Wide Field Channel of the ACS instrument on the Hubble Space Telescope
(HST). The scientific goals of this survey include an exploration of the
three-dimensional structure of the Virgo Cluster and a critical examination of
the usefulness of the globular cluster luminosity function as a distance
indicator. Both of these issues require accurate distances for the full sample
of 100 program galaxies. In this paper, we describe our data reduction
procedures and examine the feasibility of accurate distance measurements using
the method of surface brightness fluctuations (SBF) applied to the ACS Virgo
Cluster Survey F850LP imaging. The ACS exhibits significant geometrical
distortions due to its off-axis location in the HST focal plane; correcting for
these distortions by resampling the pixel values onto an undistorted frame
results in pixel correlations that depend on the nature of the interpolation
kernel used for the resampling. This poses a major challenge for the SBF
technique, which normally assumes a flat power spectrum for the noise. We
investigate a number of different interpolation kernels and show through an
analysis of simulated galaxy images having realistic noise properties that it
is possible, depending on the kernel, to measure SBF distances using
distortion-corrected ACS images without introducing significant additional
error from the resampling. We conclude by showing examples of real image power
spectra from our survey.Comment: ApJS, in press, complete version of the paper at the link:
http://www.physics.rutgers.edu/~pcote/acs/publications.htm
Application of Doppler Broadened Gamma Spectroscopy to Study the Surface of Graphene
We present Doppler broadened gamma spectra, obtained using the newly
developed advanced positron beam at the University of Texas at Arlington, from
a sample consisting of 6 to 8 layers of graphene (MLG) on polycrystalline Cu.
The kinetic energy of the positron beam was varied form 2 eV to 20 keV allowing
for a depth resolved measurement. The ratio curves formed by dividing the
measured Doppler broadened gamma spectra obtained at low positron kinetic
energies (~2eV) to the gamma spectra obtained at 20 keV were compared to ratio
curves found by dividing the calculated spectra of bulk graphite to bulk Cu.
The ratio curves obtained from the measured results show qualitative agreement
with those obtained from the calculated spectra. In particular, both sets of
curves indicate a much reduced intensity at high momentum. The agreement
between the measured and calculated curves is consistent with the hypothesis
that the 2eV spectra correspond to the Doppler broadened spectra from the thin
overlayer of Graphene (which we anticipate should be similar to the spectra
obtained from bulk graphite) and that the spectra taken at 20 keV corresponds
to bulk Cu due to the fact that most of the positrons implanted at this energy
annihilate in the Cu substrate. The results taken at 2 eV provide evidence that
it is possible to obtain chemically sensitive information from the top atomic
layers of surfaces (both internal and external) from an analysis of the high
momentum tail of the Doppler broadened gamma spectra obtained from the
annihilation of positrons at the surface.Comment: Presented at 18th International Conference on Positron Annihilation,
August 19-24, 2018 | Orlando, USA. The following article has been accepted by
AIP Conference Proceedings. After it is published, it will be found at
https://aip.scitation.org/journal/ap
Environmental correlates of aquatic macroinvertebrate diversity in garden ponds: Implications for pond management
Garden ponds are a ubiquitous feature of urban landscapes and have the potential to be an important resource for biodiversity. However, the environmental and spatial factors influencing ecological communities within garden ponds have been poorly quantified to date, despite such evidence being critical to the development of effective management strategies that support biodiversity. This study aims to identify the environmental and spatial factors influencing macroinvertebrate richness, abundance and compositional variation among garden ponds and provide management recommendations at the local and landscape scale. In total, 99 macroinvertebrate taxa were recorded from 30 garden ponds. A negative association was recorded between ecological uniqueness (measured as the Local Contribution to Beta-Diversity) and taxonomic richness among the garden ponds, and those ponds with high ecological uniqueness typically displayed high replacement (turnover) values. Surface area, total plant richness and non-native plant richness were positively associated with macroinvertebrate richness. Taxonomic richness and abundance predominantly displayed a negative association with conductivity levels. Current management practices for garden ponds are highly variable, often focussed on individual ponds. Based on the findings of this study, we recommend that management should focus on making garden pond surface areas as large as possible, ensure that there is a wide variety of native aquatic plant species present and manage conductivity levels. Garden ponds likely comprise a significant component of the urban freshwater network, and considering their management at both local and landscape scales will ensure that biotic communities inhabiting urban landscapes can be more effectively supported
Fabrication and electrical characterization of three-dimensional graphitic microchannels in single crystal diamond
We report on the systematic characterization of conductive micro-channels
fabricated in single-crystal diamond with direct ion microbeam writing. Focused
high-energy (~MeV) helium ions are employed to selectively convert diamond with
micrometric spatial accuracy to a stable graphitic phase upon thermal
annealing, due to the induced structural damage occurring at the end-of-range.
A variable-thickness mask allows the accurate modulation of the depth at which
the microchannels are formed, from several {\mu}m deep up to the very surface
of the sample. By means of cross-sectional transmission electron microscopy
(TEM) we demonstrate that the technique allows the direct writing of amorphous
(and graphitic, upon suitable thermal annealing) microstructures extending
within the insulating diamond matrix in the three spatial directions, and in
particular that buried channels embedded in a highly insulating matrix emerge
and electrically connect to the sample surface at specific locations. Moreover,
by means of electrical characterization both at room temperature and variable
temperature, we investigate the conductivity and the charge-transport
mechanisms of microchannels obtained by implantation at different ion fluences
and after subsequent thermal processes, demonstrating that upon
high-temperature annealing, the channels implanted above a critical damage
density convert to a stable graphitic phase. These structures have significant
impact for different applications, such as compact ionizing radiation
detectors, dosimeters, bio-sensors and more generally diamond-based devices
with buried three-dimensional all-carbon electrodes
VX Hydrolysis by Human Serum Paraoxonase 1: A Comparison of Experimental and Computational Results
Human Serum paraoxonase 1 (HuPON1) is an enzyme that has been shown to hydrolyze a variety of chemicals including the nerve agent VX. While wildtype HuPON1 does not exhibit sufficient activity against VX to be used as an in vivo countermeasure, it has been suggested that increasing HuPON1's organophosphorous hydrolase activity by one or two orders of magnitude would make the enzyme suitable for this purpose. The binding interaction between HuPON1 and VX has recently been modeled, but the mechanism for VX hydrolysis is still unknown. In this study, we created a transition state model for VX hydrolysis (VXts) in water using quantum mechanical/molecular mechanical simulations, and docked the transition state model to 22 experimentally characterized HuPON1 variants using AutoDock Vina. The HuPON1-VXts complexes were grouped by reaction mechanism using a novel clustering procedure. The average Vina interaction energies for different clusters were compared to the experimentally determined activities of HuPON1 variants to determine which computational procedures best predict how well HuPON1 variants will hydrolyze VX. The analysis showed that only conformations which have the attacking hydroxyl group of VXts coordinated by the sidechain oxygen of D269 have a significant correlation with experimental results. The results from this study can be used for further characterization of how HuPON1 hydrolyzes VX and design of HuPON1 variants with increased activity against VX.United States. Defense Threat Reduction Agenc
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