3,986 research outputs found
High-Redshift SDSS Quasars with Weak Emission Lines
We identify a sample of 74 high-redshift quasars (z>3) with weak emission
lines from the Fifth Data Release of the Sloan Digital Sky Survey and present
infrared, optical, and radio observations of a subsample of four objects at
z>4. These weak emission-line quasars (WLQs) constitute a prominent tail of the
Lya+NV equivalent width distribution, and we compare them to quasars with more
typical emission-line properties and to low-redshift active galactic nuclei
with weak/absent emission lines, namely BL Lac objects. We find that WLQs
exhibit hot (T~1000 K) thermal dust emission and have rest-frame 0.1-5 micron
spectral energy distributions that are quite similar to those of normal
quasars. The variability, polarization, and radio properties of WLQs are also
different from those of BL Lacs, making continuum boosting by a relativistic
jet an unlikely physical interpretation. The most probable scenario for WLQs
involves broad-line region properties that are physically distinct from those
of normal quasars.Comment: Updated to match version published in ApJ. 20 pages, 12 figure
The interface between silicon and a high-k oxide
The ability to follow Moore's Law has been the basis of the tremendous
success of the semiconductor industry in the past decades. To date, the
greatest challenge for device scaling is the required replacement of silicon
dioxide-based gate oxides by high-k oxides in transistors. Around 2010 high-k
oxides are required to have an atomically defined interface with silicon
without any interfacial SiO2 layer. The first clean interface between silicon
and a high-K oxide has been demonstrated by McKee et al. Nevertheless, the
interfacial structure is still under debate. Here we report on first-principles
calculations of the formation of the interface between silicon and SrTiO3 and
its atomic structure. Based on insights into how the chemical environment
affects the interface, a way to engineer seemingly intangible electrical
properties to meet technological requirements is outlined. The interface
structure and its chemistry provide guidance for the selection process of other
high-k gate oxides and for controlling their growth. Our study also shows that
atomic control of the interfacial structure can dramatically improve the
electronic properties of the interface. The interface presented here serves as
a model for a variety of other interfaces between high-k oxides and silicon.Comment: 10 pages, 2 figures (one color
Stochastic Acceleration in the Western Hotspot of Pictor A
Chandra's high resolution observations of radio galaxies require a revisit of
the relevant electron acceleration processes. Although the diffusive shock
particle acceleration model may explain spectra of spatially unresolved
sources, it encounters difficulties in explaining the structure and spectral
properties of recently discovered Chandra X-ray features in several low-power
radio sources. We argue that these observations strongly suggest stochastic
electron acceleration by magnetized turbulence, and show that the simplest
stochastic particle acceleration model with energy independent acceleration and
escape timescales can overcome most of these difficulties. We use the bright
core of the western hotspot of Pictor A as an example to demonstrate the model
characteristics, which may be tested with high energy observations.Comment: 12 pages, 2 figures. Accepted by ApJ Letter
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Design of the Subpopulations and Intermediate Outcome Measures in COPD (SPIROMICS) AIR Study.
IntroductionPopulation-based epidemiological evidence suggests that exposure to ambient air pollutants increases hospitalisations and mortality from chronic obstructive pulmonary disease (COPD), but less is known about the impact of exposure to air pollutants on patient-reported outcomes, morbidity and progression of COPD.Methods and analysisThe Subpopulations and Intermediate Outcome Measures in COPD (SPIROMICS) Air Pollution Study (SPIROMICS AIR) was initiated in 2013 to investigate the relation between individual-level estimates of short-term and long-term air pollution exposures, day-to-day symptom variability and disease progression in individuals with COPD. SPIROMICS AIR builds on a multicentre study of smokers with COPD, supplementing it with state-of-the-art air pollution exposure assessments of fine particulate matter, oxides of nitrogen, ozone, sulfur dioxide and black carbon. In the parent study, approximately 3000 smokers with and without airflow obstruction are being followed for up to 3 years for the identification of intermediate biomarkers which predict disease progression. Subcohorts undergo daily symptom monitoring using comprehensive daily diaries. The air monitoring and modelling methods employed in SPIROMICS AIR will provide estimates of individual exposure that incorporate residence-specific infiltration characteristics and participant-specific time-activity patterns. The overarching study aim is to understand the health effects of short-term and long-term exposures to air pollution on COPD morbidity, including exacerbation risk, patient-reported outcomes and disease progression.Ethics and disseminationThe institutional review boards of all the participating institutions approved the study protocols. The results of the trial will be presented at national and international meetings and published in peer-reviewed journals
SWISS MADE: Standardized WithIn Class Sum of Squares to Evaluate Methodologies and Dataset Elements
Contemporary high dimensional biological assays, such as mRNA expression microarrays, regularly involve multiple data processing steps, such as experimental processing, computational processing, sample selection, or feature selection (i.e. gene selection), prior to deriving any biological conclusions. These steps can dramatically change the interpretation of an experiment. Evaluation of processing steps has received limited attention in the literature. It is not straightforward to evaluate different processing methods and investigators are often unsure of the best method. We present a simple statistical tool, Standardized WithIn class Sum of Squares (SWISS), that allows investigators to compare alternate data processing methods, such as different experimental methods, normalizations, or technologies, on a dataset in terms of how well they cluster a priori biological classes. SWISS uses Euclidean distance to determine which method does a better job of clustering the data elements based on a priori classifications. We apply SWISS to three different gene expression applications. The first application uses four different datasets to compare different experimental methods, normalizations, and gene sets. The second application, using data from the MicroArray Quality Control (MAQC) project, compares different microarray platforms. The third application compares different technologies: a single Agilent two-color microarray versus one lane of RNA-Seq. These applications give an indication of the variety of problems that SWISS can be helpful in solving. The SWISS analysis of one-color versus two-color microarrays provides investigators who use two-color arrays the opportunity to review their results in light of a single-channel analysis, with all of the associated benefits offered by this design. Analysis of the MACQ data shows differential intersite reproducibility by array platform. SWISS also shows that one lane of RNA-Seq clusters data by biological phenotypes as well as a single Agilent two-color microarray
A Deep HST Search for Escaping Lyman Continuum Flux at z~1.3: Evidence for an Evolving Ionizing Emissivity
We have obtained deep Hubble Space Telescope far-UV images of 15 starburst
galaxies at z~1.3 in the GOODS fields to search for escaping Lyman continuum
photons. These are the deepest far-UV images m_{AB}=28.7, 3\sigma, 1" diameter)
over this large an area (4.83 arcmin^2) and provide the best escape fraction
constraints for any galaxy at any redshift. We do not detect any individual
galaxies, with 3\sigma limits to the Lyman Continuum (~700 \AA) flux 50--149
times fainter (in f_nu) than the rest-frame UV (1500 \AA) continuum fluxes.
Correcting for the mean IGM attenuation (factor ~2), as well as an intrinsic
stellar Lyman Break (~3), these limits translate to relative escape fraction
limits of f_{esc,rel}<[0.03,0.21]. The stacked limit is
f_{esc,rel}(3\sigma)<0.02. We use a Monte Carlo simulation to properly account
for the expected distribution of IGM opacities. When including constraints from
previous surveys at z~1.3 we find that, at the 95% confidence level, no more
than 8% of star--forming galaxies at z~1.3 can have relative escape fractions
greater than 0.50. Alternatively, if the majority of galaxies have low, but
non-zero, escaping Lyman Continuum, the escape fraction can not be more than
0.04. Both the stacked limits, and the limits from the Monte Carlo simulation
suggest that the average ionizing emissivity (relative to non-ionizing UV
emissivity) at z~1.3 is significantly lower than has been observed in Lyman
Break Galaxies (LBGs) at z~3. If the ionizing emissivity of star-forming
galaxies is in fact increasing with redshift, it would help to explain the high
photoionization rates seen in the IGM at z>4 and reionization of the
intergalactic medium at z>6. [Abridged]Comment: Submitted to ApJ (Nov. 6) Comments Welcome. 11 pages, 8 figure
Colors of 2625 Quasars at 0<z<5 Measured in the Sloan Digital Sky Survey Photometric System
We present an empirical investigation of the colors of quasars in the Sloan
Digital Sky Survey (SDSS) photometric system. The sample studied includes 2625
quasars with SDSS photometry. The quasars are distributed in a 2.5 degree wide
stripe centered on the Celestial Equator covering square degrees.
Positions and SDSS magnitudes are given for the 898 quasars known prior to SDSS
spectroscopic commissioning. New SDSS quasars represent an increase of over
200% in the number of known quasars in this area of the sky. The ensemble
average of the observed colors of quasars in the SDSS passbands are well
represented by a power-law continuum with (). However, the contributions of the bump
and other strong emission lines have a significant effect upon the colors. The
color-redshift relation exhibits considerable structure, which may be of use in
determining photometric redshifts for quasars. The range of colors can be
accounted for by a range in the optical spectral index with a distribution
(95% confidence), but there is a red tail in the
distribution. This tail may be a sign of internal reddening. Finally, we show
that there is a continuum of properties between quasars and Seyfert galaxies
and we test the validity of the traditional division between the two classes of
AGN.Comment: 66 pages, 15 figures (3 color), accepted by A
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