4,091 research outputs found
Many-body theory of gamma spectra from positron-atom annihilation
A many-body theory approach to the calculation of gamma spectra of positron
annihilation on many-electron atoms is developed. We evaluate the first-order
correlation correction to the annihilation vertex and perform numerical
calculations for the noble gas atoms. Extrapolation with respect to the maximal
orbital momentum of the intermediate electron and positron states is used to
achieve convergence. The inclusion of correlation corrections improves
agreement with experimental gamma spectra.Comment: 25 pages, 9 figures, submitted to J. Phys.
Cabinet Gorge Dam Spillway Modifications for TDG Abatement - Design Evolution and Field Performance
Avista is implementing spillway modifications to reduce TDG supersaturation downstream of Cabinet Gorge Dam.The key feature of the modifications is the addition of roughness elements, similar to supercavitating baffle blocks,to break up the spillway jet thereby reducing the depth of plunge and TDG supersaturation. The work is progressing in a step-wise manner. A single bay was modified in 2012 and field tested in 2013. Following the initial field tests, aCFD model was developed to aid in design refinements for the prototype and to improve the design prior to modification of subsequent spillway bays. The prototype demonstrated that spillway modifications are an effective method to reduce TDG downstream of a spillway discharging freely into a deep plunge pool. The CFD model has allowed the design to be simplified while maintaining the plunge depth improvements of the initial prototype and reducing the effect of the modifications on spillway capacity. This paper presents the prototype design and the design that will be implemented for the next two bays, summarizes the results from the prototype field tests, and describes the CFD model and results
The Global Star Formation Rate from the 1.4 GHz Luminosity Function
The decimetric luminosity of many galaxies appears to be dominated by
synchrotron emission excited by supernova explosions. Simple models suggest
that the luminosity is directly proportional to the rate of supernova
explosions of massive stars averaged over the past 30 Myr. The proportionality
may be used together with models of the evolving 1.4 GHz luminosity function to
estimate the global star formation rate density in the era z < 1. The local
value is estimated to be 0.026 solar masses per year per cubic megaparsec, some
50% larger than the value inferred from the Halpha luminosity density. The
value at z ~ 1 is found to be 0.30 solar masses per year per cubic megaparsec.
The 10-fold increase in star formation rate density is consistent with the
increase inferred from mm-wave, far-infrared, ultra-violet and Halpha
observations.Comment: 10 pages, 2 figures, Astrophysical Journal Letters (in press); new PS
version has improved figure placemen
Spectacular Shells in the Host Galaxy of the QSO MC2 1635+119
We present deep HST/ACS images and Keck spectroscopy of MC2 1635+119, a QSO
hosted by a galaxy previously classified as an undisturbed elliptical. Our new
images reveal dramatic shell structure indicative of a merger event in the
relatively recent past. The brightest shells in the central regions of the host
are distributed alternately in radius, with at least two distinct shells on one
side of the nucleus and three on the other, out to a distance of ~13 kpc. The
light within the five shells comprises ~6% of the total galaxy light. Lower
surface brightness ripples or tails and other debris extend out to a distance
of ~65 kpc. A simple N-body model for a merger reproduces the inner shell
structure and gives an estimate for the age of the merger between ~30 Myr and
~1.7 Gyr, depending on a range of reasonable assumptions. While the inner shell
structure is suggestive of a minor merger, the total light contribution from
the shells and extended structures are more indicative of a major merger. The
spectrum of the host galaxy is dominated by a population of intermediate age
(~1.4 Gyr), indicating a strong starburst episode that may have occurred at the
time of the merger event. We speculate that the current QSO activity may have
been triggered in the recent past by either a minor merger, or by debris from
an older (~Gyr) major merger that is currently ``raining'' back into the
central regions of the merger remnant.Comment: 14 pages, 5 figures. Accepted for publication in the Astrophysical
Journa
Avoiding spurious breaks in binned luminosity functions
We show that using either the method of Page & Carrera or the well-known
method to construct the binned luminosity function (LF) of a flux
limited sample of Active Galactic Nuclei (AGN) can produce an artificial
flattening (or steepening in the case of negative evolution) of the binned LF
for bins intersected by the flux cutoff of the sample. This effect is more
pronounced for samples with steep and strongly evolving parent LFs but is still
present even for non-evolving LFs. As a result of this distortion of the true
LF, fitting a model LF to binned data may lead to errors in the estimation of
the parameters and may even prompt the erroneous use of broken power law
functions. We compute the expected positions of apparent breaks in the binned
LF. We show that these spurious breaks in the binned LFs can be avoided if the
binning is done in the flux--redshift plane instead of the typically used
luminosity--redshift plane. Binning in the flux--redshift plane can be used in
conjunction with the binning in the luminosity--redshift plane to test for real
breaks in the binned LFs and to identify the features that are the result of
binning biases. We illustrate this effect for most typical forms of luminosity
dependence and redshift evolution and show how the proposed method helps
address this problem. We also apply this method to the MOJAVE AGN sample and
show that it eliminates an apparent break in the binned LF.Comment: 7 pages and 7 figures. Accepted for publication in Ap
Histopathologic parameters as predictors of response to endoscopic sinus surgery in nonallergic patients with chronic rhinosinusitis
OBJECTIVE: To estimate the predictable value of histopathologic parameters in chronic rhinosinusitis (CRS) for response to endoscopic sinus surgery (ESS). - - - - - STUDY DESIGN: Symptomatology was rated in 100 patients prior to as well as 12 and 24 months after surgery. Specimens taken during the procedure were examined and scored for goblet cells, subepithelial thickening, mast cells, and eosinophils. Multiple regression analysis was performed to predict the total score of subjective symptoms before treatment by histopathologic parameters. The correlation between histopathologic parameters and postoperative symptoms was then evaluated. - - - - - RESULTS: Goblet cells were the best predictor correlating with 5 symptoms. Subepithelial thickening correlated with 4 symptoms. Mast cell infiltration correlated with 3 symptoms. Eosinophilic infiltration correlated with only one symptom (P<0.05). - - - - - CONCLUSION: Certain histopathologic parameters in CRS are predictive of favorable response to ESS. - - - - - SIGNIFICANCE: Pathologic evaluation may help the ENT surgeon to predict the persistence of certain CRS symptoms after ESS, even in patients at low risk for surgical failure. - - - - - EBM rating: C-4
Linking stellar mass and star formation in Spitzer/MIPS 24 micron galaxies
We present deep Ks<21.5 (Vega) identifications, redshifts and stellar masses
for most of the sources composing the bulk of the 24 micron background in the
GOODS/CDFS. Our identified sample consists of 747 Spitzer/MIPS 24 micron
objects, and includes ~94% of all the 24 micron sources in the GOODS-South
field which have fluxes Snu(24)>83 microJy (the 80% completeness limit of the
Spitzer/GTO 24 micron catalog). 36% of our galaxies have spectroscopic
redshifts (mostly at z<1.5) and the remaining ones have photometric redshifts
of very good quality, with a median of |dz|=|zspec-zphot|/(1+zspec)=0.02. We
find that MIPS 24 micron galaxies span the redshift range z~0-4, and that a
substantial fraction (28%) lie at high redshifts z>1.5. We determine the
existence of a bump in the redshift distribution at z~1.9, indicating the
presence of a significant population of galaxies with PAH emission at these
redshifts. Massive (M>10^11 Msun) star-forming galaxies at redshifts 2<z<3 are
characterized by very high star-formation rates (SFR>500 Msun/yr), and some of
them are able to construct a mass of 10^10-10^11 Msun in a single burst
lifetime (~0.01-0.1 Gyr). At lower redshifts z<2, massive star-forming galaxies
are also present, but appear to be building their stars on long timescales,
either quiescently or in multiple modest burst-like episodes. At redshifts
z~1-2, the ability of the burst-like mode to produce entire galaxies in a
single event is limited to some lower (M<7x10^10 Msun) mass systems, and it is
basically negligible at z<1. Our results support a scenario where
star-formation activity is differential with assembled stellar mass and
redshift, and where the relative importance of the burst-like mode proceeds in
a down-sizing way from high to low redshifts. (abridged)Comment: Accepted for publication in the ApJ. 19 pages, 10 figures. Uses
emulateap
Correcting for Distortions due to Ionization in the STAR TPC
Physics goals of the STAR Experiment at RHIC in recent (and future) years
drive the need to operate the STAR TPC at ever higher luminosities, leading to
increased ionization levels in the TPC gas. The resulting ionic space charge
introduces field distortions in the detector which impact tracking performance.
Further complications arise from ionic charge leakage into the main TPC volume
from the high gain anode region. STAR has implemented corrections for these
distortions based on measures of luminosity, which we present here.
Additionally, we highlight a novel approach to applying the corrections on an
event-by-event basis applicable in conditions of rapidly varying ionization
sources.Comment: 6 pages, 7 figures, proceedings of the Workshop on Tracking in High
Multiplicity Environments (TIME 05) in Zurich, Switzerland, submitted to
Nucl. Instr. and Meth.
Optical/Near-Infrared Imaging of Infrared-Excess Palomar-Green QSOs
Ground-based high spatial-resolution (FWHM < 0.3-0.8") optical and
near-infrared imaging (0.4-2.2um) is presented for a complete sample of
optically selected Palomar-Green QSOs with far-infrared excesses at least as
great as those of "warm" AGN-like ultraluminous infrared galaxies
(L_ir/L_big-blue-bump > 0.46). In all cases, the host galaxies of the QSOs were
detected and most have discernable two-dimensional structure. The QSO host
galaxies and the QSO nuclei are similar in magnitude at H-band. H-band
luminosities of the hosts range from 0.5-7.5 L* with a mean of 2.3 L*, and are
consistent with those found in ULIGs. Both the QSO nuclei and the host galaxies
have near-infrared excesses, which may be the result of dust associated with
the nucleus and of recent dusty star formation in the host. These results
suggest that some, but not all, optically-selected QSOs may have evolved from
an infrared-active state triggered by the merger of two similarly-sized L*
galaxies, in a manner similar to that of the ultraluminous infrared galaxies.Comment: Aastex format, 38 pages, 4 tables, 10 figures. Higher quality figures
are available in JPG forma
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