218 research outputs found
Heat transfer and pressure drop correlations for laminar flow in an in-line and staggered array of circular cylinders
Enhanced heat transfer surfaces based on cylindrically shaped pin fins with wire diameters in the range of 100 µm were analyzed. The design is based on a high pin length to diameter ratio in the range of 20–100. Correlations for thermal and fluid dynamic characteristics of these fine wire structures are not available in literature. An in-line and staggered arrangement of pins were simulated for a variety of operational and geometrical conditions with a twodimensional computational thermal and fluid dynamics model. Correlations for Nusselt number and friction factor with respect to Reynolds number and geometry were derived thereby. Reynolds numbers based on the wire diameter are in the range of 3–60. The correlations for the Nusselt number and friction factor can predict 93% and 97% of the simulated data within ±10%
Gravitational Lensing as Signal and Noise in Lyman-alpha Forest Measurements
In Lyman-alpha forest measurements it is generally assumed that quasars are
mere background light sources which are uncorrelated with the forest.
Gravitational lensing of the quasars violates this assumption. This effect
leads to a measurement bias, but more interestingly it provides a valuable
signal. The lensing signal can be extracted by correlating quasar magnitudes
with the flux power spectrum and with the flux decrement. These correlations
will be challenging to measure but their detection provides a direct measure of
how features in the Lyman-alpha forest trace the underlying mass density field.
Observing them will test the fundamental hypothesis that fluctuations in the
forest are predominantly driven by fluctuations in mass, rather than in the
ionizing background, helium reionization or winds. We discuss ways to
disentangle the lensing signal from other sources of such correlations,
including dust, continuum and background residuals. The lensing-induced
measurement bias arises from sample selection: one preferentially collects
spectra of magnified quasars which are behind overdense regions. This
measurement bias is ~0.1-1% for the flux power spectrum, optical depth and the
flux probability distribution. Since the effect is systematic, quantities such
as the amplitude of the flux power spectrum averaged across scales should be
interpreted with care.Comment: 22 pages, 8 figures; v2: references added, discussion expanded,
matches PRD accepted versio
Acceleration and Classical Electromagnetic Radiation
Classical radiation from an accelerated charge is reviewed along with the
reciprocal topic of accelerated observers detecting radiation from a static
charge. This review commemerates Bahram Mashhoon's 60th birthday.Comment: To appear in Gen. Rel. Gra
Galaxy-Quasar correlations between APM galaxies and Hamburg-ESO QSOs
We detect angular galaxy-QSO cross-correlations between the APM Galaxy
Catalogue and a preliminary release (consisting of roughly half of the
anticipated final catalogue) of the Hamburg-ESO Catalogue of Bright QSOs as a
function of source QSO redshift using multiple cross-correlation estimators.
Each of the estimators yield very similar results, implying that the APM
catalogue and the Hamburg-ESO survey are both fair samples of the respective
true galaxy and QSO populations. Though the signal matches the expectations of
gravitational lensing qualitatively, the strength of the measured
cross-correlation signal is significantly greater than the CDM models of
lensing by large scale structure would suggest. This same disagreement between
models and observation has been found in several earlier studies. We estimate
our confidence in the correlation detections versus redshift by generating 1000
random realizations of the Hamburg-ESO QSO survey: We detect physical
associations between galaxies and low-redshift QSOs at 99% confidence and
detect lensing associations at roughly 95% confidence for QSOs with redshifts
between 0.6 and 1. Control cross-correlations between Galactic stars and QSOs
show no signal. Finally, the overdensities (underdensities) of galaxies near
QSO positions relative to those lying roughly 135 - 150 arcmin away are
uncorrelated with differences in Galactic extinction between the two regions,
implying that Galactic dust is not significantly affecting the QSO sample.Comment: 35 pages total, including 9 figures. Accepted by the Astrophysical
Journa
Cosmological Information from Quasar-Galaxy Correlations induced by Weak Lensing
The magnification bias of large-scale structures, combined with galaxy
biasing, leads to a cross-correlation of distant quasars with foreground
galaxies on angular scales of the order of arc minutes and larger. The
amplitude and angular shape of the cross-correlation function w_QG contain
information on cosmological parameters and the galaxy bias factor. While the
existence of this cross-correlation has firmly been established, existing data
did not allow an accurate measurement of w_QG yet, but wide area surveys like
the Sloan Digital Sky Survey now provide an ideal database for measuring it.
However, w_QG depends on several cosmological parameters and the galaxy bias
factor. We study in detail the sensitivity of w_QG to these parameters and
develop a strategy for using the data. We show that the parameter space can be
reduced to the bias factor b, Omega_0 and sigma_8, and compute the accuracy
with which these parameters can be deduced from SDSS data. Under reasonable
assumptions, it should be possible to reach relative accuracies of the order of
5%-15% for b, Omega_0, and sigma_8. This method is complementary to other
weak-lensing analyses based on cosmic shear.Comment: 11 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
Functional Glycosylation of Dystroglycan Is Crucial for Thymocyte Development in the Mouse
BACKGROUND: Alpha-dystroglycan (alpha-DG) is a cell surface receptor providing a molecular link between the extracellular matrix (ECM) and the actin-based cytoskeleton. During its biosynthesis, alpha-DG undergoes specific and unusual O-glycosylation crucial for its function as a high-affinity cellular receptor for ECM proteins. METHODOLOGY/PRINCIPAL FINDINGS: We report that expression of functionally glycosylated alpha-DG during thymic development is tightly regulated in developing T cells and largely confined to CD4(-)CD8(-) double negative (DN) thymocytes. Ablation of DG in T cells had no effect on proliferation, migration or effector function but did reduce the size of the thymus due to a significant loss in absolute numbers of thymocytes. While numbers of DN thymocytes appeared normal, a marked reduction in CD4(+)CD8(+) double positive (DP) thymocytes occurred. In the periphery mature naïve T cells deficient in DG showed both normal proliferation in response to allogeneic cells and normal migration, effector and memory T cell function when tested in acute infection of mice with either lymphocytic choriomeningitis virus (LCMV) or influenza virus. CONCLUSIONS/SIGNIFICANCE: Our study demonstrates that DG function is modulated by glycosylation during T cell development in vivo and that DG is essential for normal development and differentiation of T cells
Correlation between galaxies and QSO in the SDSS:na signal from gravitational lensing magnification?
We report a detection of galaxy-QSO cross-correlation w_GQ in the Sloan
Digital Sky Survey (SDSS) Early Data Release (EDR) over 0.2-30 arc-minute
scales. We cross-correlate galaxy samples of different mean depths r'=19-22
(z_G =0.15-0.35) with the main QSO population (i'<19.1) at mean z_Q=1.6. We
find significant positive correlation in all cases except for the faintest
QSOs, as expected if the signal were due to weak lensing magnification. The
amplitude of the signal on arc-minute scales is about 20%. This is a few times
larger than currently expected from weak lensing in LCDM but confirms, at a
higher significance, previous measurements by several groups. When compared to
the galaxy-galaxy correlation w_GG, a weak lensing interpretation indicates a
strong and steep non-linear amplitude for the underlaying matter fluctuations:
sigma=400 on scales of 0.2 Mpc/h, in contradiction with non-linear modeling of
LCDM fluctuations. We also detect a normalized skewness (galaxy-galaxy-QSO
correlation) of S_3=21 +/-6 at z=0.15 (S_3= 14 +/- 4 at z=0.35), which several
sigma low, as compared to LCDM expectations. These observational trends can be
reconciled with lensing in a flat Lambda universe with sigma_8=1, provided the
linear spectrum is steeper (n = 1) than in the LCDM model on these small
(cluster) scales. Under this interpretation, the galaxy distribution traces the
matter variance with an amplitude that is 100 times smaller: ie galaxies are
anti-bias with b=0.1 on small scales, increasing to b=1 at 10 Mpc/h.Comment: Final version accepted in ApJ. Minor changes and ref
Large Scale QSO-galaxy correlations for radio loud and optically selected QSO samples
We have studied the distribution of galaxies from the ROE/NRL
COSMOS/UKST catalogue around two samples of QSOs with similar redshift
distributions. The first sample is formed by 144 radio-loud QSOs from the
Parkes Catalogue, and the other contains 167 optically selected QSOs extracted
from the Large Bright Quasar Survey. It is found that there is a significance level excess of COSMOS/UKST galaxies around the PKS QSOs,
whereas there is a marginal defect of galaxies around the LBQS QSOs. When the
distribution of galaxies around both samples is compared, we found that there
is an overdensity of galaxies around the PKS sample with respect to the LBQS
sample anticorrelated with the distance from the QSOs at a significance
level. Although this result apparently agrees with the predictions of the
double magnification bias, it is difficult to explain by gravitational lensing
effects alone; dust in the foreground galaxies and selection effects in the
detection of LBQS QSOs should be taken into account. It has been established
that the lines of sight to PKS flat-spectrum QSOs go through significatively
higher foreground galaxy densities than the directions to LBQS quasars, what
may be partially related with the reported reddening of PKS QSOs.Comment: 18 pages, 8 figures, AASTeX, accepted for publication in Ap
Thermal Degradation of Adsorbed Bottle-Brush Macromolecules: Molecular Dynamics Simulation
The scission kinetics of bottle-brush molecules in solution and on an
adhesive substrate is modeled by means of Molecular Dynamics simulation with
Langevin thermostat. Our macromolecules comprise a long flexible polymer
backbone with segments, consisting of breakable bonds, along with two side
chains of length , tethered to each segment of the backbone. In agreement
with recent experiments and theoretical predictions, we find that bond cleavage
is significantly enhanced on a strongly attractive substrate even though the
chemical nature of the bonds remains thereby unchanged.
We find that the mean bond life time decreases upon adsorption by
more than an order of magnitude even for brush molecules with comparatively
short side chains $N=1 \div 4$. The distribution of scission probability along
the bonds of the backbone is found to be rather sensitive regarding the
interplay between length and grafting density of side chains. The life time
declines with growing contour length as ,
and with side chain length as . The probability
distribution of fragment lengths at different times agrees well with
experimental observations. The variation of the mean length of the
fragments with elapsed time confirms the notion of the thermal degradation
process as a first order reaction.Comment: 15 pages, 7 figure
Understanding how the V(D)J recombinase catalyzes transesterification: distinctions between DNA cleavage and transposition
The Rag1 and Rag2 proteins initiate V(D)J recombination by introducing site-specific DNA double-strand breaks. Cleavage occurs by nicking one DNA strand, followed by a one-step transesterification reaction that forms a DNA hairpin structure. A similar reaction allows Rag transposition, in which the 3′-OH groups produced by Rag cleavage are joined to target DNA. The Rag1 active site DDE triad clearly plays a catalytic role in both cleavage and transposition, but no other residues in Rag1 responsible for transesterification have been identified. Furthermore, although Rag2 is essential for both cleavage and transposition, the nature of its involvement is unknown. Here, we identify basic amino acids in the catalytic core of Rag1 specifically important for transesterification. We also show that some Rag1 mutants with severe defects in hairpin formation nonetheless catalyze substantial levels of transposition. Lastly, we show that a catalytically defective Rag2 mutant is impaired in target capture and displays a novel form of coding flank sensitivity. These findings provide the first identification of components of Rag1 that are specifically required for transesterification and suggest an unexpected role for Rag2 in DNA cleavage and transposition
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