291 research outputs found
Thermally activated breakdown in a simple polymer model
We consider the thermally activated fragmentation of a homopolymer chain. In
our simple model the dynamics of the intact chain is a Rouse one until a bond
breaks and bond breakdown is considered as a first passage problem over a
barrier to an absorbing boundary. Using the framework of the Wilemski-Fixman
approximation we calculate activation times of individual bonds for free and
grafted chains. We show that these times crucially depend on the length of the
chain and the location of the bond yielding a minimum at the free chain ends.
Theoretical findings are qualitatively confirmed by Brownian dynamics
simulations
Lensing Corrections to Features in the Angular Two-Point Correlation Function and Power Spectrum
It is well known that magnification bias, the modulation of galaxy or quasar
source counts by gravitational lensing, can change the observed angular
correlation function. We investigate magnification-induced changes to the shape
of the observed correlation function w(\theta) and the angular power spectrum
C_{\ell}, paying special attention to the matter-radiation equality peak and
the baryon wiggles. Lensing mixes the correlation function of the source
galaxies with the matter correlation at the lower redshifts of the lenses.
Since the lenses probe structure nearer to the observer, the angular scale
dependence of the lensing terms is different from that of the sources, thus the
observed correlation function is distorted. We quantify how the lensing
corrections depend on the width of the selection function, the galaxy bias b,
and the number count slope s. The correction increases with redshift and larger
corrections are present for sources with steep number count slopes and/or broad
redshift distributions. The most drastic changes to C_{\ell} occur for
measurements at z >~1.5 and \ell <~ 100. For the source distributions we
consider, magnification bias can shift the matter-radiation equality scale by
1-6% at z ~ 1.5 and by z ~ 3.5 the shift can be as large as 30%. The baryon
bump in \theta^2w(\theta) is shifted by <~ 1% and the width is typically
increased by ~10%. Shifts of >~ 0.5% and broadening of >~ 20% occur only for
very broad selection functions and/or galaxies with (5s-2)/b>~2. However, near
the baryon bump the magnification correction is not constant but a gently
varying function which depends on the source population. Depending on how the
w(\theta) data is fitted, this correction may need to be accounted for when
using the baryon acoustic scale for precision cosmology.Comment: v2: 8 pages, 5 figures, text and figures condensed, references adde
Large-Scale QSO-Galaxy Correlations and Weak Lensing
Several recent studies show that bright, intermediate and high redshift
optically and radio selected QSOs are positively correlated with nearby
galaxies on a range of angular scales up to a degree. Obscuration by unevenly
distributed Galactic dust can be ruled out as the cause, leaving weak
statistical lensing as the physical process responsible. However the amplitude
of correlations on < 1 degree scales is at least a factor of a few larger than
lensing model predictions. A possible way to reconcile the observations and
theory is to revise the weak lensing formalism. We extend the standard lensing
formulation to include the next higher order term (second order) in the
geodesic equation of motion for photons. We derive relevant equations
applicable in the weak lensing regime, and discuss qualitative properties of
the updated formulation. We then perform numerical integrations of the revised
equation and study the effect of the extra term using two different types of
cosmic mass density fluctuations. We find that nearby large-scale coherent
structures increase the amplitude of the predicted lensing-induced correlations
between QSOs and foreground galaxies by ~ 10% (not a factor of several required
by observations), while the redshift of the optimal, i.e. `most correlated'
structures is moved closer to the observer compared to what is predicted using
the standard lensing equation.Comment: extended Section 2; 20 pages, including 4 figures, accepted to Ap
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
Resonance-like phenomena of the mobility of a chain of nonlinear coupled oscillators in a two-dimensional periodic potential
We study the Langevin dynamics of a two-dimensional discrete oscillator chain
absorbed on a periodic substrate and subjected to an external localized point
force. Going beyond the commonly used harmonic bead-spring model, we consider a
nonlinear Morse interaction between the next-nearest-neighbors. We focus
interest on the activation of directed motion instigated by thermal
fluctuations and the localized point force. In this context the local
transition states are identified and the corresponding activation energies are
calculated. As a novel feature it is found that the transport of the chain in
point force direction is determined by stepwise escapes of a single unit or
segments of the chain due to the existence of multiple locally stable
attractors. The non-vanishing net current of the chain is quantitatively
assessed by the value of the mobility of the center of mass. It turns out that
the latter as a function of the ratio of the competing length scales of the
system, that is the period of the substrate potential and the equilibrium
distance between two chain units, shows a resonance behavior. More precisely
there exist a set of optimal parameter values maximizing the mobility.
Interestingly, the phenomenon of negative resistance is found, i.e. the
mobility possesses a minimum at a finite value of the strength of the thermal
fluctuations for a given overcritical external driving force.Comment: 25 pages, 12 figure
Automated optical identification of a large complete northern hemisphere sample of flat spectrum radio sources with S_6cm > 200 mJy
This paper describes the automated optical APM identification of radio
sources from the Jodrell Bank - VLA Astrometric Survey (JVAS), as used for the
search for distant radio-loud quasars. The sample has been used to investigate
possible relations between optical and radio properties of flat spectrum radio
sources. From the 915 sources in the sample, 756 have an optical APM
identification at a red (e) and/or blue (o) plate,resulting in an
identification fraction of 83% with a completeness and reliability of 98% and
99% respectively. About 20% are optically identified with extended APM objects
on the red plates, e.g. galaxies. However the distinction between galaxies and
quasars can not be done properly near the magnitude limit of the POSS-I plates.
The identification fraction appears to decrease from >90% for sources with a 5
GHz flux density of >1 Jy, to <80% for sources at 0.2 Jy. The identification
fraction, in particular that for unresolved quasars, is found to be lower for
sources with steeper radio spectra. In agreement with previous studies, we find
that the quasars at low radio flux density levels also tend to have fainter
optical magnitudes, although there is a large spread. In addition, objects with
a steep radio-to-optical spectral index are found to be mainly highly polarised
quasars, supporting the idea that in these objects the polarised synchrotron
component is more prominent. It is shown that the large spread in
radio-to-optical spectral index is possibly caused by source to source
variations in the Doppler boosting of the synchrotron component [Abridged].Comment: LaTex, 17 pages, 5 gif figures, 4 tables. Accepted for publication in
MNRAS. High resolution figures can be found at http://www.roe.ac.uk/~ignas
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
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
The Pearson-Readhead Survey of Compact Extragalactic Radio Sources From Space. II. Analysis of Source Properties
We have performed a multi-dimensional correlation analysis on the observed
properties of a statistically complete core-selected sample of compact
radio-loud active galactic nuclei, based on data from the VLBI Space Observing
Programme (Paper I) and previously published studies. Our sample is drawn from
the well-studied Pearson-Readhead (PR) survey, and is ideally suited for
investigating the general effects of relativistic beaming in compact radio
sources. In addition to confirming many previously known correlations, we have
discovered several new trends that lend additional support to the beaming
model. These trends suggest that the most highly beamed sources in
core-selected samples tend to have a) high optical polarizations; b) large
pc/kpc-scale jet misalignments; c) prominent VLBI core components; d)
one-sided, core, or halo radio morphology on kiloparsec scales; e) narrow
emission line equivalent widths; and f) a strong tendency for intraday
variability at radio wavelengths. We have used higher resolution space and
ground-based VLBI maps to confirm the bi-modality of the jet misalignment
distribution for the PR survey, and find that the sources with aligned parsec-
and kiloparsec-scale jets generally have arcsecond-scale radio emission on both
sides of the core. The aligned sources also have broader emission line widths.
We find evidence that the BL Lacertae objects in the PR survey are all highly
beamed, and have very similar properties to the high-optically polarized
quasars, with the exception of smaller redshifts. A cluster analysis on our
data shows that after partialing out the effects of redshift, the luminosities
of our sample objects in various wave bands are generally well-correlated with
each other, but not with other source properties.Comment: 21 pages, 10 figures. Accepted for publication in the Astrophysical
Journal. Part I can be found at astro-ph/010227
Multi-frequency VLBA observations of compact sources from the Peacock & Wall catalogue
VLBA observations are presented for 6 compact radio sources selected from the
Peacock & Wall catalogue. From the new morphological and spectral information 2
objects that in the Peacock and Wall catalogue are flat spectrum (alpha < 0.5)
sources, appear to be double sided objects with linear sizes of the order of
one kpc. Three are core-jet sources and the last one is still an ``enigmatic''
object. These data complete the sample of small double compact sources in the
Peacock & Wall catalogue and the complete list is given.Comment: 11 pages, 14 figures, aa.cls Accepted by A&
- …