4,190 research outputs found
Mg II Absorption Systems in SDSS QSO Spectra
We present the results of a MgII absorption-line survey using QSO spectra
from the SDSS EDR. Over 1,300 doublets with rest equivalent widths greater than
0.3\AA and redshifts were identified and measured. We
find that the rest equivalent width ()
distribution is described very well by an exponential function , with
and \AA. Previously reported power law
fits drastically over-predict the number of strong lines. Extrapolating our
exponential fit under-predicts the number of \AA systems,
indicating a transition in near \AA. A combination of
two exponentials reproduces the observed distribution well, suggesting that
MgII absorbers are the superposition of at least two physically distinct
populations of absorbing clouds. We also derive a new redshift parameterization
for the number density of \AA lines:
and \AA. We find that the distribution steepens with decreasing redshift,
with decreasing from \AA at to \AA at
. The incidence of moderately strong MgII lines does not
show evidence for evolution with redshift. However, lines stronger than
\AA show a decrease relative to the no-evolution prediction with
decreasing redshift for . The evolution is stronger for
increasingly stronger lines. Since in saturated absorption lines is an
indicator of the velocity spread of the absorbing clouds, we interpret this as
an evolution in the kinematic properties of galaxies from moderate to low z.Comment: 50 pages, 26 figures, accepted for publication in Ap
Star Formation, Supernovae Feedback and the Angular Momentum Problem in Numerical CDM Cosmogony: Half Way There?
We present a smoothed particle hydrodynamic (SPH) simulation that reproduces
a galaxy that is a moderate facsimile of those observed. The primary failing
point of previous simulations of disk formation, namely excessive transport of
angular momentum from gas to dark matter, is ameliorated by the inclusion of a
supernova feedback algorithm that allows energy to persist in the model ISM for
a period corresponding to the lifetime of stellar associations. The inclusion
of feedback leads to a disk at a redshift , with a specific angular
momentum content within 10% of the value required to fit observations. An
exponential fit to the disk baryon surface density gives a scale length within
17% of the theoretical value. Runs without feedback, with or without star
formation, exhibit the drastic angular momentum transport observed elsewhere.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter
Longitudinal thalamic white and gray matter changes associated with visual hallucinations in Parkinson’s disease
Objective: Visual hallucinations are common in
Parkinson’s disease (PD) and associated with worse
outcomes. Large-scale network imbalance is seen in
PD-associated hallucinations, but mechanisms remain
unclear. As the thalamus is critical in controlling cortical
networks, structural thalamic changes could underlie
network dysfunction in PD hallucinations.
Methods: We used whole-brain fixel-based analysis
and cortical thickness measures to examine longitudinal
white and grey matter changes in 76 patients with PD
(15 hallucinators, 61 non-hallucinators) and 26 controls
at baseline, and after 18 months. We compared white
matter and cortical thickness, adjusting for age, gender,
time-between-scans and intracranial volume. To assess
thalamic changes, we extracted volumes for 50 thalamic
subnuclei (25 each hemisphere) and mean fibre crosssection (FC) for white matter tracts originating in each
subnucleus and examined longitudinal change in PDhallucinators versus non-hallucinators.
Results: PD hallucinators showed white matter changes
within the corpus callosum at baseline and extensive
posterior tract involvement over time. Less extensive
cortical thickness changes were only seen after followup. White matter connections from the right medial
mediodorsal magnocellular thalamic nucleus showed
reduced FC in PD hallucinators at baseline followed
by volume reductions longitudinally. After follow-up,
almost all thalamic subnuclei showed tract losses in PD
hallucinators compared with non-hallucinators.
Interpretation: PD hallucinators show white matter
loss particularly in posterior connections and in thalamic
nuclei, over time with relatively preserved cortical
thickness. The right medial mediodorsal thalamic
nucleus shows both connectivity and volume loss in
PD hallucinations. Our findings provide mechanistic
insights into the drivers of network imbalance in PD
hallucinations and potential therapeutic targets
Chemoselective cysteine or disulfide modification via single atom substitution in chloromethyl acryl reagents
Visual dysfunction predicts cognitive impairment and white matter degeneration in Parkinson's disease
Visual dysfunction predicts dementia in Parkinsons disease (PD), but whether this translates to structural change is not known. We aimed to identify longitudinal white matter changes in patients with Parkinsons disease and low visual function and also in those who developed mild cognitive impairment (MCI). We used fixel-based analysis to examine longitudinal white matter change in PD. Diffusion MRI and clinical assessments were performed in 77 patients at baseline (22 low visual function /55 intact vision; and 13 MCI, 13 MCI converters /51 normal cognition) and 25 controls and again after 18 months. We compared micro-structural changes in fibre density, macro-structural changes in fibre bundle cross-section (FC) and combined fibre density and cross-section across white matter, adjusting for age, gender and intracranial volume. Patients with Parkinsons and visual dysfunction showed worse cognitive performance at follow up and were more likely to develop MCI compared with those with normal vision (p=0.008). Parkinsons with poor visual function showed diffuse micro-structural and macro-structural changes at baseline, whereas those with MCI showed fewer baseline changes. At follow-up, Parkinsons with low visual function showed widespread macrostructural changes, involving the fronto-occipital fasciculi, external capsules, and middle cerebellar peduncles bilaterally. No longitudinal change was seen in baseline MCI or in MCI converters, even when the two groups were combined. Parkinsons patients with poor visual function show increased white matter damage over time, providing further evidence for visual function as a marker of imminent cognitive decline
Fibre-specific white matter reductions in Parkinson’s hallucinations and visual dysfunction
Objective: To investigate the microstructural and macrostructural white matter changes that accompany visual hallucinations and low visual performance in Parkinson’s disease, a risk factor for Parkinson’s dementia.
Methods: We performed fixel-based analysis, a novel technique that provides metrics of specific fibre-bundle populations within a voxel (or fixel). Diffusion MRI data was acquired from patients with Parkinson’s disease (n=105, of which 34 low visual performers and 19 hallucinators) and age-matched controls (n=35). We used whole brain fixel-based analysis to compare micro-structural differences in fibre density (FD), macro-structural differences in fibre bundle cross-section (FC) and the combined fibre density and cross-section metric (FDC) across all white matter fixels. We then performed a tract of interest analysis comparing the most sensitive FDC metric across 11 tracts within the visual system.
Results: Patients with Parkinson’s disease hallucinations exhibited macrostructural changes (reduced FC) within the splenium of the corpus callosum and the left posterior thalamic radiation compared to patients without hallucinations. Whilst there were no significant changes in FD, we found large reductions in the combined FDC metric in Parkinson’s hallucinators within the splenium (>50% reduction compared to non-hallucinators). Patients with Parkinson’s disease and low visual performance showed widespread microstructural and macrostructural changes within the genu and splenium of the corpus callosum, bilateral posterior thalamic radiations and the left inferior fronto-occipital fasciculus.
Conclusions: We demonstrate specific white matter tract degeneration affecting posterior thalamic tracts in patients with Parkinson’s disease with hallucinations and low visual performance, providing direct mechanistic support for attentional models of visual hallucinations
On compression of Bruhat-Tits buildings
We obtain an analog of the compression of angles theorem in symmetric spaces
for Bruhat--Tits buildings of the type .
More precisely, consider a -adic linear space and the set of
all lattices in . The complex distance in is a complete system of
invariants of a pair of points of under the action of the complete
linear group. An element of a Nazarov semigroup is a lattice in the duplicated
linear space . We investigate behavior of the complex distance under
the action of the Nazarov semigroup on the set .Comment: 6 page
High cooperativity coupling of electron-spin ensembles to superconducting cavities
Electron spins in solids are promising candidates for quantum memories for
superconducting qubits because they can have long coherence times, large
collective couplings, and many quantum bits can be encoded into the spin-waves
of a single ensemble. We demonstrate the coupling of electron spin ensembles to
a superconducting transmission-line resonator at coupling strengths greatly
exceeding the cavity decay rate and comparable to spin linewidth. We also use
the enhanced coupling afforded by the small cross-section of the transmission
line to perform broadband spectroscopy of ruby at millikelvin temperatures at
low powers. In addition, we observe hyperfine structure in diamond P1 centers
and time domain saturation-relaxation of the spins.Comment: 4pgs, 4 figure
Organisational and neuromodulatory underpinnings of structural-functional connectivity decoupling in patients with Parkinson's disease
Parkinson's dementia is characterised by changes in perception and thought, and preceded by visual dysfunction, making this a useful surrogate for dementia risk. Structural and functional connectivity changes are seen in humans with Parkinson's disease, but the organisational principles are not known. We used resting-state fMRI and diffusion-weighted imaging to examine changes in structural-functional connectivity coupling in patients with Parkinson's disease, and those at risk of dementia. We identified two organisational gradients to structural-functional connectivity decoupling: anterior-to-posterior and unimodal-to-transmodal, with stronger structural-functional connectivity coupling in anterior, unimodal areas and weakened towards posterior, transmodal regions. Next, we related spatial patterns of decoupling to expression of neurotransmitter receptors. We found that dopaminergic and serotonergic transmission relates to decoupling in Parkinson's overall, but instead, serotonergic, cholinergic and noradrenergic transmission relates to decoupling in patients with visual dysfunction. Our findings provide a framework to explain the specific disorders of consciousness in Parkinson's dementia, and the neurotransmitter systems that underlie these
Patchy Amphiphilic Dendrimers Bind Adenovirus and Control Its Host Interactions and in Vivo Distribution
The surface of proteins is heterogeneous with sophisticated but precise hydrophobic and hydrophilic patches, which is essential for their diverse biological functions. To emulate such distinct surface patterns on macromolecules, we used rigid spherical synthetic dendrimers (polyphenylene dendrimers) to provide controlled amphiphilic surface patches with molecular precision. We identified an,. I optimal spatial arrangement of these patches on certain dendrimers that enabled their interaction with human adenovirus 5 (Ads). Patchy dendrimers bound to the surface of Ads formed a synthetic polymer corona that greatly altered various host interactions of Ads as well as in vivo distribution. The dendrimer corona (1) improved the ability of Ad5-derived gene transfer vectors to transduce cells deficient for the primary Ad5 cell membrane receptor and (2) modulated the binding of Ads to blood coagulation factor X, one of the most critical virus host interactions in the bloodstream. It significantly enhanced the transduction efficiency of Ad5 while also protecting it from neutralization by natural antibodies and the complement system in human whole blood. Ads with a synthetic dendrimer corona revealed profoundly altered in vivo distribution, improved transduction of heart, and dampened vector sequestration by liver and spleen. We propose the design of bioactive polymers that bind protein surfaces solely based on their amphiphilic surface patches and protect against a naturally occurring protein corona, which is highly attractive to improve Ad5-based in vivo gene therapy applications
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