3,890 research outputs found
Experiments to investigate particulate materials in reduced gravity fields
Study investigates agglomeration and macroscopic behavior in reduced gravity fields of particles of known properties by measuring and correlating thermal and acoustical properties of particulate materials. Experiment evaluations provide a basis for a particle behavior theory and measure bulk properties of particulate materials in reduced gravity
The Ultimate Halo Mass in a LCDM Universe
In the far future of an accelerating LCDM cosmology, the cosmic web of
large-scale structure consists of a set of increasingly isolated halos in
dynamical equilibrium. We examine the approach of collisionless dark matter to
hydrostatic equilibrium using a large N-body simulation evolved to scale factor
a = 100, well beyond the vacuum--matter equality epoch, a_eq ~ 0.75, and 53/h
Gyr into the future for a concordance model universe (Omega_m ~ 0.3,
Omega_Lambda ~ 0.7). The radial phase-space structure of halos -- characterized
at a < a_eq by a pair of zero-velocity surfaces that bracket a dynamically
active accretion region -- simplifies at a > 10 a_eq when these surfaces merge
to create a single zero-velocity surface, clearly defining the halo outer
boundary, rhalo, and its enclosed mass, mhalo. This boundary approaches a fixed
physical size encompassing a mean interior density ~ 5 times the critical
density, similar to the turnaround value in a classical Einstein-deSitter
model. We relate mhalo to other scales currently used to define halo mass
(m200, mvir, m180b) and find that m200 is approximately half of the total
asymptotic cluster mass, while m180b follows the evolution of the inner zero
velocity surface for a < 2 but becomes much larger than the total bound mass
for a > 3. The radial density profile of all bound halo material is well fit by
a truncated Hernquist profile. An NFW profile provides a somewhat better fit
interior to r200 but is much too shallow in the range r200 < r < rhalo.Comment: 5 pages, 3 figures, submitted to MNRAS letter
The complete amino acid sequence of the antenna polypeptide B806-866-β from the cytoplasmic membrane of the green bacterium Chloroflexus auranliacus
AbstractThe bacteriochlorophyll a-binding polypeptide B806â866-β was extracted from membranes of the green thermophilic bacterium Chloroflexus aurantiacus with chloroform/methanol/ammonium acetate. Purification of the antenna polypeptide (6.3 kDa) was achieved by chromatography on Sephadex LH-60, Whatman DE-32 and by FPLC. The complete amino acid sequence (53 amino acid residues) was determined. The B806â866-β polypeptide is sequence homologous to the antenna β-polypeptides of purple bacteria (27â40%) and exhibits the characteristic three domain structure of the B870, B800â850 and B800â820 antenna complexes. The two typical His residues, conserved in all antenna β-polypeptides of purple bacteria, were found: His-24 lies within the N-terminal hydrophilic domain and His-42 within the central hydrophobic domain. This polypeptide together with the previously described Îą-polypeptide form the basic structural unit of the B806â866 antenna complex from C. aurantiacus
Concentrations of Dark Halos from their Assembly Histories
(abridged) We study the relation between the density profiles of dark matter
halos and their mass assembly histories, using a statistical sample of halos in
a high-resolution N-body simulation of the LCDM cosmology. For each halo at
z=0, we identify its merger-history tree, and determine concentration
parameters c_vir for all progenitors, thus providing a structural merger tree
for each halo. We fit the mass accretion histories by a universal function with
one parameter, the formation epoch a_c, defined when the log mass accretion
rate dlogM/dloga falls below a critical value S. We find that late forming
galaxies tend to be less concentrated, such that c_vir ``observed'' at any
epoch a_o is strongly correlated with a_c via c_vir=c_1*a_o/a_c. Scatter about
this relation is mostly due to measurement errors in c_v and a_c, implying that
the actual spread in c_vir for halos of a given mass can be mostly attributed
to scatter in a_c. We demonstrate that this relation can also be used to
predict the mass and redshift dependence of c_v, and the scatter about the
median c_vir(M,z), using accretion histories derived from the Extended
Press-Schechter (EPS) formalism, after adjusting for a constant offset between
the formation times as predicted by EPS and as measured in the simulations;this
new ingredient can thus be easily incorporated into semi-analytic models of
galaxy formation. The correlation found between halo concentration and mass
accretion rate suggests a physical interpretation: for high mass infall rates
the central density is related to the background density; when the mass infall
rate slows, the central density stays approximately constant and the halo
concentration just grows as R_vir. The tight correlation demonstrated here
provides an essential new ingredient for galaxy formation modeling.Comment: 19 pages, 18 figures, uses emulateapj5.tex. ApJ, in press; revised to
match accepted versio
How Common are the Magellanic Clouds?
We introduce a probabilistic approach to the problem of counting dwarf
satellites around host galaxies in databases with limited redshift information.
This technique is used to investigate the occurrence of satellites with
luminosities similar to the Magellanic Clouds around hosts with properties
similar to the Milky Way in the object catalog of the Sloan Digital Sky Survey.
Our analysis uses data from SDSS Data Release 7, selecting candidate
Milky-Way-like hosts from the spectroscopic catalog and candidate analogs of
the Magellanic Clouds from the photometric catalog. Our principal result is the
probability for a Milky-Way-like galaxy to host N_{sat} close satellites with
luminosities similar to the Magellanic Clouds. We find that 81 percent of
galaxies like the Milky Way are have no such satellites within a radius of 150
kpc, 11 percent have one, and only 3.5 percent of hosts have two. The
probabilities are robust to changes in host and satellite selection criteria,
background-estimation technique, and survey depth. These results demonstrate
that the Milky Way has significantly more satellites than a typical galaxy of
its luminosity; this fact is useful for understanding the larger cosmological
context of our home galaxy.Comment: Updated to match published version. Added referenc
Halpha rotation curves: the soft core question
We present high resolution Halpha rotation curves of 4 late-type dwarf
galaxies and 2 low surface brightness galaxies (LSB) for which accurate HI
rotation curves are available from the literature. Observations are carried out
at Telescopio Nazionale Galileo (TNG). For LSB F583-1 an innovative dispersing
element was used, the Volume Phase Holographic (VPH) with a dispersion of about
0.35 A/pxl. We find good agreement between the Halpha data and the HI
observations and conclude that the HI data for these galaxies suffer very
little from beam smearing. We show that the optical rotation curves of these
dark matter dominated galaxies are best fitted by the Burkert profile. In the
centers of galaxies, where the N-body simulations predict cuspy cores and fast
rising rotation curves, our data seem to be in better agreement with the
presence of soft cores.Comment: Accepted for Publication in ApJ with minor changes require
Residual cognitive deficits 50 years after lead poisoning during childhood
The long term neurobehavioural consequences of childhood lead poisoning are not known. In this study adult subjects with a documented history of lead poisoning before age 4 and matched controls were examined with an abbreviated battery of neuropsychological tests including measures of attention, reasoning, memory, motor speed, and current mood. The subjects exposed to lead were inferior to controls on almost all of the cognitive tasks. This pattern of widespread deficits resembles that found in children evaluated at the time of acute exposure to lead rather than the more circumscribed pattern typically seen in adults exposed to lead. Despite having completed as many years of schooling as controls, the subjects exposed to lead were lower in lifetime occupational status. Within the exposed group, performance on the neuropsychological battery and occupational status were related, consistent with the presumed impact of limitations in neuropsychological functioning on everyday life. The results suggest that many subjects exposed to lead suffered acute encephalopathy in childhood which resolved into a chronic subclinical encephalopathy with associated cognitive dysfunction still evident in adulthood. These findings lend support to efforts to limit exposure to lead in childhood
Age-related Differences in Prestimulus Subsequent Memory Effects Assessed with Event-related Potentials
Prestimulus subsequent memory effects (preSMEs)âdifferences in neural activity elicited by a task cue at encoding that are predictive of later memory performanceâare thought to reflect differential engagement of preparatory processes that benefit episodic memory encoding. We investigated age differences in preSMEs indexed by differences in ERP amplitude just before the onset of a study item. Young and older adults incidentally encoded words for a subsequent memory test. Each study word was preceded by a task cue that signaled a judgment to perform on the word. Words were presented for either a short (300 msec) or long (1000 msec) duration with the aim of placing differential benefits on engaging preparatory processes initiated by the task cue. ERPs associated with subsequent successful and unsuccessful recollection, operationalized here by source memory accuracy, were estimated time-locked to the onset of the task cue. In a late time window (1000â2000 msec after onset of the cue), young adults demonstrated frontally distributed preSMEs for both the short and long study durations, albeit with opposite polarities in the two conditions. This finding suggests that preSMEs in young adults are sensitive to perceived task demands. Although older adults showed no evidence of preSMEs in the same late time window, significant preSMEs were observed in an earlier time window (500â1000 msec) that was invariant with study duration. These results are broadly consistent with the proposal that older adults differ from their younger counterparts in how they engage preparatory processes during memory encoding
Parents' postnatal depressive symptoms and their children's academic attainment at 16 years: Pathways of risk transmission
This is the author accepted manuscript. The final version is available from Wiley via the DOI in this record.The aim of the study was to examine whether parentsâ increased postnatal depressive symptoms predicted
childrenâs academic attainment over time, and whether the parent-child relationship, childrenâs prior
academic attainment and mental health mediated this association.
We conducted secondary analyses on the Avon Longitudinal Study of Parents and Children data (12,607
mothers, 9,456 fathers). Each parent completed the Edinburgh-Postnatal Depression Scale at 8 weeks after
the childâs birth (predictor) and a questionnaire about the mother-child and father-child relationship at 7
years and 1 month (mediator). The childrenâs mental health problems were assessed with the teacher version
of the Strengths and Difficulties Questionnaire at 10-11 years (mediator). We used data on the childrenâs
academic attainment on UK Key Stage 1 (5-7 years; mediator) and Key Stage 4 (General Certificate of
Secondary Education (GCSE)16 years) (outcome). We adjusted for the parentsâ education, and child gender
and cognitive ability.
The results revealed that parentsâ depressive symptoms at 8 weeks predicted lower academic performance in
children at 16 years. Mothersâ postnatal depressive symptoms had an indirect effect through childrenâs
mental health problems on academic outcomes at 16 years via negative mother-child relationship, and prior
academic attainment. There was a significant negative indirect effect of fathersâ postnatal depressive
symptoms on academic attainment at 16 years via negative father-child relationship on child mental health.
The findings suggest that the family environment (parental mental health and parent-child relationship) and
childrenâs mental health should be potential targets for support programmes for children of depressed
parents.Medical Research Council (MRC)Wellcome Trus
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