502 research outputs found
Perpendicular-current Studies of Electron Transport Across Metal/Metal Interfaces
We review what we have learned about the scattering of electrons by the
interfaces between two different metals (M1/M2) in the
current-perpendicular-to-plane (CPP) geometry. In this geometry, the intrinsic
quantity is the specific resistance, AR, the product of the area through which
the CPP current flows times the CPP resistance. We describe results for both
non-magnetic/non-magnetic (N1/N2) and ferromagnetic/non-magnetic (F/N) pairs.
We focus especially upon cases where M1/M2 are lattice matched (i.e., have the
same crystal structure and the same lattice parameters to within ~ 1%), because
in these cases no-free-parameter calculations of 2AR agree surprisingly well
with measured values. But we also list and briefly discuss cases where M1/M2
are not lattice matched, either having different crystal structures, or lattice
parameters that differ by several percent. The published calculations of 2AR in
these latter cases do not agree so well with measured values.Comment: 6 pages, 2 figures, 2 tables. In Press: Applied Surface Scienc
Merging clusters of galaxies observed with XMM-Newton
We present results from the XMM-Newton observations of our ongoing program on
merging clusters. To date three clusters have been observed, covering the
temporal sequence from early to late stage mergers: A1750, A2065 and A3921.
Using spatially-resolved spectroscopy of discrete regions, hardness ratio and
temperature maps, we show that all three clusters display a complex temperature
structure. In the case of A1750, a double cluster, we argue that the observed
temperature structure is not only related to the ongoing merger but also to
previous merger events. A2065 seems an excellent example of a `compact merger',
i.e. when the centres of the two clusters have just started to interact,
producing a shock in the ICM. Using comparisons with numerical simulations and
complementary optical data, the highly complex temperature structure evident in
A3921 is interpreted as an off-axis merger between two unequal mass components.
These results illustrate the complex physics of merger events. The relaxation
time can be larger than the typical time between merger events, so that the
present day morphology of clusters depends not only on on-going interaction but
also on the more ancient formation history.Comment: 10 pages, 3 figures. Use elsart.cls. Accepted for publication in
Advances in Space Research. A version with full resolution figures can be
found at http://www.star.bris.ac.uk/elena/cospar_3clusters.pd
Self Consistent Molecular Field Theory for Packing in Classical Liquids
Building on a quasi-chemical formulation of solution theory, this paper
proposes a self consistent molecular field theory for packing problems in
classical liquids, and tests the theoretical predictions for the excess
chemical potential of the hard sphere fluid. Results are given for the self
consistent molecular fields obtained, and for the probabilities of occupancy of
a molecular observation volume. For this system, the excess chemical potential
predicted is as accurate as the most accurate prior theories, particularly the
scaled particle (Percus-Yevick compressibility) theory. It is argued that the
present approach is particularly simple, and should provide a basis for a
molecular-scale description of more complex solutions.Comment: 6 pages and 5 figure
Factorbook: an Updated Catalog of Transcription Factor Motifs and Candidate Regulatory Motif Sites [preprint]
The human genome contains roughly 1,600 transcription factors (TFs) (1), DNA-binding proteins recognizing characteristic sequence motifs to exert regulatory effects on gene expression. The binding specificities of these factors have been profiled both in vitro, using techniques such as HT-SELEX (2), and in vivo, using techniques including ChIP-seq (3, 4). We previously developed Factorbook, a TF-centric database of annotations, motifs, and integrative analyses based on ChIP-seq data from Phase II of the ENCODE Project. Here we present an update to Factorbook which significantly expands the breadth of cell type and TF coverage. The update includes an expanded motif catalog derived from thousands of ENCODE Phase II and III ChIP-seq experiments and HT-SELEX experiments; this motif catalog is integrated with the ENCODE registry of candidate cis-regulatory elements to annotate a comprehensive collection of genome-wide candidate TF binding sites. The database also offers novel tools for applying the motif models within machine learning frameworks and using these models for integrative analysis, including annotation of variants and disease and trait heritability. We will continue to expand the resource as ENCODE Phase IV data are released
Substructure of the galaxy clusters in the REXCESS sample: observed statistics and comparison to numerical simulations
We study the substructure statistics of a representative sample of galaxy
clusters by means of two currently popular substructure characterisation
methods, power ratios and centroid shifts. We use the 31 clusters from the
REXCESS sample, compiled from the southern ROSAT All-Sky cluster survey REFLEX
with a morphologically unbiased selection in X-ray luminosity and redshift, all
of which have been reobserved with XMM-Newton. We investigate the uncertainties
of the substructure parameters and examine the dependence of the results on
projection effects, finding that the uncertainties of the parameters can be
quite substantial. Thus while the quantification of the dynamical state of
individual clusters with these parameters should be treated with extreme
caution, these substructure measures provide powerful statistical tools to
characterise trends of properties in large cluster samples. The centre shift
parameter, w, is found to be more sensitive in general. For the REXCESS sample
neither the occurence of substructure nor the presence of cool cores depends on
cluster mass. There is a significant anti-correlation between the existence of
substantial substructure and cool cores. The simulated clusters show on average
larger substructure parameters than the observed clusters, a trend that is
traced to the fact that cool regions are more pronounced in the simulated
clusters, leading to stronger substructure measures in merging clusters and
clusters with offset cores. Moreover, the frequency of cool regions is higher
in the simulations than in the observations, implying that the description of
the physical processes shaping cluster formation in the simulations requires
further improvement.Comment: Mauscript submitted to Astronomy and Astrophysics, 20 figure
The Representative XMM-Newton Cluster Structure Survey (REXCESS) of an X-ray Luminosity Selected Galaxy Cluster Sample
The largest uncertainty for cosmological studies using clusters of galaxies
is introduced by our limited knowledge of the statistics of galaxy cluster
structure, and of the scaling relations between observables and cluster mass.
To improve on this situation we have started an XMM-Newton Large Programme for
the in-depth study of a representative sample of 33 galaxy clusters, selected
in the redshift range z=0.055 to 0.183 from the REFLEX Cluster Survey, having
X-ray luminosities above 0.4 X 10^44 h_70^-2 erg s^-1 in the 0.1 - 2.4 keV
band. This paper introduces the sample, compiles properties of the clusters,
and provides detailed information on the sample selection function. We describe
the selection of a nearby galaxy cluster sample that makes optimal use of the
XMM-Newton field-of-view, and provides nearly homogeneous X-ray luminosity
coverage for the full range from poor clusters to the most massive objects in
the Universe. For the clusters in the sample, X-ray fluxes are derived and
compared to the previously obtained fluxes from the ROSAT All-Sky Survey. We
find that the fluxes and the flux errors have been reliably determined in the
ROSAT All-Sky Survey analysis used for the REFLEX Survey. We use the sample
selection function documented in detail in this paper to determine the X-ray
luminosity function, and compare it with the luminosity function of the entire
REFLEX sample. We also discuss morphological peculiarities of some of the
sample members. The sample and some of the background data given in this
introductory paper will be important for the application of these data in the
detailed studies of cluster structure, to appear in forthcoming publications.Comment: 17 pages, 17 figures; to appear in A&A. A pdf version with
full-quality figures can be found at
ftp://ftp.xray.mpe.mpg.de/people/gwp/xmmlp/xmmlp.pd
Exploring the galaxy cluster-group transition regime at high redshifts: Physical properties of two newly detected z > 1 systems
Context: Multi-wavelength surveys for clusters of galaxies are opening a
window on the elusive high-redshift (z>1) cluster population. Well controlled
statistical samples of distant clusters will enable us to answer questions
about their cosmological context, early assembly phases and the thermodynamical
evolution of the intracluster medium. Aims: We report on the detection of two
z>1 systems, XMMU J0302.2-0001 and XMMU J1532.2-0836, as part of the XMM-Newton
Distant Cluster Project (XDCP) sample. We investigate the nature of the
sources, measure their spectroscopic redshift and determine their basic
physical parameters. Methods: The results of the present paper are based on the
analysis of XMM-Newton archival data, optical/near-infrared imaging and deep
optical follow-up spectroscopy of the clusters. Results: We confirm the X-ray
source XMMU J0302.2-0001 as a gravitationally bound, bona fide cluster of
galaxies at spectroscopic redshift z=1.185. We estimate its M500 mass to
(1.6+/-0.3) times 10^{14} Msun from its measured X-ray luminosity. This ranks
the cluster among intermediate mass system. In the case of XMMU J1532.2-0836 we
find the X-ray detection to be coincident with a dynamically bound system of
galaxies at z=1.358. Optical spectroscopy reveals the presence of a central
active galactic nucleus, which can be a dominant source of the detected X-ray
emission from this system. We provide upper limits of X-ray parameters for the
system and discuss cluster identification challenges in the high-redshift
low-mass cluster regime. A third, intermediate redshift (z=0.647) cluster, XMMU
J0302.1-0000, is serendipitously detected in the same field as XMMU
J0302.2-0001. We provide its analysis as well.Comment: Accepted to A&A, 13/04/2011. 15 pages, 18 figures, 5 tables, 2
appendice
The large-scale shock in the cluster of galaxies Hydra A
We analyzed a deep XMM-Newton observation of the cluster of galaxies Hydra A,
focusing on the large-scale shock discovered as a surface brightness
discontinuity in Chandra images. The shock front can be seen both in the
pressure map and in temperature profiles in several sectors. The Mach numbers
determined from the temperature jumps are in good agreement with the Mach
numbers derived from EPIC/pn surface brightness profiles and previously from
Chandra data and are consistent with M~1.3. The estimated shock age in the
different sectors using a spherically symmetric point explosion model ranges
between 130 and 230 Myr and the outburst energy between 1.5 and 3e61 ergs. The
shape of the shock seen in the pressure map can be approximated with an ellipse
centered 70 kpc towards the NE from the cluster center. We aimed to develop a
better model that can explain the offset between the shock center and the AGN
and give a consistent result on the shock age and energy. To this end, we
performed 3D hydrodynamical simulations in which the shock is produced by a
symmetrical pair of AGN jets launched in a spherical galaxy cluster. As an
explanation of the observed offset of the shock center, we consider large-scale
bulk flows in the intracluster medium. The simulation successfully reproduces
the size, ellipticity, and average Mach number of the observed shock front. The
predicted age of the shock is 160 Myr and the total input energy 3e61 erg. Both
values are within the range determined by the spherically symmetric model.
Matching the observed 70 kpc offset of the shock ellipse from the cluster
center requires large-scale coherent motions with a high velocity of 670 km/s.
We discuss the feasibility of this scenario and offer alternative ways to
produce the offset and to further improve the simulation.Comment: 14 pages, accepted for publication in A&A, minor revision compared to
previous versio
Differential analysis of chromatin accessibility and histone modifications for predicting mouse developmental enhancers
Enhancers are distal cis-regulatory elements that modulate gene expression. They are depleted of nucleosomes and enriched in specific histone modifications; thus, calling DNase-seq and histone mark ChIP-seq peaks can predict enhancers. We evaluated nine peak-calling algorithms for predicting enhancers validated by transgenic mouse assays. DNase and H3K27ac peaks were consistently more predictive than H3K4me1/2/3 and H3K9ac peaks. DFilter and Hotspot2 were the best DNase peak callers, while HOMER, MUSIC, MACS2, DFilter and F-seq were the best H3K27ac peak callers. We observed that the differential DNase or H3K27ac signals between two distant tissues increased the area under the precision-recall curve (PR-AUC) of DNase peaks by 17.5-166.7% and that of H3K27ac peaks by 7.1-22.2%. We further improved this differential signal method using multiple contrast tissues. Evaluated using a blind test, the differential H3K27ac signal method substantially improved PR-AUC from 0.48 to 0.75 for predicting heart enhancers. We further validated our approach using postnatal retina and cerebral cortex enhancers identified by massively parallel reporter assays, and observed improvements for both tissues. In summary, we compared nine peak callers and devised a superior method for predicting tissue-specific mouse developmental enhancers by reranking the called peaks
LoCuSS: Comparison of Observed X-ray and Lensing Galaxy Cluster Scaling Relations with Simulations
The Local Cluster Substructure Survey (LoCuSS, Smith et al.) is a systematic
multi-wavelength survey of >100 X-ray luminous galaxy clusters (0.14<z<0.3)
selected from the ROSAT all sky survey. We used data on 37 LoCuSS clusters from
the XMM-Newton archive to investigate the global scaling relations of galaxy
clusters. The scaling relations based solely on the X-ray data obey empirical
self-similarity and reveal no additional evolution beyond the LSS growth. Weak
lensing mass measurements are also available in the literature for 19 of the
clusters with XMM-Newton data. The average of the weak lensing mass to X-ray
based mass ratio is 1.09+/-8, setting the limit of the non-thermal pressure
support to 9+/-8%. The mean of the weak lensing mass to X-ray based mass ratio
of these clusters is ~1 with 31-51% scatter. The scatter in the
mass--observable relations (M-Y_X, M-M_{gas} and M-T) is smaller using X-ray
based masses than using weak lensing masses by a factor of 2. Using the scaled
radius defined by the Y_X profile, we obtain lower scatter in the weak lensing
mass based mass--observable relations. The normalization of the M-Y_X relation
(also M-M_{gas} and M-T relations) using X-ray (weak lensing) mass estimates is
lower than the one from simulations by up to 20% at ~3 sigma (~2 sigma)
significance. Despite the large scatter in the X-ray to lensing comparison, the
agreement between these two completely independent observational methods is an
important step towards controlling astrophysical and measurement systematics in
cosmological scaling relations.Comment: 56 pages, 32 figure, 2008A&A...482..451Z, typos corrected in Table
A.
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