1,945 research outputs found
The Origin of the Spatial Distribution of X-ray luminous AGN in Massive Galaxy Clusters
We study the spatial distribution of a 95% complete sample of 508 X-ray point
sources (XPS) detected in the 0.5-2.0 keV band in Chandra ACIS-I observations
of 51 massive galaxy clusters found in the MACS survey. Covering the redshift
range z=0.3-0.7, our cluster sample is statistically complete and comprises all
MACS clusters with X-ray luminosities in excess of 4.5 x 10^44 erg/s (0.1-2.4
keV, h_0=0.7, LCDM). Also studied are 20 control fields that do not contain
clusters. We find the XPS surface density, computed in the cluster restframe,
to exhibit a pronounced excess within 3.5 Mpc of the cluster centers. The
excess, believed to be caused by AGN in the cluster, is significant at the 8.0
sigma confidence level compared to the XPS density observed at the field edges.
No significant central excess is found in the control fields. To investigate
the physical origin of the AGN excess, we study the radial AGN density profile
for a subset of 24 virialized clusters. We find a pronounced central spike
(r<0.5 Mpc), followed by a depletion region at about 1.5 Mpc, and a broad
secondary excess centered at approximately the virial radius of the host
clusters (~2.5 Mpc). We present evidence that the central AGN excess reflects
increased nuclear activity triggered by close encounters between infalling
galaxies and the giant cD-type elliptical occupying the very cluster center. By
contrast, the secondary excess at the cluster-field interface is likely due to
black holes being fueled by galaxy mergers. In-depth spectroscopic and
photometric follow-up observations of the optical counterparts of the XPS in a
subset of our sample are being conducted to confirm this picture.Comment: ApJ Letters, accepted (4 pages, 3 figures, uses emulateapj
Entropy and Long range correlations in literary English
Recently long range correlations were detected in nucleotide sequences and in
human writings by several authors. We undertake here a systematic investigation
of two books, Moby Dick by H. Melville and Grimm's tales, with respect to the
existence of long range correlations. The analysis is based on the calculation
of entropy like quantities as the mutual information for pairs of letters and
the entropy, the mean uncertainty, per letter. We further estimate the number
of different subwords of a given length . Filtering out the contributions
due to the effects of the finite length of the texts, we find correlations
ranging to a few hundred letters. Scaling laws for the mutual information
(decay with a power law), for the entropy per letter (decay with the inverse
square root of ) and for the word numbers (stretched exponential growth with
and with a power law of the text length) were found.Comment: 8 page
Thermodynamics of hot dense H-plasmas: Path integral Monte Carlo simulations and analytical approximations
This work is devoted to the thermodynamics of high-temperature dense hydrogen
plasmas in the pressure region between and Mbar. In particular
we present for this region results of extensive calculations based on a
recently developed path integral Monte Carlo scheme (direct PIMC). This method
allows for a correct treatment of the thermodynamic properties of hot dense
Coulomb systems. Calculations were performed in a broad region of the
nonideality parameter and degeneracy parameter . We give a comparison with a few available results from
other path integral calculations (restricted PIMC) and with analytical
calculations based on Pade approximations for strongly ionized plasmas. Good
agreement between the results obtained from the three independent methods is
found.Comment: RevTex file, 21 pages, 5 ps-figures include
Deformable self-propelled particles
A theory of self-propelled particles is developed in two dimensions assuming
that the particles can be deformed from a circular shape when the propagating
velocity is increased. A coupled set of equations in terms of the velocity and
a tensor variable to represent the deformation is introduced to show that there
is a bifurcation from a straight motion to a circular motion of a single
particle. Dynamics of assembly of the particles is studied numerically where
there is a global interaction such that the particles tend to cause an
orientational order.Comment: 4pages, 4figure
A New Radio - X-Ray Probe of Galaxy Cluster Magnetic Fields
Results are presented of a new VLA-ROSAT study that probes the magnetic field
strength and distribution over a sample of 16 ``normal'' low redshift (z < 0.1)
galaxy clusters. The clusters span two orders of magnitude in X-ray luminosity,
and were selected to be free of (unusual) strong radio cluster halos, and
widespread cooling flows. Consistent with these criteria, most clusters show a
relaxed X-ray morphology and little or no evidence for recent merger activity.
Analysis of the rotation measure (RM) data shows cluster-generated Faraday RM
excess out to ~0.5 Mpc from cluster centers. The results, combined with RM
imaging of cluster-embedded sources and ROSAT X-ray profiles indicates that the
hot intergalactic gas within these ``normal'' clusters is permeated with a high
filling factor by magnetic fields at levels of = 5-10 (l/10 kpc)^{-1/2}
microGauss, where l is the field correlation length. These results lead to a
global estimate of the total magnetic energy in clusters, and give new insight
into the ultimate energy origin, which is likely gravitational. These results
also shed some light on the cluster evolutionary conditions that existed at the
onset of cooling flows.Comment: 6 pages, 1 figure, uses emulateapj5.sty, accepted by ApJ
The spatial distribution of galaxies of different spectral types in the massive intermediate-redshift cluster MACSJ0717.5+3745
We present the results of a wide-field spectroscopic analysis of the galaxy
population of the massive cluster MACSJ0717.5+3745 and the surrounding
filamentary structure (z=0.55), as part of our systematic study of the 12 most
distant clusters in the MACS sample. Of 1368 galaxies spectroscopically
observed in this field, 563 are identified as cluster members; of those, 203
are classified as emission-line galaxies, 260 as absorption-line galaxies, and
17 as E+A galaxies (defined by \AA and no
detection of [OII] and in emission). The variation of the fraction
of emission- and absorption-line galaxies as a function of local projected
galaxy density confirms the well-known morphology-density relation, and becomes
flat at projected galaxy densities less than $\sim 20Mpc^{-2}. Interestingly,
16 out of 17 E+A galaxies lie (in projection) within the ram-pressure stripping
radius around the cluster core, which we take to be direct evidence of
ram-pressure stripping being the primary mechanism that terminates
star-formation in the E+A population of galaxy clusters. This conclusion is
supported by the rarity of E+A galaxies in the filament which rules out galaxy
mergers as the dominant driver of evolution for E+A galaxies in clusters. In
addition, we find the 42 e(a) and 27 e(b) member galaxies, i.e., the
dusty-starburst and starburst galaxies respectively, to be spread out across
almost the entire study area. Their spatial distribution, which shows a strong
preference for the filament region, suggests that starbursts are triggered in
relatively low-density environments as galaxies are accreted from the field
population.Comment: 16 pages, 15 figures, accepted by Ap
Statistical Mechanics of Canonical-Dissipative Systems and Applications to Swarm Dynamics
We develop the theory of canonical-dissipative systems, based on the
assumption that both the conservative and the dissipative elements of the
dynamics are determined by invariants of motion. In this case, known solutions
for conservative systems can be used for an extension of the dynamics, which
also includes elements such as the take-up/dissipation of energy. This way, a
rather complex dynamics can be mapped to an analytically tractable model, while
still covering important features of non-equilibrium systems. In our paper,
this approach is used to derive a rather general swarm model that considers (a)
the energetic conditions of swarming, i.e. for active motion, (b) interactions
between the particles based on global couplings. We derive analytical
expressions for the non-equilibrium velocity distribution and the mean squared
displacement of the swarm. Further, we investigate the influence of different
global couplings on the overall behavior of the swarm by means of
particle-based computer simulations and compare them with the analytical
estimations.Comment: 14 pages incl. 13 figures. v2: misprints in Eq. (40) corrected, ref.
updated. For related work see also:
http://summa.physik.hu-berlin.de/~frank/active.htm
Agent-Based Modeling of Intracellular Transport
We develop an agent-based model of the motion and pattern formation of
vesicles. These intracellular particles can be found in four different modes of
(undirected and directed) motion and can fuse with other vesicles. While the
size of vesicles follows a log-normal distribution that changes over time due
to fusion processes, their spatial distribution gives rise to distinct
patterns. Their occurrence depends on the concentration of proteins which are
synthesized based on the transcriptional activities of some genes. Hence,
differences in these spatio-temporal vesicle patterns allow indirect
conclusions about the (unknown) impact of these genes.
By means of agent-based computer simulations we are able to reproduce such
patterns on real temporal and spatial scales. Our modeling approach is based on
Brownian agents with an internal degree of freedom, , that represents
the different modes of motion. Conditions inside the cell are modeled by an
effective potential that differs for agents dependent on their value .
Agent's motion in this effective potential is modeled by an overdampted
Langevin equation, changes of are modeled as stochastic transitions
with values obtained from experiments, and fusion events are modeled as
space-dependent stochastic transitions. Our results for the spatio-temporal
vesicle patterns can be used for a statistical comparison with experiments. We
also derive hypotheses of how the silencing of some genes may affect the
intracellular transport, and point to generalizations of the model
Recommended from our members
Flow measurement using micro-PIV and related temperature distributions within evaporating sessile drops of self-rewetting mixtures of 1-pentanol and water
This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.Recently interest has arisen in the use of so-called self-rewetting mixtures for micro-scale heat
transfer systems. Such fluids, in which the surface tension can increase with increasing temperature, are
expected to offer superior evaporative cooling performance by extending the region of operation before dryout
of the heated surface sets in. Whilst improved performance has been shown in some practical situations
using these fluids, it is not entirely clear as to the mechanism of such improvements.
We have studied the flow within evaporating sessile drops of 1-pentanol-water mixtures using micro-PIV
and have observed three stages in the evaporation process. During the first stage there appears to be a single
toroidal vortex with flow inwards along the base of the drop. The vortex only occupies the central region of
the drop and appears to pulsate, reducing in size during evaporation. This is followed by a second transition
stage to a third stage in which the flow is directed radially outward, as observed by us for pure water droplet
evaporation and in the latter stages of ethanol/water drop evaporation. Temperature measurements, using IR
thermography suggest that the initial stage of evaporation may be controlled by thermal Marangoni effects as
opposed to the concentration driven Marangoni flows postulated for ethanol-water mixtures
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