564 research outputs found
Calibration of the Mass-Temperature Relation for Clusters of Galaxies Using Weak Gravitational Lensing
The main uncertainty in current determinations of the power spectrum
normalization, sigma_8, from abundances of X-ray luminous galaxy clusters
arises from the calibration of the mass-temperature relation. We use our weak
lensing mass determinations of 30 clusters from the hitherto largest sample of
clusters with lensing masses, combined with X-ray temperature data from the
literature, to calibrate the normalization of this relation at a temperature of
8 keV, M_{500c,8 keV}=(8.7 +/- 1.6) h^{-1} 10^{14} M_sun. This normalization is
consistent with previous lensing-based results based on smaller cluster
samples, and with some predictions from numerical simulations, but higher than
most normalizations based on X-ray derived cluster masses. Assuming the
theoretically expected slope alpha=3/2 of the mass-temperature relation, we
derive sigma_8 = 0.88 +/-0.09 for a spatially-flat LambdaCDM universe with
Omega_m = 0.3. The main systematic errors on the lensing masses result from
extrapolating the cluster masses beyond the field-of-view used for the
gravitational lensing measurements, and from the separation of
cluster/background galaxies, contributing each with a scatter of 20%. Taking
this into account, there is still significant intrinsic scatter in the
mass-temperature relation indicating that this relation may not be very tight,
at least at the high mass end. Furthermore, we find that dynamically relaxed
clusters are 75 +/-40% hotter than non-relaxed clusters.Comment: 8 pages, 4 figures, revised version submitted to Ap
The Density Profile of Cluster-scale Dark Matter Halos
We measure the average gravitational shear profile of 6 massive clusters
(M_vir ~ 10^15 M_sun) at z=0.3 out to a radius ~2h^-1 Mpc. The measurements are
fitted to a generalized NFW-like halo model \rho(r) with an arbitrary r -> 0
slope \alpha. The data are well fitted by such a model with a central cusp with
\alpha ~ 0.9 - 1.6 (68% confidence interval). For the standard-NFW case \alpha
= 1.0, we find a concentration parameter c_vir that is consistent with recent
predictions from high-resolution CDM N-body simulations. Our data are also well
fitted by an isothermal sphere model with a softened core. For this model, our
1\sigma upper limit for the core radius corresponds to a limit \sigma_star \leq
0.1 cm^2 g^-1 on the elastic collision cross-section in a self-interacting dark
matter model.Comment: 4 pages, 3 figures; version accepted for publication by ApJ Letters.
Three figures omitted to allow space for new fig. 3 and expanded results and
discussion sections, including NSIS model fi
ROSAT PSPC Observations of the Richest () ACO Clusters
We have compiled an X-ray catalog of optically selected rich clusters of
galaxies observed by the PSPC during the pointed GO phase of the ROSAT mission.
This paper contains a systematic X-ray analysis of 150 clusters with an optical
richness classification of from the ACO catalog (Abell, Corwin, and
Olowin 1989). All clusters were observed within 45' of the optical axis of the
telescope during pointed PSPC observations. For each cluster, we calculate: the
net 0.5-2.0 keV PSPC count rate (or upper limit) in a 1 Mpc radius
aperture, 0.5-2.0 keV flux and luminosity, bolometric luminosity, and X-ray
centroid. The cluster sample is then used to examine correlations between the
X-ray and optical properties of clusters, derive the X-ray luminosity function
of clusters with different optical classifications, and obtain a quantitative
estimate of contamination (i.e, the fraction of clusters with an optical
richness significantly overestimated due to interloping galaxies) in the ACO
catalog
Active Brownian Motion Models and Applications to Ratchets
We give an overview over recent studies on the model of Active Brownian
Motion (ABM) coupled to reservoirs providing free energy which may be converted
into kinetic energy of motion. First, we present an introduction to a general
concept of active Brownian particles which are capable to take up energy from
the source and transform part of it in order to perform various activities. In
the second part of our presentation we consider applications of ABM to ratchet
systems with different forms of differentiable potentials. Both analytical and
numerical evaluations are discussed for three cases of sinusoidal,
staircase-like and Mateos ratchet potentials, also with the additional loads
modeled by tilted potential structure. In addition, stochastic character of the
kinetics is investigated by considering perturbation by Gaussian white noise
which is shown to be responsible for driving the directionality of the
asymptotic flux in the ratchet. This \textit{stochastically driven
directionality} effect is visualized as a strong nonmonotonic dependence of the
statistics of the right versus left trajectories of motion leading to a net
current of particles. Possible applications of the ratchet systems to molecular
motors are also briefly discussedComment: 12 pages, 17 figure
The X-ray Luminosity Function of Nearby Rich and Poor Clusters of Galaxies: A Cosmological Probe
In this letter, we present a new determination of the local (z<0.09) X-ray
luminosity function (XLF) using a large, statistical sample of 294 Abell
clusters and the ROSAT All-Sky-Survey. Given our large sample size, we have
reduced errors by a factor of two for L(X)(0.5-2keV)>10^43 ergs/sec. We combine
our data with previous work in order to explore possible constraints imposed by
the shape of the XLF on cosmological models. A set of currently viable
cosmologies is used to construct theoretical XLFs assuming Lx is proportional
to M^p and a sigma_8-Omega_0 constraint (from Viana & Liddle 1996) based on the
local X-ray temperature function. We fit these models to our observed XLF and
verify that the simplest adiabatic, analytic scaling relation (e.g. Kaiser
1986) disagrees strongly with observations. If we assume that clusters can be
described by the pre-heated, constant core-entropy models of Evrard & Henry
(1991) then the observed XLF is consistent only with 0.1 < Omega_0 < 0.4 if the
energy per unit mass in galaxies is roughly equal to the gas energy (ie if
beta=1). (abridged)Comment: 4 pages, 2 figures, accepted for publication in ApJ Letters. uses
emulateapj.st
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
Collective motion of active Brownian particles in one dimension
We analyze a model of active Brownian particles with non-linear friction and
velocity coupling in one spatial dimension. The model exhibits two modes of
motion observed in biological swarms: A disordered phase with vanishing mean
velocity and an ordered phase with finite mean velocity. Starting from the
microscopic Langevin equations, we derive mean-field equations of the
collective dynamics. We identify the fixed points of the mean-field equations
corresponding to the two modes and analyze their stability with respect to the
model parameters. Finally, we compare our analytical findings with numerical
simulations of the microscopic model.Comment: submitted to Eur. Phys J. Special Topic
Noise-Induced Transition from Translational to Rotational Motion of Swarms
We consider a model of active Brownian agents interacting via a harmonic
attractive potential in a two-dimensional system in the presence of noise. By
numerical simulations, we show that this model possesses a noise-induced
transition characterized by the breakdown of translational motion and the onset
of swarm rotation as the noise intensity is increased. Statistical properties
of swarm dynamics in the weak noise limit are further analytically
investigated.Comment: 7 pages, 7 figure
Discovery of a large-scale filament connected to the massive galaxy cluster MACSJ0717.5+3745 at z=0.55
We report the detection of a 4/h70 Mpc long large-scale filament leading into
the massive galaxy cluster MACS J0717.5+3745. The extent of this object well
beyond the cluster's nominal virial radius (~2.3 Mpc) rules out prior
interaction between its constituent galaxies and the cluster and makes it a
prime candidate for a genuine filament as opposed to a merger remnant or a
double cluster. The structure was discovered as a pronounced overdensity of
galaxies selected to have V-R colors close to the cluster red sequence.
Extensive spectroscopic follow-up of over 300 of these galaxies in a region
covering the filament and the cluster confirms that the entire structure is
located at the cluster redshift of z=0.545. Featuring galaxy surface densities
of typically 15/Mpc^2 down to luminosities of 0.10 L(*,V), the most diffuse
parts of the filament are comparable in density to the clumps of red galaxies
found around A851 in the only similar study carried out to date (Kodama et
al.). Our direct detection of an extended large-scale filament funneling matter
onto a massive, distant cluster provides a superb target for in-depth studies
of the evolution of galaxies in environments of greatly varying density, and
supports the predictions from theoretical models and numerical simulations of
structure formation in a hierarchical picture.Comment: ApJL, in pres
SPEAR Far Ultraviolet Spectral Images of the Cygnus Loop
We present far-ultraviolet (FUV) spectral images, measured at C IV 1550, He
II 1640, Si IV+O IV] 1400, and O III] 1664, of the entire Cygnus Loop, observed
with the Spectroscopy of Plasma Evolution from Astrophysical Radiation (SPEAR)
instrument, also known as FIMS. The spatial distribution of FUV emission
generally corresponds with a limb-brightened shell, and is similar to optical,
radio and X-ray images. The features found in the present work include a
``carrot'', diffuse interior, and breakout features, which have not been seen
in previous FUV studies. Shock velocities of 140-160 km/s is found from a line
ratio of O IV] to O III], which is insensitive not only to resonance scattering
but also to elemental abundance. The estimated velocity indicates that the fast
shocks are widespread across the remnant. By comparing various line ratios with
steady-state shock models, it is also shown that the resonance scattering is
widespread.Comment: 13 pages, 3 figures, 1 table, accepted for publication in ApJ
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