1,480 research outputs found
Comment on "A test-tube model for rainfall" by Wilkinson M., EPL 106 (2014) 40001
This paper is a comment to M Wilkinson, EPL 106 (2014) 40001, arXiv:1401.4620
[physics.ao-ph,cond-mat.soft], which draws conclusion from our data that are at
variance with our observations
Hot gas in Mach cones around Virgo Cluster spiral galaxies
The detailed comparison between observations and simulations of ram pressure
stripped spiral galaxies in the Virgo cluster has led to a three dimensional
view of the galaxy orbits within the hot intracluster medium. The 3D velocities
and Mach numbers derived from simulations can be used to derive simple Mach
cone geometries for Virgo spiral galaxies. We search for indications of hot gas
within Mach cones in X-ray observations of selected Virgo Cluster spiral
galaxies (NGC 4569, NGC 4388, and NGC 4501). We find extraplanar diffuse X-ray
emission in all galaxies. Based on the 3D velocity vectors from dynamical
modelling a simple Mach cone is fitted to the triangular shape of NGC 4569's
diffuse X-ray emission. Assuming that all extraplanar diffuse X-ray emission
has to be located inside the Mach cone, we also fit Mach cones to NGC 4388's
and NGC 4501's extraplanar X-ray emission. For NGC 4569 it is hard to reconcile
the derived Mach cone opening angle with a Mach number based on the sound speed
alone. Instead, a Mach number involving the Alfv\'enic speed seems to be more
appropriate, yielding a magnetic field strength of -6 G for a
intracluster medium density of cm. Whereas the
temperature of the hot component of NGC 4569's X-ray halo (0.5 keV) is at the
high end but typical for a galactic outflow, the temperature of the hot gas
tails of NGC 4388 and NGC 4501 are significantly hotter (0.7-0.9 keV). In NGC
4569 we find direct evidence for a Mach cone which is filled with hot gas from
a galactic superwind. We suggest that the high gas temperatures in the X-ray
tails of NGC 4388 and NGC 4501 are due to the mixing of the stripped ISM into
the hot intracluster medium of the Virgo cluster.Comment: 11 pages, 10 figures, 5 tables. Accepted for publication in Astronomy
and Astrophysic
Label-free optical detection of single enzyme-reactant reactions and associated conformational changes
Monitoring the kinetics and conformational dynamics of single enzymes is
crucial in order to better understand their biological functions as these
motions and structural dynamics are usually unsynchronized among the molecules.
Detecting the enzyme-reactant interactions and associated conformational
changes of the enzyme on a single molecule basis, however, remain as a
challenge with established optical techniques due to the commonly required
labeling of the reactants or the enzyme itself. The labeling process is usually
non-trivial and the labels themselves might skew the physical properties of the
enzyme. Here we demonstrate an optical, label-free method capable of observing
enzymatic interactions and the associated conformational changes on the single
molecule level. We monitor polymerase/DNA interactions via the strong
near-field enhancement provided by plasmonic nanorods resonantly coupled to
whispering gallery modes in microcavities. Specifically, we employ two
different recognition schemes: one in which the kinetics of polymerase/DNA
interactions are probed in the vicinity of DNA-functionalized nanorods, and the
other in which these interactions are probed via the magnitude of
conformational changes in the polymerase molecules immobilized on nanorods. In
both approaches we find that low and high polymerase activities can be clearly
discerned via their characteristic signal amplitude and signal length
distributions. Furthermore, the thermodynamic study of the monitored
interactions suggests the occurrence of DNA polymerization. This work
constitutes a proof-of-concept study of enzymatic activities via plasmonically
enhanced microcavities and establishes an alternative and label-free method
capable of investigating structural changes in single molecules
Model Checking CTL is Almost Always Inherently Sequential
The model checking problem for CTL is known to be P-complete (Clarke, Emerson, and Sistla (1986), see Schnoebelen (2002)). We consider fragments of CTL obtained by restricting the use of temporal modalities or the use of negations—restrictions already studied for LTL by Sistla and Clarke (1985) and Markey (2004). For all these fragments, except for the trivial case without any temporal operator, we systematically prove model checking to be either inherently sequential (P-complete) or very efficiently parallelizable (LOGCFL-complete). For most fragments, however, model checking for CTL is already P-complete. Hence our results indicate that in most applications, approaching CTL model checking by parallelism will not result in the desired speed up. We also completely determine the complexity of the model checking problem for all fragments of the extensions ECTL, CTL +, and ECTL +
The effects of ram-pressure stripping on the internal kinematics of simulated spiral galaxies
We investigate the influence of ram-pressure stripping on the internal gas
kinematics of simulated spiral galaxies. Additional emphasis is put on the
question of how the resulting distortions of the gaseous disc are visible in
the rotation curve and/or the full 2D velocity field of galaxies at different
redshifts. A Milky-Way type disc galaxy is modelled in combined
N-body/hydrodynamic simulations with prescriptions for cooling, star formation,
stellar feedback, and galactic winds. This model galaxy moves through a
constant density and temperature gas, which has parameters similar to the
intra-cluster medium (ICM). Rotation curves (RCs) and 2D velocity fields of the
gas are extracted from these simulations in a way that follows the procedure
applied to observations of distant, small, and faint galaxies as closely as
possible. We find that the appearance of distortions of the gaseous disc due to
ram-pressure stripping depends on the direction of the acting ram pressure. In
the case of face-on ram pressure, the distortions mainly appear in the outer
parts of the galaxy in a very symmetric way. In contrast, in the case of
edge-on ram pressure we find stronger distortions. The 2D velocity field also
shows signatures of the interaction in the inner part of the disc. At angles
smaller than 45 degrees between the ICM wind direction and the disc, the
velocity field asymmetry increases significantly compared to larger angles.
Compared to distortions caused by tidal interactions, the effects of
ram-pressure stripping on the velocity field are relatively low in all cases
and difficult to observe at intermediate redshift in seeing-limited
observations. (abridged)Comment: 9 pages, 11 figures, accepted for publication in A&
Spectral gaps for a Metropolis-Hastings algorithm in infinite dimensions
We study the problem of sampling high and infinite dimensional target
measures arising in applications such as conditioned diffusions and inverse
problems. We focus on those that arise from approximating measures on Hilbert
spaces defined via a density with respect to a Gaussian reference measure. We
consider the Metropolis-Hastings algorithm that adds an accept-reject mechanism
to a Markov chain proposal in order to make the chain reversible with respect
to the target measure. We focus on cases where the proposal is either a
Gaussian random walk (RWM) with covariance equal to that of the reference
measure or an Ornstein-Uhlenbeck proposal (pCN) for which the reference measure
is invariant. Previous results in terms of scaling and diffusion limits
suggested that the pCN has a convergence rate that is independent of the
dimension while the RWM method has undesirable dimension-dependent behaviour.
We confirm this claim by exhibiting a dimension-independent Wasserstein
spectral gap for pCN algorithm for a large class of target measures. In our
setting this Wasserstein spectral gap implies an -spectral gap. We use
both spectral gaps to show that the ergodic average satisfies a strong law of
large numbers, the central limit theorem and nonasymptotic bounds on the mean
square error, all dimension independent. In contrast we show that the spectral
gap of the RWM algorithm applied to the reference measures degenerates as the
dimension tends to infinity.Comment: Published in at http://dx.doi.org/10.1214/13-AAP982 the Annals of
Applied Probability (http://www.imstat.org/aap/) by the Institute of
Mathematical Statistics (http://www.imstat.org
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