162 research outputs found
Influence of β-diiminato Ligand Design in Transition Metal Mediated E4 (E = P, As) Activation
This work provides a detailed insight into the reactivity of late transition metal (Fe, Co, Cu) complexes towards white phosphorus (P4) and yellow arsenic (As4). Of particular interest is the application of differently substituted β-diiminato supporting ligands and their influence on the En ligand complex formation (E = P, As)
Conduction and cooling flows
Chandra and XMM-Newton observations have confirmed the presence of large
temperature gradients within the cores of many relaxed clusters of galaxies.
Here we investigate whether thermal conduction operating over those gradients
can supply sufficient heat to offset radiative cooling. Narayan & Medvedev
(2001) and Gruzinov (2002) have noted, using published results on cluster
temperatures, that conduction within a factor of a few of the Spitzer rate is
sufficient to balance bremsstrahlung cooling. From a detailed study of the
temperature and emission measure profiles of Abell 2199 and Abell 1835, we find
that the heat flux required by conduction is consistent with or below the rate
predicted by Spitzer in the outer regions of the core. Conduction may therefore
explain the lack of observational evidence for large mass cooling rates
inferred from arguments based simply on radiative cooling, provided that
conductivity is suppressed by no more than a factor of three below the full
Spitzer rate. To stem cooling in the cluster centre, however, would necessitate
conductivity values at least a factor of two larger than the Spitzer values,
which we consider implausible. This may provide an explanation for the observed
star formation and optical nebulosities in cluster cores. The solution is
likely to be time dependent. We briefly discuss the possible origin of the
cooler gas and the implications for massive galaxies.Comment: 5 pages, 4 figures, accepted by MNRAS. Minor changes following
referee's comment
On viscosity, conduction and sound waves in the intracluster medium
Recent X-ray and optical observations of the Perseus cluster indicate that
the viscous and conductive dissipation of sound waves is the mechanism
responsible for heating the intracluster medium and thus balancing radiative
cooling of cluster cores. We discuss this mechanism more generally and show how
the specific heating and cooling rates vary with temperature and radius. It
appears that the heating mechanism is most effective above 10^7K, which allows
for radiative cooling to proceed within normal galaxy formation but will stifle
the growth of very massive galaxies. The scaling of the wavelength of sound
waves with cluster temperature and feedback in the system are investigated.Comment: 5 pages, 4 figures, MNRAS accepte
On conduction, cooling flows and galaxy formation
On the basis of the universal gas fraction in clusters of galaxies, we
estimate that the effective thermal conductivity required to balance radiative
cooling in the cores, where the gas temperature is 3-10keV, is about one tenth
of the Spitzer rate. This confirms that thermal conduction can be important for
the energy balance provided that it is not highly suppressed by magnetic fields
in the gas. We determine the global effective conductivity in a sample of 29
clusters using published X-ray data on the inferred cooling rates and show that
most lie between one and one tenth of the Spitzer rate. More work on the
profiles in cooling flow clusters is required to test the conduction hypothesis
further. We examine the possibility that conduction operates during galaxy
formation, and show that it provides a simple explanation for the upper-mass
cutoff in galaxy masses.Comment: 4 pages, 3 figures, submitted to MNRA
Radio pulsar binaries in globular clusters: their orbital eccentricities and stellar interactions
High sensitivity searches of globular clusters (GC) for radio pulsars by
improved pulsar search algorithms and sustained pulsar timing observations have
so far yielded some 140 pulsars in more than two dozen GCs. The observed
distribution of orbital eccentricity and period of binary radio pulsars in GCs
have imprints of the past interaction between single pulsars and binary systems
or of binary pulsars and single passing non-compact stars. It is seen that GCs
have different groups of pulsars. These may have arisen out of exchange or
merger of a component of the binary with the incoming star or a "fly-by" in
which the original binary remains intact but undergoes a change of eccentricity
and orbital period. We consider the genesis of the distribution of pulsars
using analytical and computational tools such as STARLAB, which performs
numerical scattering experiments with direct N-body integration. Cluster
pulsars with intermediate eccentricities can mostly be accounted for by fly-bys
whereas those with high eccentricities are likely to be the result of exchanges
and/or mergers of single stars with the binary companion of the pulsar,
although there are a few objects which do not easily fit into this description.
The corresponding distribution for galactic field pulsars shows notable
differences from the GC pulsar orbital period and eccentricity distribution.
The long orbital period pulsars in the galactic field with frozen out low
eccentricities are largely missing from the globular clusters, and we show that
ionization of these systems in GCs cannot alone account for the peculiarities.Comment: Accepted for publication in Ap
Cold filaments in galaxy clusters: effects of heat conduction
We determine the critical size l_crit of a filament of cold (T~10^4 K) gas
that is in radiative equilibrium with X-ray emitting gas at temperatures
T_out~10^6 - 10^8 K. Filaments smaller than l_crit will be rapidly evaporated,
while longer ones will induce the condensation of the ambient medium. At fixed
pressure P, l_crit increases as T_out^(11/4), while at fixed T_out it scales as
1/P. It scales as f^(1/2), where f is the factor by which the magnetic field
depresses the thermal conductivity below Spitzer's benchmark value. For
plausible values of f, l_crit is similar to the lengths of observed filaments.
In a cluster such as Perseus, the value of l_crit increases by over an order of
magnitude between the centre and a radius of 100 kpc. If the spectrum of seed
filament lengths l is strongly falling with l, as is natural, then these
results explain why filaments are only seen within a few kiloparsecs of the
centres of clusters, and are not seen in clusters that have no cooling flow. We
calculate the differential emission measure as a function of temperature for
the interface between filaments and ambient gas of various temperatures. We
discuss the implications of our results for the origin of the galaxy luminosity
function.Comment: 8 pages with 5 figures, MNRAS (submitted
Noise transfer for unsupervised domain adaptation of retinal OCT images
Optical coherence tomography (OCT) imaging from different camera devices
causes challenging domain shifts and can cause a severe drop in accuracy for
machine learning models. In this work, we introduce a minimal noise adaptation
method based on a singular value decomposition (SVDNA) to overcome the domain
gap between target domains from three different device manufacturers in retinal
OCT imaging. Our method utilizes the difference in noise structure to
successfully bridge the domain gap between different OCT devices and transfer
the style from unlabeled target domain images to source images for which manual
annotations are available. We demonstrate how this method, despite its
simplicity, compares or even outperforms state-of-the-art unsupervised domain
adaptation methods for semantic segmentation on a public OCT dataset. SVDNA can
be integrated with just a few lines of code into the augmentation pipeline of
any network which is in contrast to many state-of-the-art domain adaptation
methods which often need to change the underlying model architecture or train a
separate style transfer model. The full code implementation for SVDNA is
available at https://github.com/ValentinKoch/SVDNA.Comment: published at MICCAI 202
Ram pressure stripping in a viscous intracluster medium
In the recent literature there is circumstantial evidence that the viscosity
of the intracluster medium may not be too far from the Spitzer value. In this
letter, we present two-dimensional hydrodynamical simulations of ram pressure
stripping of disc galaxies in a viscous intracluster medium. The values of
viscosity explored range between 0.1 and 1.0 times the Spitzer value. We find
that viscosity affects the appearance and the dimensions of the galactic wakes
but has very little effect on the evolution of the gas mass of the galaxy.Comment: 5 pages, 2 figures, accepted by MNRAS letter
A Chandra Study of Temperature Substructures in Intermediate-Redshift Galaxy Clusters
By analyzing the gas temperature maps created from the Chandra archive data,
we reveal the prevailing existence of temperature substructures on ~100 kpc
scales in the central regions of nine intermediate-redshift (z~0.1) galaxy
clusters, which resemble those found in the Virgo and Coma Clusters. Each
substructure contains a clump of hot plasma whose temperature is about 2-3 keV
higher than the environment, corresponding to an excess thermal energy of
~1E58-1E60 erg per clump. Since if there were no significant non-gravitational
heating sources, these substructures would have perished in 1E8-1E9 yrs due to
thermal conduction and turbulent flows, whose velocity is found to range from
about 200 to 400 km/s, we conclude that the substructures cannot be created and
sustained by inhomogeneous radiative cooling. We also eliminate the
possibilities that the temperature substructures are caused by supernova
explosions, or by the non-thermal X-ray emission due to the
inverse-Comptonization of the CMB photons. By calculating the rising time of
AGN-induced buoyant bubbles, we speculate that the intermittent AGN outbursts
(~ 1E60 erg per burst) may have played a crucial role in the forming of the
high temperature substructures. Our results are supported by recent study of
McNamara & Nulsen (2007), posing a tight observational constraint on future
theoretical and numerical studies.Comment: 31 pages, 7 figures, ApJ accepte
The relationship between the optical Halpha filaments and the X-ray emission in the core of the Perseus cluster
NGC 1275 in the centre of the Perseus cluster of galaxies, Abell 426, is
surrounded by a spectacular filamentary Halpha nebula. Deep Chandra X-ray
imaging has revealed that the brighter outer filaments are also detected in
soft X-rays. This can be due to conduction and mixing of the cold gas in the
filaments with the hot, dense intracluster medium. We show the correspondence
of the filaments in both wavebands and draw attention to the relationship of
two prominent curved NW filaments to an outer, buoyant radio bubble seen as a
hole in the X-ray image. There is a strong resemblance in the shape of the hole
and the disposition of the filaments to the behaviour of a large air bubble
rising in water. If this is a correct analogy, then the flow is laminar and the
intracluster gas around this radio source is not turbulent. We obtain a limit
on the viscosity of this gas.Comment: Accepted for publication in MNRA
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