5,312 research outputs found
Cores and Cusps in the Dwarf Spheroidals
We consider the problem of determining the structure of the dark halo of
nearby dwarf spheroidal galaxies (dSphs) from the spherical Jeans equations.
Whether the dark halos are cusped or cored at the centre is an important
strategic problem in modern astronomy. The observational data comprise the
line-of-sight velocity dispersion of a luminous tracer population. We show that
when such data are analysed to find the dark matter density with the spherical
Poisson and Jeans equations, then the generic solution is a dark halo density
that is cusped like an isothermal. Although milder cusps (like the
Navarro-Frenk-White 1/r cusp and even cores are possible, they are not generic.
Such solutions exist only if the anisotropy parameter beta and the logarithmic
slope of the stellar density gamma satisfy the constraint gamma = 2 x beta at
the centre or if the radial velocity dispersion falls to zero at the centre.
This surprisingly strong statement is really a consequence of the assumption of
spherical symmetry, and the consequent coordinate singularity at the origin.
So, for example, a dSph with an exponential light profile can exist in
Navarro-Frenk- White halo and have a flat velocity dispersion, but anisotropy
in general drives the dark halo solution to an isothermal cusp. The identified
cusp or core is therefore a consequence of the assumptions (particularly of
spherical symmetry and isotropy), and not the data.Comment: MNRAS, in pres
A Survey for EHB Stars in the Galactic Bulge
We present a progress report on an extensive survey to find and characterize
all types of blue horizontal-branch stars in the nuclear bulge of the Galaxy.
We have obtained wide, shallow imaging in UBV of ~12 square degrees in the
bulge, with follow-up spectroscopy for radial velocities and metal abundance
determinations. We have discovered a number of metal-rich blue HB stars, whose
presence in the bulge is expected by the interpretation of the extragalactic
ultraviolet excess. Very deep images have been obtained in UBV and SDSS u along
the bulge minor axis, which reveal a significant number of EHB candidates
fainter than B = 19, i.e., with the same absolute magnitudes as EHB stars in
several globular clusters.Comment: To appear in "Extreme Horizontal Branch Stars and Related Objects",
Astrophysics and Space Science, Kluwer Academic Publishers, proceedings of
the meeting held in Keele, UK, June 16-20, 200
Analytical Solutions to the Mass-Anisotropy Degeneracy with Higher Order Jeans Analysis: A General Method
The Jeans analysis is often used to infer the total density of a system by
relating the velocity moments of an observable tracer population to the
underlying gravitational potential. This technique has recently been applied in
the search for Dark Matter in objects such as dwarf spheroidal galaxies where
the presence of Dark Matter is inferred via stellar velocities. A precise
account of the density is needed to constrain the expected gamma ray flux from
DM self-annihilation and to distinguish between cold and warm dark matter
models. Unfortunately the traditional method of fitting the second order Jeans
equation to the tracer dispersion suffers from an unbreakable degeneracy of
solutions due to the unknown velocity anisotropy of the projected system. To
tackle this degeneracy one can appeal to higher moments of the Jeans equation.
By introducing an analog to the Binney anisotropy parameter at fourth order,
beta' we create a framework that encompasses all solutions to the fourth order
Jeans equations rather than those in the literature that impose unnecessary
correlations between anisotropy of second and fourth order moments. The
condition beta' = f(beta) ensures that the degeneracy is lifted and we
interpret the separable augmented density system as the order-independent case
beta'= beta. For a generic choice of beta' we present the line of sight
projection of the fourth moment and how it could be incorporated into a joint
likelihood analysis of the dispersion and kurtosis. Having presented the
mathematical framework, we then use it to develop a statistical method for the
purpose of placing constraints on dark matter density parameters from discrete
velocity data. The method is tested on simulated dwarf spheroidal data sets
leading to results which motivate study of real dwarf spheroidal data sets.Comment: 21 pages, 15 figures. Accepted by MNRAS. Typo corrected in eq. 3
The Algorithmic Origins of Life
Although it has been notoriously difficult to pin down precisely what it is
that makes life so distinctive and remarkable, there is general agreement that
its informational aspect is one key property, perhaps the key property. The
unique informational narrative of living systems suggests that life may be
characterized by context-dependent causal influences, and in particular, that
top-down (or downward) causation -- where higher-levels influence and constrain
the dynamics of lower-levels in organizational hierarchies -- may be a major
contributor to the hierarchal structure of living systems. Here we propose that
the origin of life may correspond to a physical transition associated with a
shift in causal structure, where information gains direct, and
context-dependent causal efficacy over the matter it is instantiated in. Such a
transition may be akin to more traditional physical transitions (e.g.
thermodynamic phase transitions), with the crucial distinction that determining
which phase (non-life or life) a given system is in requires dynamical
information and therefore can only be inferred by identifying causal
architecture. We discuss some potential novel research directions based on this
hypothesis, including potential measures of such a transition that may be
amenable to laboratory study, and how the proposed mechanism corresponds to the
onset of the unique mode of (algorithmic) information processing characteristic
of living systems.Comment: 13 pages, 1 tabl
Characterisation and imaging of cortical impedance changes during interictal and ictal activity in the anaesthetised rat.
Epilepsy affects approximately 50 million people worldwide, and 20-30% of these cases are refractory to antiepileptic drugs. Many patients with intractable epilepsy can benefit from surgical resection of the tissue generating the seizures; however, difficulty in precisely localising seizure foci has limited the number of patients undergoing surgery as well as potentially lowered its effectiveness. Here we demonstrate a novel imaging method for monitoring rapid changes in cerebral tissue impedance occurring during interictal and ictal activity, and show that it can reveal the propagation of pathological activity in the cortex. Cortical impedance was recorded simultaneously to ECoG using a 30-contact electrode mat placed on the exposed cortex of anaesthetised rats, in which interictal spikes (IISs) and seizures were induced by cortical injection of 4-aminopyridine (4-AP), picrotoxin or penicillin. We characterised the tissue impedance responses during IISs and seizures, and imaged these responses in the cortex using Electrical Impedance Tomography (EIT). We found a fast, transient drop in impedance occurring as early as 12ms prior to the IISs, followed by a steep rise in impedance within ~120ms of the IIS. EIT images of these impedance changes showed that they were co-localised and centred at a depth of 1mm in the cortex, and that they closely followed the activity propagation observed in the surface ECoG signals. The fast, pre-IIS impedance drop most likely reflects synchronised depolarisation in a localised network of neurons, and the post-IIS impedance increase reflects the subsequent shrinkage of extracellular space caused by the intense activity. EIT could also be used to picture a steady rise in tissue impedance during seizure activity, which has been previously described. Thus, our results demonstrate that EIT can detect and localise different physiological changes during interictal and ictal activity and, in conjunction with ECoG, may in future improve the localisation of seizure foci in the clinical setting
Modified Virial Formulae and the Theory of Mass Estimators
We show how to estimate the enclosed mass from the observed motions of an
ensemble of test particles. Traditionally, this problem has been attacked
through virial or projected mass estimators. Here, we examine and extend these
systematically, and show how to construct an optimal estimator for any given
assumption as to the potential. The estimators do not explicitly depend on any
properties of the density of the test objects, which is desirable as in
practice such information is dominated by selection effects. As particular
examples, we also develop estimators tailored for the problem of estimating the
mass of the Hernquist or NFW dark matter haloes from the projected positions
and velocities of stars.Comment: 9 pages, MNRAS, in pres
Strong Electron-Phonon Coupling in Superconducting MgB: A Specific Heat Study
We report on measurements of the specific heat of the recently discovered
superconductor MgB in the temperature range between 3 and 220 K. Based on a
modified Debye-Einstein model, we have achieved a rather accurate account of
the lattice contribution to the specific heat, which allows us to separate the
electronic contribution from the total measured specific heat. From our result
for the electronic specific heat, we estimate the electron-phonon coupling
constant to be of the order of 2, significantly enhanced compared to
common weak-coupling values . Our data also indicate that the
electronic specific heat in the superconducting state of MgB can be
accounted for by a conventional, s-wave type BCS-model.Comment: 4 pages, 4 figure
Distribution of Endogenous Farnesyl Pyrophosphate and Four Species of Lysophosphatidic Acid in Rodent Brain
Lysophosphatidic acid (LPA) is the umbrella term for lipid signaling molecules that share structural homology and activate the family of LPA receptors. Farnesyl Pyrophosphate (FPP) is commonly known as an intermediate in the synthesis of steroid hormones; however, its function as a signaling lipid is beginning to be explored. FPP was recently shown to an activator of the G-protein coupled receptor 92 (also known as LPA5) of the calcium channel TRPV3. The LPA receptors (including GPR92) are associated with the signal transduction of noxious stimuli, however, very little is known about the distribution of their signaling ligands (LPAs and FPP) in the brain. Here, using HPLC/MS/MS, we developed extraction and analytical methods for measuring levels of FPP and 4 species of LPA (palmitoyl, stearoyl, oleoyl and arachidonoyl-sn-glycerol-3 phosphate) in rodent brain. Relative distributions of each of the five compounds was significantly different across the brain suggesting divergent functionality for each as signaling molecules based on where and how much of each is being produced. Brainstem, midbrain, and thalamus contained the highest levels measured for each compound, though none in the same ratios while relatively small amounts were produced in cortex and cerebellum. These data provide a framework for investigations into functional relationships of these lipid ligands in specific brain areas, many of which are associated with the perception of pain
Articular contact in a three-dimensional model of the knee
This study is aimed at the analysis of articular contact in a three-dimensional mathematical model of the human knee-joint. In particular the effect of articular contact on the passive motion characteristics is assessed in relation to experimentally obtained joint kinematics. Two basically different mathematical contact descriptions were compared for this purpose. One description was for rigid contact and one for deformable contact. The description of deformable contact is based on a simplified theory for contact of a thin elastic layer on a rigid foundation. The articular cartilage was described either as a linear elastic material or as a non-linear elastic material. The contact descriptions were introduced in a mathematical model of the knee. The locations of the ligament insertions and the geometry of the articular surfaces were obtained from a joint specimen of which experimentally determined kinematic data were available, and were used as input for the model. The ligaments were described by non-linear elastic line elements. The mechanical properties of the ligaments and the articular cartilage were derived from literature data. Parametric model evaluations showed that, relative to rigid articular contact, the incorporation of deformable contact did not alter the motion characteristics in a qualitative sense, and that the quantitative changes were small. Variation of the elasticity of the elastic layer revealed that decreasing the surface stiffness caused the ligaments to relax and, as a consequence, increased the joint laxity, particularly for axial rotation. The difference between the linear and the non-linear deformable contact in the knee model was very small for moderate loading conditions. The motion characteristics simulated with the knee model compared very well with the experiments. It is concluded that for simulation of the passive motion characteristics of the knee, the simplified description for contact of a thin linear elastic layer on a rigid foundation is a valid approach when aiming at the study of the motion characteristics for moderate loading conditions. With deformable contact in the knee model, geometric conformity between the surfaces can be modelled as opposed to rigid contact which assumed only point contact
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