34,693,936 research outputs found
Calculation of statistical entropic measures in a model of solids
In this work, a one-dimensional model of crystalline solids based on the
Dirac comb limit of the Kronig-Penney model is considered. From the wave
functions of the valence electrons, we calculate a statistical measure of
complexity and the Fisher-Shannon information for the lower energy electronic
bands appearing in the system. All these magnitudes present an extremal value
for the case of solids having half-filled bands, a configuration where in
general a high conductivity is attained in real solids, such as it happens with
the monovalent metals.Comment: 9 pages, 4 figure
Microlensing induced spectral variability in Q2237+0305
We present both photometry and spectra of the individual images of the
quadruple gravitational lens system Q2237+0305. Comparison of spectra obtained
at two epochs, separated by years, shows evidence for significant
changes in the emission line to continuum ratio of the strong ultraviolet
CIV~1549, CIII]~1909 and MgII~2798 lines. The short,
day, light--travel time differences between the sight lines to the
four individual quasar images rule out any explanation based on intrinsic
variability of the source. The spectroscopic differences thus represent direct
detection of microlensing--induced spectroscopic differences in a quasar. The
observations allow constraints to be placed on the relative spatial scales in
the nucleus of the quasar, with the ultra--violet continuum arising in a region
of \la~0.05~{\rm pc} in extent, while the broad emission line material is
distributed on scales much greater than this.Comment: Accepted for Publication in MNRAS. Paper with 11 figure
Gamma-Rays and the Far-Infrared-Radio Continuum Correlation Reveal a Powerful Galactic Centre Wind
We consider the thermal and non-thermal emission from the inner 200 pc of the
Galaxy. The radiation from this almost star-burst-like region is ultimately
driven dominantly by on-going massive star formation. We show that this
region's radio continuum (RC) emission is in relative deficit with respect to
the expectation afforded by the Far- infrared-Radio Continuum Correlation
(FRC). Likewise we show that the region's gamma-ray emission falls short of
that expected given its star formation and resultant supernova rates. These
facts are compellingly explained by positing that a powerful (400-1200 km/s)
wind is launched from the region. This wind probably plays a number of
important roles including advecting positrons into the Galactic bulge thus
explaining the observed ~kpc extension of the 511 keV positron annihilation
signal around the GC. We also show that the large-scale GC magnetic field falls
in the range ~100-300 microG and that - in the time they remain in the region -
GC cosmic rays do not penetrate into the region's densest molecular material.Comment: Version accepted for publication in MNRAS Letters. Discussion
extended and references adde
Structure of Protocluster Galaxies: Accelerated Structural Evolution in Overdense Environments?
We present a high spatial-resolution HST/NICMOS imaging survey in the field
of a known protocluster surrounding the powerful radio galaxy MRC1138-262 at
z=2.16. Previously, we have shown that this field exhibits a substantial
surface overdensity of red J-H galaxies. Here we focus on the stellar masses
and galaxy effective radii in an effort to compare and contrast the properties
of likely protocluster galaxies with their field counterparts and to look for
correlations between galaxy structure and (projected) distance relative to the
radio galaxy.
We find a hint that quiescent, cluster galaxies are on average less dense
than quiescent field galaxies of similar stellar mass and redshift. In fact, we
find only two (of nine) quiescent protocluster galaxies are of simliar density
to the majority of the massive, quiescent compact galaxies (SEEDs) found in
several field surveys. Furthermore, there is some indication that the
structural Sersic n parameter is higher (n ~ 3-4) on average for cluster
galaxies compared to the field SEEDs (n ~ 1-2) This result may imply that the
accelerated galaxy evolution expected (and observed) in overdense regions also
extends to structural evolution presuming that massive galaxies began as dense
(low n) SEEDs and have already evolved to be more in line with local galaxies
of the same stellar mass.Comment: 11 pages, 7 figures, 1 table, Accepted for publication in Ap
A solution to the problems of cusps and rotation curves in dark matter halos in the cosmological standard model
We discuss various aspects of the inner structure formation in virialized
dark matter (DM) halos that form as primordial density inhomogeneities evolve
in the cosmological standard model. The main focus is on the study of central
cusps/cores and of the profiles of DM halo rotation curves, problems that
reveal disagreements among the theory, numerical simulations, and observations.
A method that was developed by the authors to describe equilibrium DM systems
is presented, which allows investigating these complex nonlinear structures
analytically and relating density distribution profiles within a halo both to
the parameters of the initial small-scale inhomogeneity field and to the
nonlinear relaxation characteristics of gravitationally compressed matter. It
is shown that cosmological random motions of matter `heat up' the DM particles
in collapsing halos, suppressing cusp-like density profiles within developing
halos, facilitating the formation of DM cores in galaxies, and providing an
explanation for the difference between observed and simulated galactic rotation
curves. The analytic conclusions obtained within this approach can be confirmed
by the N-body model simulation once improved spatial resolution is achieved for
central halo regions.Comment: 44 pages, 16 figures, 1 tabl
Infinite dimensional non-positively curved symmetric spaces of finite rank
This paper concerns a study of three families of non-compact type symmetric
spaces of infinite dimension. Although they have infinite dimension they have
finite rank. More precisely, we show they have finite telescopic dimension. We
also show the existence of Furstenberg maps for some group actions on these
spaces. Such maps appear as a first step toward superrigidity results.Comment: Some references have been adde
Oscillatory Finite-Time Singularities in Finance, Population and Rupture
We present a simple two-dimensional dynamical system where two nonlinear
terms, exerting respectively positive feedback and reversal, compete to create
a singularity in finite time decorated by accelerating oscillations. The power
law singularity results from the increasing growth rate. The oscillations
result from the restoring mechanism. As a function of the order of the
nonlinearity of the growth rate and of the restoring term, a rich variety of
behavior is documented analytically and numerically. The dynamical behavior is
traced back fundamentally to the self-similar spiral structure of trajectories
in phase space unfolding around an unstable spiral point at the origin. The
interplay between the restoring mechanism and the nonlinear growth rate leads
to approximately log-periodic oscillations with remarkable scaling properties.
Three domains of applications are discussed: (1) the stock market with a
competition between nonlinear trend-followers and nonlinear value investors;
(2) the world human population with a competition between a
population-dependent growth rate and a nonlinear dependence on a finite
carrying capacity; (3) the failure of a material subjected to a time-varying
stress with a competition between positive geometrical feedback on the damage
variable and nonlinear healing.Comment: Latex document of 59 pages including 20 eps figure
HST, radio and infrared observations of 28 3CR radio galaxies at redshift z ~ 1: I. Old stellar populations in central cluster galaxies
Hubble Space Telescope images of 3CR radio galaxies at redshifts 0.6 < z <
1.8 have shown a remarkable variety of structures, generally aligned along the
radio axis, indicating that the radio source strongly influences the optical
appearance of these galaxies. In this paper we investigate the host galaxies
underlying this aligned emission, combining the HST data with ground-based
infrared images. An investigation of the spectral energy distributions of the
galaxies shows that the contribution of the aligned blue component to the
K--band light is generally small (about 10%). The radial intensity profiles of
the galaxies are well matched at radii <~ 35 kpc by de Vaucouleurs' law,
demonstrating that the K--band light is dominated by that of an elliptical
galaxy. There is no evidence for a nuclear point source, in addition to the de
Vaucouleurs profile, with a contribution >~15% of the total K--band flux
density, except in two cases, 3C22 and 3C41. Large characteristic radii are
derived, indicating that the 3CR galaxies must be highly evolved dynamically,
even at a redshift of one. At radii > 35 kpc, a combined galaxy profile clearly
shows an excess of emission reminiscent of cD--type halos. This supports other
independent evidence for the hypothesis that the distant 3CR galaxies lie in
moderately rich (proto--)clusters. Since the nearby FR II galaxies in the 3CR
catalogue lie in more diffuse environments and do not possess cD halos, the
galactic environments of the 3CR galaxies must change with redshift. The K-z
relation of the 3CR galaxies cannot, therefore, be interpreted using a standard
`closed-box, passive stellar evolution' model. We offer a new interpretation,
and compare the model with the K-z relations of lower power radio galaxies and
brightest cluster galaxies. (abridged)Comment: 21 pages including 13 figures, LaTeX. To appear in MNRA
Synaptobrevin cleavage by the tetanus toxin light chain is linked to the inhibition of exocytosis in chromaffin cells
Exocytosis of secretory granules by adrenal chromaffin cells is blocked by the tetanus toxin light chain in a zinc specific manner. Here we show that cellular synaptobrevin is almost completely degraded by the tetanus toxin light chain within 15 min. We used highly purified adrenal secretory granules to show that synaptobrevin, which can be cleaved by the tetanus toxin light chain, is localized in the vesicular membrane. Proteolysis of synaptobrevin in cells and in secretory granules is reversibly inhibited by the zinc chelating agent dipicolinic acid. Moreover, cleavage of synaptobrevin present in secretory granules by the tetanus toxin light chain is blocked by the zinc peptidase inhibitor captopril and by synaptobrevin derived peptides. Our data indicate that the tetanus toxin light chain acts as a zinc dependent protease that cleaves synaptobrevin of secretory granules, an essential component of the exocytosis machinery in adrenal chromaffin cells
Star Formation and Gas Dynamics in Galactic Disks: Physical Processes and Numerical Models
Star formation depends on the available gaseous "fuel" as well as galactic
environment, with higher specific star formation rates where gas is
predominantly molecular and where stellar (and dark matter) densities are
higher. The partition of gas into different thermal components must itself
depend on the star formation rate, since a steady state distribution requires a
balance between heating (largely from stellar UV for the atomic component) and
cooling. In this presentation, I discuss a simple thermal and dynamical
equilibrium model for the star formation rate in disk galaxies, where the basic
inputs are the total surface density of gas and the volume density of stars and
dark matter, averaged over ~kpc scales. Galactic environment is important
because the vertical gravity of the stars and dark matter compress gas toward
the midplane, helping to establish the pressure, and hence the cooling rate. In
equilibrium, the star formation rate must evolve until the gas heating rate is
high enough to balance this cooling rate and maintain the pressure imposed by
the local gravitational field. In addition to discussing the formulation of
this equilibrium model, I review the current status of numerical simulations of
multiphase disks, focusing on measurements of quantities that characterize the
mean properties of the diffuse ISM. Based on simulations, turbulence levels in
the diffuse ISM appear relatively insensitive to local disk conditions and
energetic driving rates, consistent with observations. It remains to be
determined, both from observations and simulations, how mass exchange processes
control the ratio of cold-to-warm gas in the atomic ISM.Comment: 8 pages, 1 figure; to appear in "IAU Symposium 270: Computational
Star formation", Eds. J. Alves, B. Elmegreen, J. Girart, V. Trimbl
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