9,518 research outputs found
A comparison of the distribution of actin and tubulin in the mammalian mitotic spindle as seen by indirect immunofluorescence
Rabbit antibodies against actin and tubulin were used in an indirect immunofluorescence study of the structure of the mitotic spindle of PtK1 cells after lysis under conditions that preserve anaphase chromosome movement. During early prophase there is no antiactin staining associated with the mitotic centers, but by late prophase, as the spindle is beginning to form, a small ball of actin antigenicity is found beside the nucleus; After nuclear envelope breakdown, the actiactin stains the region around each mitotic center, and becomes organized into fibers that run between the chromosomes and the poles. Colchicine blocks this organization, but does not disrupt the staining at the poles. At metaphase the antiactin reveals a halo of ill-defined radius around each spindle pole and fibers that run from the poles to the metaphase plate. Antitubulin shows astral rays, fibers running from chromosomes to poles, and some fibers that run across the metaphase plate. At anaphase, there is a shortening of the antiactin-stained fibers, leaving a zone which is essentially free of actin-staining fluorescence between the separating chromosomes. Antitubulin stains the region between chromosomes and poles, but also reveals substantial fibers running through the zone between separating chromosomes. Cells fixed during cytokinesis show actin in the region of the cleavage furrow, while antitubulin reveals the fibrous spindle remnant that runs between daughter cells. These results suggest that actin is a component of the mammalian mitotic spindle, that the distribution of actin differs from that of tubulin and that the distributions of these two fibrous proteins change in different ways during anaphase
`Mass without mass' from thin shells in Gauss-Bonnet gravity
Five tensor equations are obtained for a thin shell in Gauss-Bonnet gravity.
There is the well known junction condition for the singular part of the stress
tensor intrinsic to the shell, which we also prove to be well defined. There
are also equations relating the geometry of the shell (jump and average of the
extrinsic curvature as well as the intrinsic curvature) to the non-singular
components of the bulk stress tensor on the sides of the thin shell.
The equations are applied to spherically symmetric thin shells in vacuum. The
shells are part of the vacuum, they carry no energy tensor. We classify these
solutions of `thin shells of nothingness' in the pure Gauss-Bonnet theory.
There are three types of solutions, with one, zero or two asymptotic regions
respectively. The third kind of solution are wormholes. Although vacuum
solutions, they have the appearance of mass in the asymptotic regions. It is
striking that in this theory, exotic matter is not needed in order for
wormholes to exist- they can exist even with no matter.Comment: 13 pages, RevTex, 8 figures. Version 2: includes discussion on the
well-defined thin shell limit. Version 3: typos fixed, a reference added,
accepted for publication in Phys. Rev.
Clustering of the Diffuse Infrared Light from the COBE DIRBE maps. III. Power spectrum analysis and excess isotropic component of fluctuations
The cosmic infrared background (CIB) radiation is the cosmic repository for
energy release throughout the history of the universe. Using the all-sky data
from the COBE DIRBE instrument at wavelengths 1.25 - 100 mic we attempt to
measure the CIB fluctuations. In the near-IR, foreground emission is dominated
by small scale structure due to stars in the Galaxy. There we find a strong
correlation between the amplitude of the fluctuations and Galactic latitude
after removing bright foreground stars. Using data outside the Galactic plane
() and away from the center () we extrapolate
the amplitude of the fluctuations to cosec. We find a positive intercept
of nW/m2/sr at 1.25, 2.2,3.5 and 4.9 mic
respectively, where the errors are the range of 92% confidence limits. For
color subtracted maps between band 1 and 2 we find the isotropic part of the
fluctuations at nW/m2/sr. Based on detailed numerical and
analytic models, this residual is not likely to originate from the Galaxy, our
clipping algorithm, or instrumental noise. We demonstrate that the residuals
from the fit used in the extrapolation are distributed isotropically and
suggest that this extra variance may result from structure in the CIB. For
2\deg< \theta < 15^\deg, a power-spectrum analysis yields firm upper limits
of (\theta/5^\deg) \times\delta F_{\rm rms} (\theta) < 6, 2.5, 0.8, 0.5
nW/m2/sr at 1.25, 2.2, 3.5 and 4.9 mic respectively. From 10-100 mic, the upper
limits <1 nW/m2/sr.Comment: Ap.J., in press. 69 pages including 24 fig
The Star Formation Law in Nearby Galaxies on Sub-Kpc Scales
(Abridged) We present a comprehensive analysis of the relationship between
star formation rate surface density (SFR SD) and gas surface density (gas SD)
at sub-kpc resolution in a sample of 18 nearby galaxies. We use high resolution
HI data from THINGS, CO data from HERACLES and BIMA SONG, 24 micron data from
the Spitzer Space Telescope, and UV data from GALEX. We target 7 spiral
galaxies and 11 late-type/dwarf galaxies and investigate how the star formation
law differs between the H2-dominated centers of spiral galaxies, their
HI-dominated outskirts and the HI-rich late-type/dwarf galaxies.
We find that a Schmidt-type power law with index N=1.0+-0.2 relates the SFR
SD and the H2 SD across our sample of spiral galaxies, i.e., that H2 forms
stars at a constant efficiency in spirals. The average molecular gas depletion
time is ~2*10^9 yrs. We interpret the linear relation and constant depletion
time as evidence that stars are forming in GMCs with approximately uniform
properties and that the H2 SD may be more a measure of the filling fraction of
giant molecular clouds than changing conditions in the molecular gas.
The relationship between total gas SD and SFR SD varies dramatically among
and within spiral galaxies. Most galaxies show little or no correlation between
the HI SD and the SFR SD. As a result, the star formation efficiency (SFE = SFR
SD / gas SD) varies strongly across our sample and within individual galaxies.
We show that in spirals the SFE is a clear function of radius, while the dwarf
galaxies in our sample display SFEs similar to those found in the outer optical
disks of the spirals. Another general feature of our sample is a sharp
saturation of the HI SD at ~9 M_sol/pc^2 in both the spiral and dwarf galaxies.Comment: Accepted for publication in the AJ special THINGS issue. For a
high-resolution version visit: http://www.mpia.de/THINGS/Publications.htm
Supervised Learning in Multilayer Spiking Neural Networks
The current article introduces a supervised learning algorithm for multilayer
spiking neural networks. The algorithm presented here overcomes some
limitations of existing learning algorithms as it can be applied to neurons
firing multiple spikes and it can in principle be applied to any linearisable
neuron model. The algorithm is applied successfully to various benchmarks, such
as the XOR problem and the Iris data set, as well as complex classifications
problems. The simulations also show the flexibility of this supervised learning
algorithm which permits different encodings of the spike timing patterns,
including precise spike trains encoding.Comment: 38 pages, 4 figure
Mid-Infrared Emission from E+A Galaxies in the Coma Cluster
We have used ISO to observe at 12m seven E+A galaxies plus an additional
emission line galaxy, all in the Coma cluster. E+A galaxies lacking narrow
emission lines have 2.2m to 12m flux density ratios or limits similar
to old stellar populations (typical of early-type galaxies). Only galaxies with
emission lines have enhanced 12m flux density. Excess 12m emission is
therefore correlated with the presence of on-going star formation or an active
galactic nucleus (AGN).
By comparing the current star formation rates with previous rates estimated
from the Balmer absorption features, we divide the galaxies into two groups:
those for which star formation has declined significantly following a dramatic
peak 1 Gyr ago; and those with a significant level of ongoing star
formation or/and an AGN. There is no strong difference in the spatial
distribution on the sky between these two groups. However, the first group has
systemic velocities above the mean cluster value and the second group below
that value. This suggests that the two groups differ kinematically.
Based on surveys of the Coma cluster in the radio, the IRAS sources, and
galaxies detected in H emission, we sum the far infrared luminosity
function of galaxies in the cluster. We find that star formation in late type
galaxies is probably the dominant component of the Coma cluster far infrared
luminosity. The presence of significant emission from intracluster dust is not
yet firmly established. The member galaxies also account for most of the far
infrared output from nearby rich clusters in general.Comment: AAS Latex, accepted for publication in Ap
Reed-Muller codes for random erasures and errors
This paper studies the parameters for which Reed-Muller (RM) codes over
can correct random erasures and random errors with high probability,
and in particular when can they achieve capacity for these two classical
channels. Necessarily, the paper also studies properties of evaluations of
multi-variate polynomials on random sets of inputs.
For erasures, we prove that RM codes achieve capacity both for very high rate
and very low rate regimes. For errors, we prove that RM codes achieve capacity
for very low rate regimes, and for very high rates, we show that they can
uniquely decode at about square root of the number of errors at capacity.
The proofs of these four results are based on different techniques, which we
find interesting in their own right. In particular, we study the following
questions about , the matrix whose rows are truth tables of all
monomials of degree in variables. What is the most (resp. least)
number of random columns in that define a submatrix having full column
rank (resp. full row rank) with high probability? We obtain tight bounds for
very small (resp. very large) degrees , which we use to show that RM codes
achieve capacity for erasures in these regimes.
Our decoding from random errors follows from the following novel reduction.
For every linear code of sufficiently high rate we construct a new code
, also of very high rate, such that for every subset of coordinates, if
can recover from erasures in , then can recover from errors in .
Specializing this to RM codes and using our results for erasures imply our
result on unique decoding of RM codes at high rate.
Finally, two of our capacity achieving results require tight bounds on the
weight distribution of RM codes. We obtain such bounds extending the recent
\cite{KLP} bounds from constant degree to linear degree polynomials
High Resolution Mid-Infrared Imaging of Ultraluminous Infrared Galaxies
Observations of ultraluminous infrared galaxies (ULIRGs) with an achieved
resolution approaching the diffraction limit in the mid-infrared from 8 - 25
m using the Keck Telescopes are reported. We find extremely compact
structures, with spatial scales of (diameter) in six of the seven
ULIRGs observed. These compact sources emit between 30% and 100% of the
mid-infrared energy from these galaxies. We have utilized the compact
mid-infrared structures as a diagnostic of whether an AGN or a compact (100 --
300 pc) starburst is the primary power source in these ULIRGs. In Markarian
231, the upper limit on the diameter of the 12.5 m source, 0.13, shows
that the size of the infrared source must increase with increasing wavelength,
consistent with AGN models. In IRAS 05189-2524 and IRAS 08572+3915 there is
strong evidence that the source size increases with increasing wavelength. This
suggests heating by a central source rather than an extended luminosity source,
consistent with the optical classification as an AGN. The compact mid-infrared
sources seen in the other galaxies cannot be used to distinguish the ultimate
luminosity source. If these ULIRGs are powered by compact starbursts, the star
formation rates seen in the central few hundred parsecs far exceed the global
rates seen in nearby starburst galaxies, and approach the surface brightness of
individual clusters in nearby starburst galaxies.Comment: 33pages, 6 tables, 5 figures, Accepted for publication in A
Re-theorizing the “Structure–Agency” Relationship: Figurational Theory, Organizational Change and the Gaelic Athletic Association
This article illustrates how the figurational sociology associated with Norbert Elias provides an alternative theoretical framework for explaining the relationship between, ‘individualorganization- society’ and organizational change, and in so doing transverses what is conceived as a false dichotomy between structure and agency. Through an historical case study of the Gaelic Athletic Association in Ireland, the ‘individual-organization-society’ relationship is conceptualized as overlapping figurations and organizational change is explained as figurational dynamics—the shifting social interdependencies between the individuals and groups comprising an organization, between that organization and other organizations, between social groups on a higher level of integration and competition. In tandem with this, the article illustrates how changes in the sources of power and identity are connected with these figurational dynamics
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