672 research outputs found
Quantitation of the distribution and flux of myosin-II during cytokinesis
BACKGROUND: During cytokinesis, the cell's equator contracts against the cell's global stiffness. Identifying the biochemical basis for these mechanical parameters is essential for understanding how cells divide. To achieve this goal, the distribution and flux of the cell division machinery must be quantified. Here we report the first quantitative analysis of the distribution and flux of myosin-II, an essential element of the contractile ring. RESULTS: The fluxes of myosin-II in the furrow cortex, the polar cortex, and the cytoplasm were examined using ratio imaging of GFP fusion proteins expressed in Dictyostelium. The peak concentration of GFP-myosin-II in the furrow cortex is 1.8-fold higher than in the polar cortex and 2.0-fold higher than in the cytoplasm. The myosin-II in the furrow cortex, however, represents only 10% of the total cellular myosin-II. An estimate of the minimal amount of this motor needed to produce the required force for cell cleavage fits well with this 10% value. The cell may, therefore, regulate the amount of myosin-II sent to the furrow cortex in accordance with the amount needed there. Quantitation of the distribution and flux of a mutant myosin-II that is defective in phosphorylation-dependent thick filament disassembly confirms that heavy chain phosphorylation regulates normal recruitment to the furrow cortex. CONCLUSION: The analysis indicates that myosin-II flux through the cleavage furrow cortex is regulated by thick filament phosphorylation. Further, the amount of myosin-II observed in the furrow cortex is in close agreement with the amount predicted to be required from a simple theoretical analysis
The mass-metallicity gradient relation of early-type galaxies
We present a newly observed relation between galaxy mass and radial
metallicity gradients of early-type galaxies. Our sample of 51 early-type
galaxies encompasses a comprehensive mass range from dwarf to brightest cluster
galaxies. The metallicity gradients are measured out to one effective radius by
comparing nearly all of the Lick absorption-line indices to recent models of
single stellar populations. The relation shows very different behaviour at low
and high masses, with a sharp transition being seen at a mass of ~ 3.5 x 10^10
M_sun (velocity dispersion of ~140 km/s, M_B ~ -19). Low-mass galaxies form a
tight relation with mass, such that metallicity gradients become shallower with
decreasing mass and positive at the very low-mass end. Above the mass
transition point several massive galaxies have steeper gradients, but a clear
downturn is visible marked by a broad scatter. The results are interpreted in
comparison with competing model predictions. We find that an early star-forming
collapse could have acted as the main mechanism for the formation of low-mass
galaxies, with star formation efficiency increasing with galactic mass. The
high-mass downturn could be a consequence of merging and the observed larger
scatter a natural result of different merger properties. These results suggest
that galaxies above the mass threshold of ~ 3.5 x 10^10 M_sun might have formed
initially by mergers of gas-rich disc galaxies and then subsequently evolved
via dry merger events. The varying efficiency of the dissipative merger-induced
starburst and feedback processes have shaped the radial metallicity gradients
in these high-mass systems.Comment: 5 pageg, 3 figures, accepted by ApJ Lette
Spectroscopic evolution of dusty starburst galaxies
By using a one-zone chemical and spectrophotometric evolution model of a disk
galaxy undergoing a dusty starburst, we investigate, numerically, the optical
spectroscopic properties in order to explore galaxy evolution in distant
clusters. We adopt an assumption that the degree of dust extinction
(represented by ) depends on the ages of starburst populations in such a
way that younger stars have larger (originally referred to as selective
dust extinction by Poggianti & Wu 2000). In particular, we investigate how the
time evolution of the equivalent widths of [OII]3727 and H is
controlled by the adopted age dependence. This leads to three main results: (1)
If a young stellar population (with the age of yr) is more
heavily obscured by dust than an old one ( yr), the galaxy can show
an ``e(a)'' spectrum characterized by strong H absorption and
relatively modest [OII] emission. (2) A dusty starburst galaxy with an e(a)
spectrum can evolve into a poststarburst galaxy with an a+k (or k+a) spectrum
0.2 Gyr after the starburst and then into a passive one with a k-type spectrum
1 Gyr after the starburst. This result clearly demonstrates an evolutionary
link between galaxies with different spectral classes (i.e., e(b), e(a), a+k,
k+a, and k). (3) A dusty starburst galaxy can show an a+k or k+a spectrum even
in the dusty starburst phase if the age-dependence of dust extinction is rather
weak, i.e., if young starburst populations with different ages (
yr) are uniformly obscured by dust.Comment: 27 pages 12 figures,2001,ApJ,in pres
A study of central galaxy rotation with stellar mass and environment
© 2017. The American Astronomical Society. All rights reserved. We present a pilot analysis of the influence of galaxy stellar mass and cluster environment on the probability of slow rotation in 22 central galaxies at mean redshift z = 0.07. This includes new integral-field observations of five central galaxies selected from the Sloan Digital Sky Survey, observed with the SPIRAL integral-field spectrograph on the Anglo-Australian Telescope. The composite sample presented here spans a wide range of stellar masses, 10.9 < log(M∗/M⊙)lt; 12.0, and are embedded in halos ranging from groups to clusters, 12.9 < log(M 200 Ṁ) < 15.6. We find a mean probability of slow rotation in our sample of P(SR) = 54 ± 7%. Our results show an increasing probability of slow rotation in central galaxies with increasing stellar mass. However, when we examine the dependence of slow rotation on host cluster halo mass, we do not see a significant relationship. We also explore the influence of cluster dominance on slow rotation in central galaxies. Clusters with low dominance are associated with dynamically younger systems. We find that cluster dominance has no significant effect on the probability of slow rotation in central galaxies. These results conflict with a paradigm in which halo mass alone predetermines central galaxy properties
Quasi-stars, giants and the Sch\"onberg-Chandrasekhar limit
The Sch\"onberg-Chandrasekhar (SC) limit is a well-established result in the
understanding of stellar evolution. It provides an estimate of the point at
which an evolved isothermal core embedded in an extended envelope begins to
contract. We investigate contours of constant fractional mass in terms of
homology invariant variables U and V and find that the SC limit exists because
the isothermal core solution does not intersect all the contours for an
envelope with polytropic index 3. We find that this analysis also applies to
similar limits in the literature including the inner mass limit for polytropic
models of quasi-stars. Consequently, any core solution that does not intersect
all the fractional mass contours exhibits an associated limit and we identify
several relevant cases where this is so. We show that a composite polytrope is
at a fractional core mass limit when its core solution touches but does not
cross the contour of the corresponding fractional core mass. We apply this test
to realistic models of helium stars and find that stars typically expand when
their cores are near a mass limit. Furthermore, it appears that stars that
evolve into giants have always first exceeded an SC-like limit.Comment: 9 pages, 7 figures, published in MNRAS. Updated to (more closely)
match published versio
The structure and evolution of quasi-stars
The existence of bright quasars at high redshifts implies that supermassive
black holes were able to form in the early Universe. Though a number of
mechanisms to achieve this have been proposed, none yet stands out. A recent
suggestion is the formation of quasi-stars, initially stellar-mass black holes
accreting from hydrostatic giant-like envelopes of gas, formed from the
monolithic collapse of pre-galactic gas clouds. In this work, we modify the
Cambridge STARS stellar evolution package to construct detailed models of the
evolution of these objects. We find that, in all of our models, the black hole
inside the envelope is able to reach slightly more than one-tenth of the total
mass of the system before hydrostatic equilibrium breaks down. This breakdown
occurs after a few million years of evolution. We show that the mechanism which
causes the hydrostatic evolution to end is present in polytropic models. We
also show that the solutions are highly sensitive to the size of the inner
boundary radius and that no physical solutions exist if the inner boundary is
chosen to be less than about 0.3 of the Bondi radius.Comment: 12 pages, 11 figures. Published in MNRAS. Very belatedly updated to
(more closely) match published versio
The Evolution of Bat Vestibular Systems in the Face of Potential Antagonistic Selection Pressures for Flight and Echolocation
PMCID: PMC3634842This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
Spectrophotometric evolution of spiral galaxies with truncated star formation: An evolutionary link between spirals and S0s in distant clusters
A one-zone chemo-spectro-photometric model is used to investigate the time
evolution of disk galaxies whose star formation is truncated, and to determine
the dependence of this evolution on the previous star formation history and the
truncation epoch. Truncated spirals show red colors Gyr after
truncation, and evolve spectrally from an e(b) type, down through the e(a),
a+k, and k+a classes, to finally become a k type. The exact behavior in this
phase depends on the truncation epoch and the star formation history prior to
truncation. For example, earlier type disks show redder colors and do not show
a+k-type spectra after truncation of star formation. We also discuss a possible
evolutionary link between the k-type galaxies with spiral morphology found in
distant clusters, and present-day S0s, by investigating whether truncated
spirals reproduce the infrared color-magnitude relation of Coma galaxies. We
suggest that only less luminous, later-type disk galaxies whose star formation
is truncated at intermediate and high redshifts can reproduce the red
colors observed for S0s in the Coma cluster.Comment: 43 pages 19 figures, 2002, ApJ 565, 22
TESS asteroseismology of the known red-giant host stars HD 212771 and HD 203949
International audienc
Analysis of Locally Coupled 3D Manipulation Mappings Based on Mobile Device Motion
We examine a class of techniques for 3D object manipulation on mobile devices, in which the device's physical motion is applied to 3D objects displayed on the device itself. This "local coupling" between input and display creates specific challenges compared to manipulation techniques designed for monitor-based or immersive virtual environments. Our work focuses specifically on the mapping between device motion and object motion. We review existing manipulation techniques and introduce a formal description of the main mappings under a common notation. Based on this notation, we analyze these mappings and their properties in order to answer crucial usability questions. We first investigate how the 3D objects should move on the screen, since the screen also moves with the mobile device during manipulation. We then investigate the effects of a limited range of manipulation and present a number of solutions to overcome this constraint. This work provides a theoretical framework to better understand the properties of locally-coupled 3D manipulation mappings based on mobile device motion
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