535 research outputs found
Secular Evolution of Galaxy Morphologies
Today we have numerous evidences that spirals evolve dynamically through
various secular or episodic processes, such as bar formation and destruction,
bulge growth and mergers, sometimes over much shorter periods than the standard
galaxy age of 10-15 Gyr. This, coupled to the known properties of the Hubble
sequence, leads to a unique sense of evolution: from Sm to Sa. Linking this to
the known mass components provides new indications on the nature of dark matter
in galaxies. The existence of large amounts of yet undetected dark gas appears
as the most natural option. Bounds on the amount of dark stars can be given
since their formation is mostly irreversible and requires obviously a same
amount of gas.Comment: 8 pages, Latex2e, crckapb.sty macros, 1 Postscript figure, replaced
with TeX source; To be published in the proceeedings of the "Dust-Morphology"
conference, Johannesburg, 22-26 January, 1996, D. Block (ed.), (Kluwer
Dordrecht
Solar Multi-Scale Convection and Rotation Gradients Studied in Shallow Spherical Shells
The differential rotation of the sun, as deduced from helioseismology,
exhibits a prominent radial shear layer near the top of the convection zone
wherein negative radial gradients of angular velocity are evident in the low-
and mid-latitude regions spanning the outer 5% of the solar radius.
Supergranulation and related scales of turbulent convection are likely to play
a significant role in the maintenance of such radial gradients, and may
influence dynamics on a global scale in ways that are not yet understood. To
investigate such dynamics, we have constructed a series of three-dimensional
numerical simulations of turbulent compressible convection within spherical
shells, dealing with shallow domains to make such modeling computationally
tractable. These simulations are the first models of solar convection in a
spherical geometry that can explicitly resolve both the largest dynamical
scales of the system (of order the solar radius) as well as smaller-scale
convective overturning motions comparable in size to solar supergranulation
(20--40 Mm). We find that convection within these simulations spans a large
range of horizontal scales, and that the radial angular velocity gradient in
these models is typically negative, especially in low- and mid-latitude
regions. Analyses of the angular momentum transport indicates that such
gradients are maintained by Reynolds stresses associated with the convection,
transporting angular momentum inward to balance the outward transport achieved
by viscous diffusion and large-scale flows in the meridional plane. We suggest
that similar mechanisms associated with smaller-scale convection in the sun may
contribute to the maintenance of the observed radial shear layer located
immediately below the solar photosphere.Comment: 45 pages, 17 figures, ApJ in press. A preprint of paper with hi-res
figures can be found at
http://www-lcd.colorado.edu/~derosa/modelling/modelling.htm
Rings of Dark Matter in Collisions Between Clusters of Galaxies
Several lines of evidence suggest that the galaxy cluster Cl0024+17, an
apparently relaxed system, is actually a collision of two clusters, the
interaction occurring along our line of sight. Recent lensing observations
suggest the presence of a ring-like dark matter structure, which has been
interpreted as the result of such a collision. In this paper we present
-body simulations of cluster collisions along the line of sight to
investigate the detectability of such features. We use realistic dark matter
density profiles as determined from cosmological simulations. Our simulations
show a "shoulder" in the dark matter distribution after the collision, but no
ring feature even when the initial particle velocity distribution is highly
tangentially anisotropic (). Only when the initial
particle velocity distribution is circular do our simulations show such a
feature. Even modestly anisotropic velocity distributions are inconsistent with
the halo velocity distributions seen in cosmological simulations, and would
require highly fine-tuned initial conditions. Our investigation leaves us
without an explanation for the dark matter ring-like feature in Cl 0024+17
suggested by lensing observations.Comment: 7 pages (emulateapj), 9 figures. Expanded figures and text to match
accepted versio
A SAURON study of stars and gas in Sa bulges
We present results from our ongoing effort to understand the morphological
and kinematical properties of early-type galaxies using the integral-field
spectrograph SAURON. We discuss the relation between the stellar and gas
morphology and kinematics in our sub-sample of 24 representative Sa spiral
bulges. We focus on the frequency of kinematically decoupled components and on
the presence of star formation in circumnuclear rings.Comment: 6 pages, 3 figures; To appear in the proceedings of the "Island
Universes: Structure and Evolution of Disk Galaxies" conference held in
Terschelling, Netherlands, July 2005, ed. R. de Jong. A high resolution
version is available at
http://www.strw.leidenuniv.nl/~jfalcon/JFB_terschelling.pdf.g
Luminous X-Ray Sources in Arp 147
The Chandra X-Ray Observatory was used to image the collisional ring galaxy
Arp 147 for 42 ks. We detect 9 X-ray sources with luminosities in the range of
1.4 - 7 x 10^{39} ergs/sec in or near the blue knots of star formation
associated with the ring. A source with an isotropic X-ray luminosity of 1.4 x
10^{40} ergs/sec is detected in the nuclear region of the intruder galaxy.
X-ray sources associated with a foreground star and a background quasar are
used to improve the registration of the X-ray image with respect to HST high
resolution optical images. The intruder galaxy, which apparently contained
little gas before the collision, shows no X-ray sources other than the one in
the nuclear bulge which may be a poorly fed supermassive black hole. These
observations confirm the conventional wisdom that collisions of gas rich
galaxies trigger large rates of star formation which, in turn, generate
substantial numbers of X-ray sources, some of which have luminosities above the
Eddington limit for accreting stellar-massComment: 9 pages, 5 figure
A novel physiological role for ARF1 in the formation of bidirectional tubules from the Golgi.
Capitalizing on CRISPR/Cas9 gene-editing techniques and super-resolution nanoscopy, we explore the role of the small GTPase ARF1 in mediating transport steps at the Golgi. Besides its well-established role in generating COPI vesicles, we find that ARF1 is also involved in the formation of long (∼3 µm), thin (∼110 nm diameter) tubular carriers. The anterograde and retrograde tubular carriers are both largely free of the classical Golgi coat proteins coatomer (COPI) and clathrin. Instead, they contain ARF1 along their entire length at a density estimated to be in the range of close packing. Experiments using a mutant form of ARF1 affecting GTP hydrolysis suggest that ARF1[GTP] is functionally required for the tubules to form. Dynamic confocal and stimulated emission depletion imaging shows that ARF1-rich tubular compartments fall into two distinct classes containing 1) anterograde cargoes and clathrin clusters or 2) retrograde cargoes and coatomer clusters
The Formation and Survival of Discs in a Lambda-CDM Universe
We study the formation of galaxies in a Lambda-CDM Universe using high
resolution hydrodynamical simulations with a multiphase treatment of gas,
cooling and feedback, focusing on the formation of discs. Our simulations
follow eight haloes similar in mass to the Milky Way and extracted from a large
cosmological simulation without restriction on spin parameter or merger
history. This allows us to investigate how the final properties of the
simulated galaxies correlate with the formation histories of their haloes. We
find that, at z = 0, none of our galaxies contain a disc with more than 20 per
cent of its total stellar mass. Four of the eight galaxies nevertheless have
well-formed disc components, three have dominant spheroids and very small
discs, and one is a spheroidal galaxy with no disc at all. The z = 0 spheroids
are made of old stars, while discs are younger and formed from the inside-out.
Neither the existence of a disc at z = 0 nor the final disc-to-total mass ratio
seems to depend on the spin parameter of the halo. Discs are formed in haloes
with spin parameters as low as 0.01 and as high as 0.05; galaxies with little
or no disc component span the same range in spin parameter. Except for one of
the simulated galaxies, all have significant discs at z > ~2, regardless of
their z = 0 morphologies. Major mergers and instabilities which arise when
accreting cold gas is misaligned with the stellar disc trigger a transfer of
mass from the discs to the spheroids. In some cases, discs are destroyed, while
in others, they survive or reform. This suggests that the survival probability
of discs depends on the particular formation history of each galaxy. A
realistic Lambda-CDM model will clearly require weaker star formation at high
redshift and later disc assembly than occurs in our models.Comment: 14 pages, 10 figures, mn2e.cls. MNRAS in press, updated to match
published versio
Spiral shocks in the accretion disc of IP Peg during outburst maximum
In response to our recent discovery of spiral arms in the accretion disc of
IP Peg during rise to outburst, we have obtained time-resolved
spectrophotometry of IP Peg during outburst maximum. In particular, indirect
imaging of HeII 4686, using Doppler tomography, shows a two-arm spiral pattern
on the disc image, which confirms repeatability over different outbursts. The
jump in HeII intensity (a factor of more than two) and in velocity (~200--300
km/s) clarifies the shock nature of the spiral structure. The HeII shocks show
an azimuthal extent of ~90 degrees, a shallow power-law emissivity ~V^{-1}, an
upper limit of 30 degrees in opening angle, and a flux contribution of 15 per
cent of the total disc emission. We discuss the results in view of recent
simulations of accretion discs which show that spiral shocks can be raised in
the accretion disc by the secondary star.Comment: 5 pages, 4 figures, MNRAS journal paper. in pres
Lumpy Structures in Self-Gravitating Disks
Following Toomre & Kalnajs (1991), local models of slightly dissipative
self-gravitating disks show how inhomogeneous structures can be maintained over
several galaxy rotations. Their basic physical ingredients are self-gravity,
dissipation and differential rotation. In order to explore the structures
resulting from these processes on the kpc scale, local simulation of
self-gravitating disks are performed in this paper in 2D as well as in 3D. The
third dimension becomes a priori important as soon as matter clumping causes a
tight coupling of the 3D equations of motion. The physically simple and general
framework of the model permits to make conclusions beyond the here considered
scales. A time dependent affine coordinate system is used, allowing to
calculate the gravitational forces via a particle-mesh FFT-method, increasing
the performance with respect to previous direct force calculations. Persistent
patterns, formed by transient structures, whose intensity and morphological
characteristic depend on the dissipation rate are obtained and described. Some
of our simulations reveal first signs of mass-size and velocity dispersion-size
power-law relations, but a clear scale invariant behavior will require more
powerful computer techniques.Comment: 28 pages, 32 figures. Accepted for publication in A&A. Full
resolution paper available at http://obswww.unige.ch/Preprints/dyn_art.htm
Phosphatidylinositol phosphate kinase type Iγ regulates dynamics of large dense-core vesicle fusion.
Phosphatidylinositol-4,5-bisphosphate was proposed to be an important regulator of large dense-core vesicle exocytosis from neuroendocrine tissues. Here, we have examined the kinetics of secretion in chromaffin cells from mice lacking phosphatidylinositol phosphate kinase type Iγ, the major neuronal phosphatidylinositol-4-phosphate 5-kinase. Absence of this enzyme caused a reduction of the readily releasable vesicle pool and its refilling rate, with a small increase in morphologically docked vesicles, indicating a defect in vesicle priming. Furthermore, amperometry revealed a delay in fusion pore expansion. These results provide direct genetic evidence for a key role of phosphatidylinositol-4,5-bisphosphate synthesis in the regulation of large dense-core vesicle fusion dynamics
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