1,866 research outputs found
An investigation of the structure and kinematics of the spiral galaxy NGC 6503
The spiral galaxy NGC 6503 exhibits a regular kinematical structure except
for a remarkable drop of the stellar velocity dispersion values in the central
region. To investigate the dynamics of the disc a theoretical framework has
been described. This includes a mass decomposition of the galaxy into a family
of disc/halo realizations compatible with the observed photometry and rotation
curve. For this family stellar velocity dispersion values and stability
parameters were calculated, showing that the more massive discs, although
having larger dispersions, are less stable. However, a reliable theoretical
description of the inner regions where the drop occurs cannot be given.
That is why we have resorted to numerical calculations. Pure stellar 3d
simulations have been performed for the family of decompositions. A clear
result is that disc/dark halo mass ratios approaching those of the maximum disc
limit generate a large bar structure. This is incompatible with the observed
morphology of NGC 6503. For the larger radii the stellar kinematics resulting
from the simulations essentially agrees with that predicted by the theory, but
the central velocity dispersion drop could not be reproduced.
A close inspection reveals that the central nuclear region is very small and
bright. Therefore, tentatively, this nucleus was considered as an isothermal
sphere and a core fitting procedure was applied. For an adopted equal
mass-to-light ratio of disc and nucleus, a velocity dispersion of 21.5 km/s is
predicted, in excellent agreement with the observed central value. The observed
dispersion drop can thus be explained by a separate kinematically distinct
galactic component.Comment: 14 pages, Latex, use mn.sty style fil
The Global Structure and Evolution of a Self-Gravitating Multi-phase Interstellar Medium in a Galactic Disk
Using high resolution, two-dimensional hydrodynamical simulations, we
investigate the evolution of a self-gravitating multi-phase interstellar medium
in the central kiloparsec region of a galactic disk. We find that a
gravitationally and thermally unstable disk evolves, in a self-stabilizing
manner, into a globally quasi-stable disk that consists of cold (T < 100 K),
dense clumps and filaments surrounded by hot (T > 10^4 K), diffuse medium. The
quasi-stationary, filamentary structure of the cold gas is remarkable. The hot
gas, characterized by low-density holes and voids, is produced by shock
heating. The shocks derive their energy from differential rotation and
gravitational perturbations due to the formation of cold dense clumps. In the
quasi-stable phase where cold and dense clouds are formed, the effective
stability parameter, Q, has a value in the range 2-5. The dynamic range of our
multi-phase calculations is 10^6 - 10^7 in both density and temperature. Phase
diagrams for this turbulent medium are analyzed and discussed.Comment: 10 pages, 3 figures, ApJ Letters in press (vol. 516
Deviations from the Schmidt-Kennicutt relations during early galaxy evolution
We utilize detailed time-varying models of the coupled evolution of stars and
the HI, H_2, and CO-bright H_2 gas phases in galaxy-sized numerical simulations
to explore the evolution of gas-rich and/or metal-poor systems, expected to be
numerous in the Early Universe. The inclusion of the CO-bright H_2 gas phase,
and the realistic rendering of star formation as an H_2-regulated process (and
the new feedback processes that this entails) allows the most realistic
tracking of strongly evolving galaxies, and much better comparison with
observations. We find that while galaxies eventually settle into states
conforming to Schmidt-Kennicutt (S-K) relations, significant and systematic
deviations of their star formation rates (SFRs) from the latter occur,
especially pronounced and prolonged for ...
...This indicates potentially serious limitations of (S-K)-type relations as
reliable sub-grid elements of star formation physics in simulations of
structure formation in the Early Universe. We anticipate that galaxies with
marked deviations from the S-K relations will be found at high redshifts as
unbiased inventories of total gas mass become possible with ALMA and the EVLA.Comment: 13 pages, 3 figures, accepted for publication in the Astrophysical
Journa
Relationship of care staff attitudes with social well-being and challenging behavior of nursing home residents with dementia: a cross sectional study
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'Brussel' op afstand? Een verkenning naar mogelijkheden voor maatschappelijke diensten en ruimte voor zelfsturing bij agrarisch natuurbeheer in de Europese context
De landbouw speelt een belangrijke rol bij het in stand houden en ontwikkelen van een aantrekkelijk cultuurlandschap en de biodiversiteit in de Noordelijke Friese Wouden. Het behoud van deze waarden steunt op de inzet en de vele activiteiten van de Vereniging Noordelijke Friese Wouden, op de status van Nationaal Landschap en op een aantal subsidieregelingen vanuit de overheid. Voor het aanbrengen en beheren van landschapselementen kunnen agrariërs een vergoeding van de overheid krijgen. Dit project richt zich specifiek op zelfsturingarrangementen die gericht zijn op agrarisch natuur- en landschapsbeheer. Het verkent de mogelijkheden voor de Vereniging Noordelijke Friese Wouden om meer invloed te krijgen op en verantwoordelijkheid te nemen voor het verdelen, beheren en het controleren van de besteding van de overheidsgelden voor agrarisch natuur- en landschapsbeheer. Op het huidige moment zijn deze kaders onderhevig aan discussie binnen de Europese Unie. De hervorming van het Gemeenschappelijk Landbouwbeleid (GLB) - van waaruit een deel van deze vergoedingen betaald worden - en de vereenvoudiging van het GLB staan op de politieke agenda en discussies hierover zijn in volle gang
Star formation thresholds and galaxy edges: why and where
We study global star formation thresholds in the outer parts of galaxies by
investigating the stability of disk galaxies embedded in dark halos. The disks
are self-gravitating, contain metals and dust, and are exposed to UV radiation.
We find that the critical surface density for the existence of a cold
interstellar phase depends only weakly on the parameters of the model and
coincides with the empirically derived surface density threshold for star
formation. Furthermore, it is shown that the drop in the thermal velocity
dispersion associated with the transition from the warm to the cold gas phase
triggers gravitational instability on a wide range of scales. The presence of
strong turbulence does not undermine this conclusion if the disk is
self-gravitating. Models based on the hypothesis that the onset of thermal
instability determines the star formation threshold in the outer parts of
galaxies can reproduce many observations, including the threshold radii, column
densities, and the sizes of stellar disks as a function of disk scale length
and mass. Finally, prescriptions are given for implementing star formation
thresholds in (semi-)analytic models and three-dimensional hydrodynamical
simulations of galaxy formation.Comment: 16 pages, 6 figures, accepted for publication in the Astrophysical
Journal. Version 2: text significantly revised (major improvements), physics
unchanged. Version 3: minor correction
Verantwoording van de methodiek 'Kwalitatieve monitor Systeeminnovaties verduurzaaming landbouw'
Dit werkdocument geeft een beschrijving van een kwalitatieve monitoringsmethodiek die gebruikt is binnen het project ‘Monitor systeeminnovaties verduurzaming landbouw’. In dit project zijn drie systeeminnovaties gericht op een verduurzaming van de landbouw gemonitord namelijk: integraal duurzame stallen, gesloten voer-mest kringlopen en functionele agrobiodiversiteit. Het project is onderdeel van een omvangrijker project ‘Monitoring verduurzaming landbouw’ dat in opdracht van het ministerie van Landbouw, Natuur en Voedselkwaliteit onder regie van het Planbureau voor de Leefomgeving is uitgevoerd
On the Neutral Gas Content and Environment of NGC 3109 and the Antlia Dwarf Galaxy
As part of a continuing survey of nearby galaxies, we have mapped the neutral
gas content of the low surface brightness, Magellanic-type galaxy NGC 3109 ---
and its environment, including the Antlia dwarf galaxy --- at unprecedented
velocity resolution and brightness sensitivity. The HI mass of NGC 3109 is
measured to be (3.8 +/- 0.5) x 10^8 Msun. A substantial warp in the disk of NGC
3109 is detected in the HI emission image in the form of an extended low
surface brightness feature. We report a positive detection in HI of the nearby
Antlia dwarf galaxy, and measure its total neutral gas mass to be (6.8 +/- 1.4)
x 10^5 Msun. We show the warp in NGC 3109 to lie at exactly the same radial
velocity as the gas in the Antlia dwarf galaxy and speculate that Antlia
disturbed the disk of NGC 3109 during a mild encounter ~1 Gyr in the past. HI
data for a further eight galaxies detected in the background are presented.Comment: Accepted for publication in A
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