8,639 research outputs found
Dark matter cores in the Fornax and Sculptor dwarf galaxies: joining halo assembly and detailed star formation histories
We combine the detailed Star Formation Histories of the Fornax and Sculptor
dwarf Spheroidals with the Mass Assembly History of their dark matter (DM) halo
progenitors to estimate if the energy deposited by Supernova type II (SNeII) is
sufficient to create a substantial DM core. Assuming the efficiency of energy
injection of the SNeII into DM particles is , we find
that a single early episode, , that combines the
energy of all SNeII due to explode over 0.5 Gyr, is sufficient to create a core
of several hundred parsecs in both Sculptor and Fornax. Therefore, our results
suggest that it is energetically plausible to form cores in Cold Dark Matter
(CDM) halos via early episodic gas outflows triggered by SNeII. Furthermore,
based on CDM merger rates and phase-space density considerations, we argue that
the probability of a subsequent complete regeneration of the cusp is small for
a substantial fraction of dwarf-size haloes.Comment: ApJL accepted versio
Modelleren van expertise in expertsystemen
Human experts posses the ability to solve problemns without an explicity formulated algorithm. Expert systems are a way to represent this ability as a model of the expert. To build an expert system one has to solve two consecutive problems: 1. There is a human expert and his knowledge. Is it possible to capture this knowledge? What methods can be used for knowledge acquisition? 2. Experts solve problems. In what way(s) is it possible to build an expert system based on the acquired knowledge? ... Zie: Summar
Geometric Optimisation of Hinge-less Deployment System for an Active Rotorblade
The Green Rotorcraft project (part of Clean Sky JTI) is\ud
studying the Gurney flap as a demonstrator of a smart adaptive\ud
rotorblade. Deployment systems for the Gurney flap need to\ud
sustain large centrifugal loads and vibrations while maintaining\ud
precisely the displacement under aerodynamic loading. Designing\ud
such a mechanism relies on both the actuation technology\ud
and the link which transmits motion to the control surface. Flexible\ud
beams and piezoelectric patch actuators have been chosen as\ud
components to design this mechanism. Flexible beams are providing\ud
an hinge-less robust structure onto which the piezoelectric\ud
actuators are bonded. A candidate topology is determined\ud
by investigating the compliance of a simple wire structure with\ud
beam elements. A parametrized finite element model is then built\ud
and optimized for displacement and force through surrogate optimization.\ud
The whole process does not requires many finite element\ud
analyses and quickly converge to an optimized mechanism
The extensive age gradient of the Carina dwarf galaxy
The evolution of small systems such as dwarf spheroidal galaxies (dSph) is
likely to have been a balance between external environmental effects and
internal processes within their own relatively shallow potential wells.
Assessing how strong such environmental interactions may have been is therefore
an important element in understanding the baryonic evolution of dSphs and their
derived dark matter distribution.
Here we present results from a wide-area CTIO/MOSAIC II photometric survey of
the Carina dSph, reaching down to about two magnitudes below the oldest main
sequence turn-off (MSTO). This data-set enables us to trace the structure of
Carina in detail out to very large distances from its center, and as a function
of stellar age.
We observe the presence of an extended structure made up primarily of ancient
MSTO stars, at distances between 25arcmin-60arcmin from Carina's center,
confirming results in the literature that Carina extends well beyond its
nominal tidal radius.
The large number statistics of our survey reveals features such as isophote
twists and tails that had gone undetected in other previous shallower surveys.
This is the first time that such unambiguous signs of tidal disruption have
been found in a Milky Way "classical" dwarf other than Sagittarius.
We also demonstrate the presence of a negative age gradient in Carina
directly from its MSTOs, and trace it out to very large distances from the
galaxy center. The signs of interaction with the Milky Way make it unclear
whether the age gradient was already in place before Carina underwent tidal
disruption.Comment: Accepted for publication in The Astrophysical Journal Letter
The star formation & chemical evolution timescales of two nearby dwarf spheroidal galaxies
Door het bestuderen van de eigenschappen van individuele sterren in nabijgelegen sterrenstelsels is het mogelijk om de vorming van deze stelsels met ongeĂ«venaarde precisie te bestuderen. Op die manier kunnen we bepalen wat de opbouw is van verschillende generaties sterren en chemische elementen in sterrenstelsels, en hoe deze is veranderd over kosmische tijd. Gedurende mijn promotieonderzoek heb ik een nieuwe, innovatieve methode ontwikkelt om alle bestaande informatie van nabijgelegen sterrenstelsels te combineren, zoals kleur-helderheid diagrammen en datasets van spectroscopische waarnemingen, die de hoeveelheden chemische elementen in sterren meten. Deze nieuwe techniek heb ik toegepast op twee nabijgelegen dwergsterrenstelsels in de Lokale Groep, die een uitstekend âlaboratoriumâ zijn om de vorming en evolutie van sterrenstelsels te bestuderen. Ik heb de nauwkeurige stervormingsgeschiedenis van de Sculptor en Fornax dwergstelsels bepaalt, die ons vertelt wanneer elke generatie sterren is gevormd, en met welk metaalgehalte. Deze informatie is gebruikt om de leeftijd van individuele sterren te bepalen, waarvan gemeten is wat de hoeveelheden van verschillende chemische elementen zijn. Op deze manier is het mogelijk om, voor het eerst, te bestuderen hoe de chemische elementen veranderen als een functie van tijd. Met behulp van deze gegevens kon in detail bepaald worden hoe verschillende processen het sterrenstelsel gevormd hebben dat we vandaag de dag zien. Ook kon ik bepalen wat de tijdschaal is van belangrijke fases in de evolutie van een sterrenstelsel, zoals de leeftijd waarop supernovae van verschillende typen een invloed beginnen uit te oefenen op de hoeveelheden chemische elementen in het stelsel. Verder werden de resultaten voor het Sculptor dwergstelsel ook gebruikt om theoretische modellen voor sterrenstelsels te testen
Black Hole Meiosis
The enumeration of BPS bound states in string theory needs refinement.
Studying partition functions of particles made from D-branes wrapped on
algebraic Calabi-Yau 3-folds, and classifying states using split attractor flow
trees, we extend the method for computing a refined BPS index, arXiv:0810.4301.
For certain D-particles, a finite number of microstates, namely polar states,
exclusively realized as bound states, determine an entire partition function
(elliptic genus). This underlines their crucial importance: one might call them
the `chromosomes' of a D-particle or a black hole. As polar states also can be
affected by our refinement, previous predictions on elliptic genera are
modified. This can be metaphorically interpreted as `crossing-over in the
meiosis of a D-particle'. Our results improve on hep-th/0702012, provide
non-trivial evidence for a strong split attractor flow tree conjecture, and
thus suggest that we indeed exhaust the BPS spectrum. In the D-brane
description of a bound state, the necessity for refinement results from the
fact that tachyonic strings split up constituent states into `generic' and
`special' states. These are enumerated separately by topological invariants,
which turn out to be partitions of Donaldson-Thomas invariants. As modular
predictions provide a check on many of our results, we have compelling evidence
that our computations are correct.Comment: 46 pages, 8 figures. v2: minor changes. v3: minor changes and
reference adde
Chandra Detection of X-ray Absorption Associated with a Damped Lyman Alpha System
We have observed three quasars, PKS 1127-145, Q 1331+171 and Q0054+144, with
the ACIS-S aboard the Chandra X-ray Observatory, in order to measure soft X-ray
absorption associated with intervening 21-cm and damped Ly absorbers.
For PKS 1127-145, we detect absorption which, if associated with an intervening
z_{abs}=0.312 absorber, implies a metallicity of 23% solar. If the absorption
is not at z_{abs}=0.312, then the metallicity is still constrained to be less
than 23% solar. The advantage of the X-ray measurement is that the derived
metallicity is insensitive to ionization, inclusion of an atom in a molecule,
or depletion onto grains. The X-ray absorption is mostly due to oxygen, and is
consistent with the oxygen abundance of 30% solar derived from optical nebular
emission lines in a foreground galaxy at the redshift of the absorber.
For Q1331+171 and Q 0054+144, only upper limits were obtained, although the
exposure times were intentionally short, since for these two objects we were
interested primarily in measuring flux levels to plan for future observations.
The imaging results are presented in a companion paper.Comment: 23 pages, 6 figures, accepted for publication in the Astrophysical
Journa
Under Pressure: Cetaceans and Fisheries Co-occurrence off the Coasts of Ghana and CĂŽte dâIvoire (Gulf of Guinea)
Within the Gulf of Guinea high levels of fisheries-related cetacean mortality (bycatch and direct-capture) has been documented. For locally rare species such removals could potentially lead to significant population level effects. However, information on the cetacean abundance and distribution is scarce. Similarly, it remains largely unreported where fishing fleets operate offshore. A cetacean survey took place during geophysical surveys (2013â2014) along the coasts of Ghana and CĂŽte dâIvoire. This provided a unique opportunity to study both offshore cetacean and fishing communities. Due to large group-sizes, melon-headed whales were the most abundant (0.34 animals kmâ1) followed by Fraserâs dolphins and short-finned pilot whales. Range state records were confirmed for melon-headed whale and Fraserâs dolphin in Ivoirian waters and ten further species represented first at-sea sightings. The artisanal fishing canoe was most abundant (92% of all vessels) and recorded up to 99.5 km from the Ghanaian coast. Asian trawlers operated over shelf areas and tuna purse-seine vessels in deep oceanic and slope waters. Fraserâs dolphins, melon-headed whales, pantropical spotted dolphins, bottlenose dolphins, and pilot whales were recorded in areas with the highest fishing densities. Melon-headed whales, pilot whales, and rough-toothed dolphins were observed in vicinity of trawlers; bottlenose dolphins, pantropical spotted dolphins, and pilot whales in vicinity of canoes. Some notable differences were found in the species composition between the present surveys and port-based surveys of landed cetaceans (bycatch/direct-captures). These may be explained by (1) feeding strategies (nocturnal vs. diurnal; surface vs. deep water); (2) different attractions to vessels/fishing gear; (3) variable body sizes; and (4) difficulty to positively identify species. Despite these differences, both cetaceans and fishing vessels predominantly occurred in shelf and slope waters (\u3c 1000m depth contour), making fishery-related mortality likely. The poor knowledge on population trends of cetaceans in this unique upwelling region, together with a high demand for cetacean products for human consumption (as âmarine bushmeatâ) may lead to a potential decline of some species that may go unnoticed. These new insights can provide a foundation for the urgently required risk assessments of cetacean mortality in fisheries within the northern Gulf of Guinea
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