13,954 research outputs found
Galaxies with Spiral Structure up to z = 0.87 --Limits on M/L and the Stellar Velocity Dispersion
We consider seven distant galaxies with clearly evident spiral structure from
HST images. Three of these were chosen from Vogt et al. (1996) (VFP) and have
measured rotational velocities. Five were chosen from the Medium Deep Survey
and are studied in Sarajedini et al. 1996 (SGGR), and one galaxy is found in
both papers. We place upper limits on their mass-to-light ratios (M/L) by
computing M/L_B for a maximal disk. We find that these galaxies have maximal
disk mass-to-light ratios M/L_B = 1.5 - 3.5 M_sol/L_Bsol at the low end, but
within the range seen in nearby galaxies. The mass-to-light ratios are low
enough to suggest that the galaxies contain a young, rapidly formed stellar
population.
By using a Toomre stability criterion for formation of spiral structure, we
place constraints on the ratio of M/L to the stellar velocity dispersion. If
these galaxies have maximal disks they would have to be nearly unstable so as
to have small enough velocity dispersions that their disks are not
unrealistically thick. This suggests that there is a substantial amount of dark
matter present in the luminous regions of the galaxy.Comment: AAS Latex + PS Figure, accepted for publication in A
Adaptive mutation using statistics mechanism for genetic algorithms
Copyright @ 2004 Springer-Verla
Quantum-Statistical Current Correlations in Multi-Lead Chaotic Cavities
Quantum mechanics requires that identical particles are treated as
indistinguishable. This requirement leads to correlations in the fluctuating
properties of a system. Theoretical predictions are made for an experiment on a
multi-lead chaotic quantum dot which can identify exchange effects in
electronic current-current correlations. Interestingly, we find that the
ensemble averaged exchange effects are of the order of the channel number, and
are insensitive to dephasing.Comment: 4 pages REVTEX, including two figure
Relativistic and correlated calculations on the ground, excited, and ionized states of iodine
The electronic structure, spectroscopic, and bonding properties of the ground, excited, and ionized states of iodine are studied within a four-component relativistic framework using the MOLFDIR program package, The experimentally determined properties of the (1) Sigma(g)(+) ground state are well reproduced by our results calculated at the CCSD(T) level of theory, Relativistic effects and core-valence correlation need to be included in order to get reliable results, but the Gaunt interaction can be neglected. The photoelectron spectrum and the potential energy curves of the ionized and excited states are calculated using relativistic configuration interaction and coupled cluster methods. The calculated properties of the excited states are generally in good agreement with the experimental data, as well as with the earlier theoretical results of Teichteil and Pelissier, An alternative assignment of some recently measured, low lying, ionized states is proposed. (C) 1997 American Institute of Physics
Duurzame ontwikkeling van de school als professionele leergemeenschap
Vorig onderzoek: PLG op gang brengenIn een eerder afgerond onderzoek brachten we bij 14 middelbare scholen in kaart hoe zij de ontwikkeling van een school als professionele leergemeenschap (PLG) op gang brachten. Dit onderzoek liet zien dat een school als PLG zich onder meer kenmerkt door samenwerking van docenten, met een collectieve verantwoordelijkheid voor het leerproces van leerlingen, een breed gedragen visie en ondersteunend leiderschap.Nieuw onderzoek: duurzame ontwikkelingDe ontwikkeling van PLG’s is complex, omdat het niet gaat om een eenmalige herstructurering van scholen, maar om een doorlopend proces van cultuurverandering. Bij 5 van de 14 vo-scholen is daarom meer inzicht verkregen in de duurzame ontwikkeling van de school als PLG. De onderzoekers hebben hiervoor interviews gehouden met projectleiders en focusgroepgesprekken met project- en schoolleiding en docenten. Verder zijn onder meer ontwikkelplannen uit de scholen geanalyseerd
A novel method to generate unmarked gene deletions in the intracellular pathogen Rhodococcus equi using 5-fluorocytosine conditional lethality
A novel method to efficiently generate unmarked in-frame gene deletions in Rhodococcus equi was developed, exploiting the cytotoxic effect of 5-fluorocytosine (5-FC) by the action of cytosine deaminase (CD) and uracil phosphoribosyltransferase (UPRT) enzymes. The opportunistic, intracellular pathogen R. equi is resistant to high concentrations of 5-FC. Introduction of Escherichia coli genes encoding CD and UPRT conferred conditional lethality to R. equi cells incubated with 5-FC. To exemplify the use of the codA::upp cassette as counter-selectable marker, an unmarked in-frame gene deletion mutant of R. equi was constructed. The supA and supB genes, part of a putative cholesterol catabolic gene cluster, were efficiently deleted from the R. equi wild-type genome. Phenotypic analysis of the generated ΔsupAB mutant confirmed that supAB are essential for growth of R. equi on cholesterol. Macrophage survival assays revealed that the ΔsupAB mutant is able to survive and proliferate in macrophages comparable to wild type. Thus, cholesterol metabolism does not appear to be essential for macrophage survival of R. equi. The CD-UPRT based 5-FC counter-selection may become a useful asset in the generation of unmarked in-frame gene deletions in other actinobacteria as well, as actinobacteria generally appear to be 5-FC resistant and 5-FU sensitive
Stellar science from a blue wavelength range - A possible design for the blue arm of 4MOST
From stellar spectra, a variety of physical properties of stars can be
derived. In particular, the chemical composition of stellar atmospheres can be
inferred from absorption line analyses. These provide key information on large
scales, such as the formation of our Galaxy, down to the small-scale
nucleosynthesis processes that take place in stars and supernovae. By extending
the observed wavelength range toward bluer wavelengths, we optimize such
studies to also include critical absorption lines in metal-poor stars, and
allow for studies of heavy elements (Z>38) whose formation processes remain
poorly constrained. In this context, spectrographs optimized for observing blue
wavelength ranges are essential, since many absorption lines at redder
wavelengths are too weak to be detected in metal-poor stars. This means that
some elements cannot be studied in the visual-redder regions, and important
scientific tracers and science cases are lost. The present era of large public
surveys will target millions of stars. Here we describe the requirements
driving the design of the forthcoming survey instrument 4MOST, a multi-object
spectrograph commissioned for the ESO VISTA 4m-telescope. We focus here on
high-density, wide-area survey of stars and the science that can be achieved
with high-resolution stellar spectroscopy. Scientific and technical
requirements that governed the design are described along with a thorough line
blending analysis. For the high-resolution spectrograph, we find that a
sampling of >2.5 (pixels per resolving element), spectral resolution of 18000
or higher, and a wavelength range covering 393-436 nm, is the most
well-balanced solution for the instrument. A spectrograph with these
characteristics will enable accurate abundance analysis (+/-0.1 dex) in the
blue and allow us to confront the outlined scientific questions. (abridged)Comment: 14 pages, 8 figures, accepted for publication in A
Andreev Reflection In Ferromagnet-Superconductor Junctions
The transport properties of a ferromagnet-superconductor (FS) junction are
studied in a scattering formulation. Andreev reflection at the FS interface is
strongly affected by the exchange interaction in the ferromagnet. The
conductance G_FS of a ballistic point contact between F and S can be both
larger or smaller than the value G_FN with the superconductor in the normal
state, depending on the ratio of the exchange and Fermi energies. If the
ferromagnet contains a tunnel barrier (I), the conductance G_FIFS exhibits
resonances which do not vanish in linear response -- in contrast to the Tomasch
oscillations for non-ferromagnetic materials.Comment: 8 pages, RevTeX v3.0, including 3 encapsulated postscript figures;
[2017: figures included in text
Finite voltage shot noise in normal-metal - superconductor junctions
We express the low-frequency shot noise in a disordered normal-metal -
superconductor (NS) junction at finite (subgap) voltage in terms of the normal
scattering amplitudes and the Andreev reflection amplitude. In the multichannel
limit, the conductance exhibits resonances which are accompanied by an
enhancement of the (differential) shot noise. In the study of multichannel
single and double barrier junctions we discuss the noise properties of coherent
transport at low versus high voltage with respect to the Andreev level spacing.Comment: 6 pages, Latex, 2 eps-figures, to be published in PRB, Appendix on
Bogoliubov equation
Influence of Cooled Interstellar Gas on the Fundamental Plane for Elliptical Galaxies
We explore the possibly important influence of cooled interstellar gas on the
fundamental plane of elliptical galaxies. Interstellar cooling is described by
a parameterized sink term in the equation of continuity. Parameters that give
the best fits to the X-ray observations of NGC 4472 are used as a template for
the radial distribution of interstellar cooling in structurally homologous
elliptical galaxies of lower mass. Gas that cools within an effective radius
can contribute an additional 10 - 30 percent to the mass of the old stellar
population. If the cooled gas forms into stars of very low mass, , as is commonly assumed, the cooled mass is optically dark. As a
result, the mass to light ratios determined from stellar velocities
systematically overestimate that of the old stellar population. Moreover, the
total mass and spatial distribution of the optically dark young stellar
population does not scale homologously with galactic luminosity or radius and
the total stellar mass to light ratio varies with galactic radius. We
investigate the non-homologous perturbations of cooled gas on the mass to light
ratio for several idealized homologous elliptical galaxies and show that they
appear to be incompatible with the observed thinness of the fundamental plane.
If optically luminous young stars formed from the cooled gas, the disturbance
of the fundamental plane would be lessened.Comment: 10 pages with 2 figures; accepted by Astrophysical Journa
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