288 research outputs found
Determination of the dynamical structure of galaxies using optical spectra
Galaxy spectra are a rich source of kinematical information since the shapes
of the absorption lines reflect the movement of stars along the line-of-sight.
We present a technique to directly build a dynamical model for a galaxy by
fitting model spectra, calculated from a dynamical model, to the observed
galaxy spectra. Using synthetic spectra from a known galaxy model we
demonstrate that this technique indeed recovers the essential dynamical
characteristics of the galaxy model. Moreover, the method allows a
statistically meaningful error analysis on the resulting dynamical quantities.Comment: 14 pages, 14 figures, Latexfile, MNRAS, in pres
Simulations of the formation and evolution of isolated dwarf galaxies
We present new fully self-consistent models of the formation and evolution of
isolated dwarf galaxies. We have used the publicly available N-body/SPH code
HYDRA, to which we have added a set of star formation criteria, and
prescriptions for chemical enrichment (taking into account contributions from
both SNIa and SNII), supernova feedback, and gas cooling. The models follow the
evolution of an initially homogeneous gas cloud collapsing in a pre-existing
dark-matter halo. These simplified initial conditions are supported by the
merger trees of isolated dwarf galaxies extracted from the milli-Millennium
Simulation.
The star-formation histories of the model galaxies exhibit burst-like
behaviour. These bursts are a consequence of the blow-out and subsequent
in-fall of gas. The amount of gas that leaves the galaxy for good is found to
be small, in absolute numbers, ranging between 3x10^7 Msol and 6x10^7 Msol .
For the least massive models, however, this is over 80 per cent of their
initial gas mass. The local fluctuations in gas density are strong enough to
trigger star-bursts in the massive models, or to inhibit anything more than
small residual star formation for the less massive models. Between these
star-bursts there can be time intervals of several Gyrs.
We have compared model predictions with available data for the relations
between luminosity and surface brightness profile, half-light radius, central
velocity dispersion, broad band colour (B-V) and metallicity, as well as the
location relative to the fundamental plane. The properties of the model dwarf
galaxies agree quite well with those of observed dwarf galaxies.Comment: 16 pages, 20 figures, accepted for publication in MNRA
Novel nitrogen-based organosulfur electrodes for advanced intermediate temperature batteries
Advanced secondary batteries operating at intermediate temperatures (100 to 200 C) have attracted considerable interest due to their inherent advantages (reduced corrosion and safety risks) over higher temperature systems. Current work in this laboratory has involved research on a class of intermediate temperature Na/beta double prime- alumina/RSSR batteries conceptually similar to Na/S cells, but operating within a temperature range of 100 to 150 C, and having an organosulfur rather than inorganic sulfur positive electrode. The organosulfur electrodes are based on the reversible, two electron eduction of organodisulfides to the corresponding thiolate anions, RSSR + 2 electrons yield 2RS(-), where R is an organic moiety. Among the advantages of such a generic redox couple for battery research is the ability to tailor the physical, chemical, and electrochemical properties of the RSSR molecule through choice of the organic moiety. The viscosity, liquidus range, dielectric constant, equivalent weight, and redox potential can in fact be verified in a largely predictable manner. The current work concerns the use of multiple nitrogen organosulfur molecules, chosen for application in Na/RSSR cells for their expected oxidizing character. In fact, a Na/RSSR cell containing one of these materials, the sodium salt of 5-mercapto 1-methyltetrazole, yielded the highest open circuit voltage obtained yet in the laboratory; 3.0 volts in the charged state and 2.6 volts at 100 percent discharge. Accordingly, the cycling behavior of a series of multiple nitrogen organodisulfides as well as polymeric organodisulfides are presented in this manuscript
A genetic algorithm for the non-parametric inversion of strong lensing systems
We present a non-parametric technique to infer the projected-mass
distribution of a gravitational lens system with multiple strong-lensed images.
The technique involves a dynamic grid in the lens plane on which the mass
distribution of the lens is approximated by a sum of basis functions, one per
grid cell. We used the projected mass densities of Plummer spheres as basis
functions. A genetic algorithm then determines the mass distribution of the
lens by forcing images of a single source, projected back onto the source
plane, to coincide as well as possible. Averaging several tens of solutions
removes the random fluctuations that are introduced by the reproduction process
of genomes in the genetic algorithm and highlights those features common to all
solutions. Given the positions of the images and the redshifts of the sources
and the lens, we show that the mass of a gravitational lens can be retrieved
with an accuracy of a few percent and that, if the sources sufficiently cover
the caustics, the mass distribution of the gravitational lens can also be
reliably retrieved. A major advantage of the algorithm is that it makes full
use of the information contained in the radial images, unlike methods that
minimise the residuals of the lens equation, and is thus able to accurately
reconstruct also the inner parts of the lens.Comment: 11 pages, accepted for publication by MNRA
Realistic error estimates on kinematic parameters
Current error estimates on kinematic parameters are based on the assumption
that the data points in the spectra follow a Poisson distribution. For
realistic data that have undergone several steps in a reduction process, this
is generally not the case. Neither is the noise distribution independent in
adjacent pixels. Hence, the error estimates on the derived kinematic parameters
will (in most cases) be smaller than the real errors. In this paper we propose
a method that makes a diagnosis of the characteristics of the observed noise
The method also offers the possibility to calculate more realistic error
estimates on kinematic parameters. The method was tested on spectroscopic
observations of NGC3258. In this particular case, the realistic errors are
almost a factor of 2 larger than the errors based on least squares statistics.Comment: 11 pages, 11 figures, accepted for publication by MNRA
The Three-Dimensional Mass Distribution in NGC 1700
A variety of modeling techniques is used with surface photometry from the
literature and recently acquired high-accuracy stellar kinematic data to
constrain the three-dimensional mass distribution in the luminous cuspy
elliptical galaxy NGC 1700. First, we model the radial velocity field and
photometry, and, using a Bayesian technique, estimate the triaxiality T and
short-to-long axis ratio c in five concentric annuli between approximately 1
and 3 effective radii. The results are completely consistent with T being
constant inside about 2.5 r_e (36 arcsec; 6.7/h kpc). Adding an assumption of
constant T as prior information gives an upper limit of T < 0.16 (95%
confidence); this relaxes to T < 0.22 if it is also assumed that there is
perfect alignment between the angular momentum and the galaxy's intrinsic short
axis. Near axisymmetry permits us then to use axisymmetric models to constrain
the radial mass profile. Using the Jeans (moment) equations, we demonstrate
that 2-integral, constant-M/L models cannot fit the data; but a 2-integral
model in which the cumulative enclosed M/L increases by a factor of roughly 2
from the center out to 12/h kpc can. Three-integral models constructed by
quadratic programming show that, in fact, no constant-M/L model is consistent
with the kinematics. Anisotropic 3-integral models with variable M/L, while not
uniquely establishing a minimum acceptable halo mass, imply, as do the moment
models, a cumulative M/L_B approximately 10 h at 12/h kpc. We conclude that NGC
1700 represents the best stellar dynamical evidence to date for dark matter in
elliptical galaxies.Comment: 26 pages, Latex, AASTeX v4.0, with 11 eps figures. To appear in The
Astronomical Journal, January 1999. Figures 1 and 3 are color but are
readable in b/
La Formation de Jupille, nouvelle formation dans le Dévonien inférieur de la Haute-Ardenne (Belgique)
The Jupille Formation, new formation in the lower Devonian of the High-Ardenne (Belgium). A new formation named Jupille is proposed to better characterize in the High-Ardenne area the rocks interbedded between the La Roche (or Villé if La Roche is missing) and Pèrnelle Formations, at the transition between the Pragian/Emsian stages (Lower Devonian). This formation is made up of series of grey, blue grey or greenish grey sandstone layers interbedded in blue grey siltstones and slates similar to those of the La Roche Formation. Locally, the sandstones grades to quartzites. Tool marks, current ripples, lenticular and oblique or hummocky cross-stratifications and load casts (pseudonodules) are present in the sandstone layers
The dynamics of planetary nebulae in the Galaxy: evidence for a third integral
We present a dynamical analysis of 673 galactic Planetary Nebulae, using a
two-integral axisymmetric model with a Kuzmin-Kutuzov St\"{a}ckel potential.
The method fits the kinematics to the projected moments of a distribution
function, by means of Quadratic Programming. The 2.2 m COBE brightness map
has been used after correction for the interstellar extinction as a projected
star counts map in the modeling, because it constitutes a galactic distribution
view of evolved red populations which are considered to be the progenitors of
PNe. The model we have obtained provides a 2-integral distribution function for
the COBE 2.2 m map, and thus {\it a fortiori} a deprojection of it, which
allows moreover the identification of all the major Galactic components. We
derive the density laws for them. The projected velocity dispersions are not
well fitted though, especially in the disk, which points at the likely presence
of a third integral. If this result can be confirmed by additional data, this
would mean that for the first time the presence and importance of a third
integral on a global scale is demonstrated.Comment: 9 pages, uuencoded gzipped postscript file, 9 figures include
The puzzlingly large Ca II triplet absorption in dwarf elliptical galaxies
We present central CaT, PaT, and CaT* indices for a sample of fifteen dwarf
elliptical galaxies (dEs). Twelve of these have CaT* ~ 7 A and extend the
negative correlation between the CaT* index and central velocity dispersion
sigma, which was derived for bright ellipticals (Es), down to 20 < sigma < 55
km/s. For five dEs we have independent age and metallicity estimates. Four of
these have CaT* ~ 7 A, much higher than expected from their low metallicities
(-1.5 < [Z/H] < -0.5). The observed anti-correlation of CaT* as a function of
sigma or Z is in flagrant disagreement with theory. We discuss some of the
amendments that have been proposed to bring the theoretical predictions into
agreement with the observed CaT*-values of bright Es and how they can be
extended to incorporate also the observed CaT*-values of dEs. Moreover, 3 dEs
in our sample have CaT* ~ 5 A, as would be expected for metal-poor stellar
systems. Any theory for dE evolution will have to be able to explain the
co-existence of low-CaT* and high-CaT* dEs at a given mean metallicity. This
could be the first direct evidence that the dE population is not homogeneous,
and that different evolutionary paths led to morphologically and kinematically
similar but chemically distinct objects.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter
Dynamical streams in the solar neighbourhood
The true nature of the Hyades and Sirius superclusters is still an open
question. In this contribution, we confront Eggen's hypothesis that they are
cluster remnants with the results of a kinematic analysis of more than 6000 K
and M giants in the solar neighbourhood. This analysis includes new radial
velocity data from a large survey performed with the Coravel spectrometer,
complemented by Hipparcos parallaxes and Tycho-2 proper motions (Famaey et al.
2004). A maximum-likelihood method, based on a bayesian approach, has been
applied to the data, in order to make full use of all the available data
(including less precise parallaxes) and to derive the properties of the
different kinematic subgroups. Two such subgroups can be identified with the
Hyades and Sirius superclusters. Stars belonging to them span a very wide range
of age, which is difficult to account for in Eggen's scenario. These groups are
thus most probably "dynamical streams" related to the dynamical perturbation by
spiral waves rather than to cluster remnants.
In this scenario, the Hyades and Ursa Major clusters just happen to be in the
Hyades and Sirius streams, which are purely dynamical features that have
nothing to do with the remnants of more massive primordial clusters. This
mechanism could be the key to understanding the presence of an old metal-rich
population, and of many exoplanetary systems in our neighbourhood. Moreover, a
strong spiral pattern seems to be needed in order to yield such prominent
streams. Since spiral structure is usually baryonic, this would leave very
little room for dark matter. This may be an indication that the era of the
dark-matter paradigm explaining the dynamics of the Galaxy may come to an end,
and is being superseded by modified gravity.Comment: 5 pages, 1 figure, to appear in The Three Dimensional Universe with
GAIA, eds M. Perryman & C. Turo
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