29,796 research outputs found
Interactions of Satellite Galaxies in Cosmological Dark Matter Halos
We present a statistical analysis of the interactions between satellite
galaxies in cosmological dark matter halos taken from fully self-consistent
high-resolution simulations of galaxy clusters. We show that the number
distribution of satellite encounters has a tail that extends to as many as 3-4
encounters per orbit. On average 30% of the substructure population had at
least one encounter (per orbit) with another satellite galaxy. However, this
result depends on the age of the dark matter host halo with a clear trend for
more interactions in younger systems. We also report a correlation between the
number of encounters and the distance of the satellites to the centre of the
cluster: satellite galaxies closer to the centre experience more interactions.
However, this can be simply explained by the radial distribution of the
substructure population and merely reflects the fact that the density of
satellites is higher in those regions.
In order to find substructure galaxies we applied (and present) a new
technique based upon the N-body code MLAPM. This new halo finder MHF
(MLAPM's-Halo-Finder) acts with exactly the same accuracy as the N-body code
itself and is therefore free of any bias and spurious mismatch between
simulation data and halo finding precision related to numerical effects.Comment: 6 pages, 4 figures, accepted by PASA (refereed contribution to the
5th Galactic Chemodynamics workshop, July 2003
Volatiles in interplanetary dust particles and aerogels
Volatiles measured in 25 interplanetary dust particles (IDPs) are a mixture of both indigenous materials and contaminants associated with the collection and processing of the ODPs prior to analysis. Most IDPs have been collected in the stratosphere using a silicone oil/freon mixture (20:1 ratio) coated on collector plates. Studies have shown that silicone oil, freon and hexane residues remain with the ODPs, despite attempts to clean the IDPs. Analysis of the IDPs with the LMMS-technique produces spectra with a mixture of indigeneous and contaminants components. The contamination signal can be identified and removed; however, the contamination signal may obscure some of the indigeneous component's signal. Employing spectra stripping techniques, the indigenous volatile constituents associated with the IDPs can be identified. Volatiles are similar to those measured in CI or CM carbonaceous chondrites. Collection of IDPs in low-Earth orbit utilizing a Cosmic Dust Collection Facility attached to Space Station Freedom has been proposed. The low-density material aerogel has been proposed as a collection substrate for IDPs. Our studies have concentrated on identifying volatile contaminants that are associated with aerogel. We have found that solvents used for the preparation of aerogel remain in aerogel and methods must be developed for removing the entrapped solvents before aerogels can be used for an IDP collection substrate
The [?/Fe] ratios of very metal-poor stars within the integrated galactic initial mass function theory
The aim of this paper is to quantify the amplitude of the predicted plateau in [α/Fe] ratios associated with the most metal-poor stars of a galaxy. We assume that the initial mass function (IMF) in galaxies is steeper if the star formation rate (SFR) is low – as per the integrated galactic initial mass function (IGIMF) theory. A variant of the theory, in which the IGIMF depends upon the metallicity of the parent galaxy, is also considered. The IGIMF theory predicts low [α/Fe] plateaus in dwarf galaxies, characterized by small SFRs. The [α/Fe] plateau is up to 0.7 dex lower than the corresponding plateau of the Milky Way. For a universal IMF one should expect instead that the [α/Fe] plateau is the same for all the galaxies, irrespective of their masses or SFRs. Assuming a strong dependence of the IMF on the metallicity of the parent galaxy, dwarf galaxies can show values of the [α/Fe] plateau similar to those of the Milky Way, and almost independent of the SFR. The [Mg/Fe] ratios of the most metal-poor stars in dwarf galaxies satellites of the Milky Way can be reproduced either if we consider metallicity-dependent IMFs or if the early SFRs of these galaxies were larger than we presently think. Present and future observations of dwarf galaxies can help disentangle between these different IGIMF formulations
Mapping Substructures in Dark Matter Halos
We present a detailed study of the real and integrals-of-motion space
distributions of a satellite obtained from a self-consistent high-resolution
simulation of a galaxy cluster and re-simulated using various analytical halo
potentials. We found that the disrupted satellite appears as a coherent
structure in integrals-of-motion space in all models (``live'' and analytical
potential) although the distribution is significantly smeared for the live host
halo. Further the primary mechanism for this smearing is the mass growth of the
host, which changes both the energy and angular momentum of the satellite
debris. Hence, this must be considered when searching for (stellar) streams
with future observational experiments such as RAVE and GAIA.Comment: 5 pages, 6 figures, MNRAS accepted - minor editing without changing
the conclusions, a high-resolution version of the paper is available from
http://astronomy.swin.edu.au/~sgill/downloads/downloads.htm
Constraints on Early Nucleosynthesis from the Abundance Pattern of a Damped Ly-alpha System at z = 2.626
We have investigated chemical evolution in the young universe by analysing
the detailed chemical enrichment pattern of a metal-rich galaxy at high
redshift. The recent detection of over 20 elements in the gas-phase of a damped
Lyman-alpha absorber (DLA) at z = 2.626 represents an exciting new avenue for
exploring early nucleosynthesis. Given a strict upper age of ~2.5 Gyr and a
gas-phase metallicity about one third solar, we have shown the DLA abundance
pattern to be consistent with the predictions of a chemical evolution model in
which the interstellar enrichment is dominated by massive stars with a small
contribution from Type Ia supernovae. Discrepancies between the empirical data
and the models are used to highlight outstanding issues in nucleosynthesis
theory, including a tendency for Type II supernovae models to overestimate the
magnitude of the "odd-even" effect at subsolar metallicities. Our results
suggest a possible need for supplemental sources of magnesium and zinc, beyond
that provided by massive stars.Comment: 12 pages, 7 figs. Accepted for publication in ApJ (The Astrophysical
Journal
Diagnostics of Coronal Magnetic Fields Through the Hanle Effect in UV and IR Lines
The plasma thermodynamics in the solar upper atmosphere, particularly in the
corona, are dominated by the magnetic field, which controls the flow and
dissipation of energy. The relative lack of knowledge of the coronal vector
magnetic field is a major handicap for progress in coronal physics. This makes
the development of measurement methods of coronal magnetic fields a high
priority in solar physics. The Hanle effect in the UV and IR spectral lines is
a largely unexplored diagnostic. We use magnetohydrodynamic (MHD) simulations
to study the magnitude of the signal to be expected for typical coronal
magnetic fields for selected spectral lines in the UV and IR wavelength ranges,
namely the H I Ly- and the He I 10830 {\AA} lines. We show that the
selected lines are useful for reliable diagnosis of coronal magnetic fields.
The results show that the combination of polarization measurements of spectral
lines with different sensitivities to the Hanle effect may be most appropriate
for deducing coronal magnetic properties from future observations.Comment: 15 pages, 5 figures, Frontiers in Astronomy and Space Sciences, 201
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