36,285 research outputs found
On Dwarf Galaxies as the Source of Intracluster Gas
Recent observational evidence for steep dwarf galaxy luminosity functions in
several rich clusters has led to speculation that their precursors may be the
source of the majority of gas and metals inferred from intracluster medium
(ICM) x-ray observations. Their deposition into the ICM is presumed to occur
through early supernovae-driven winds, the resultant systems reflecting the
photometric and chemical properties of the low luminosity dwarf spheroidals and
ellipticals we observe locally. We consider this scenario, utilising a
self-consistent model for spheroidal photo-chemical evolution and gas ejection
via galactic superwinds. Insisting that post-wind dwarfs obey the observed
colour-luminosity-metallicity relations, we conclude that the bulk of the ICM
gas and metals does not originate within their precursors.Comment: 43 pages, 8 figures, LaTeX, also available at
http://msowww.anu.edu.au/~gibson/publications.html, to appear in ApJ, Vol
473, 1997, in pres
Are Antidepressants Overprescribed? Patients’ Experiences of the Prescribing Process
An online survey was completed by 1829 New Zealand adults prescribed antidepressants. Only 43% were experiencing self-reported “severe” depression when first prescribed antidepressants. Thus, most prescriptions were for depression that was self-reported as mild or moderate, despite studies suggesting that antidepressants are no more effective than placebo at these levels. General practitioners (GPs) prescribed at lower depression levels than psychiatrists and spent less time with patients. 35% of GPs and 42% of psychiatrists reportedly gave no information about adverse effects. Almost no prescribers gave information about adverse effects in the personal and interpersonal domains, or about withdrawal effects. Closer adherence to evidence-based prescribing and to the principle of informed consent may lead to a reduction in unnecessary, ineffective, and potentially harmful prescribing
Top-Down Fragmentation of a Warm Dark Matter Filament
We present the first high-resolution n-body simulations of the fragmentation
of dark matter filaments. Such fragmentation occurs in top-down scenarios of
structure formation, when the dark matter is warm instead of cold. In a
previous paper (Knebe et al. 2002, hereafter Paper I), we showed that WDM
differs from the standard Cold Dark Matter (CDM) mainly in the formation
history and large-scale distribution of low-mass haloes, which form later and
tend to be more clustered in WDM than in CDM universes, tracing more closely
the filamentary structures of the cosmic web. Therefore, we focus our
computational effort in this paper on one particular filament extracted from a
WDM cosmological simulation and compare in detail its evolution to that of the
same CDM filament. We find that the mass distribution of the halos forming via
fragmentation within the filament is broadly peaked around a Jeans mass of a
few 10^9 Msun, corresponding to a gravitational instability of smooth regions
with an overdensity contrast around 10 at these redshifts. Our results confirm
that WDM filaments fragment and form gravitationally bound haloes in a top-down
fashion, whereas CDM filaments are built bottom-up, thus demonstrating the
impact of the nature of the dark matter on dwarf galaxy properties.Comment: 7 pages, 7 figures, replaced with MNRAS accepted version (minor
revisions
Crystal Structure and Chemistry of Topological Insulators
Topological surface states, a new kind of electronic state of matter, have
recently been observed on the cleaved surfaces of crystals of a handful of
small band gap semiconductors. The underlying chemical factors that enable
these states are crystal symmetry, the presence of strong spin orbit coupling,
and an inversion of the energies of the bulk electronic states that normally
contribute to the valence and conduction bands. The goals of this review are to
briefly introduce the physics of topological insulators to a chemical audience
and to describe the chemistry, defect chemistry, and crystal structures of the
compounds in this emergent field.Comment: Submitted to Journal of Materials Chemistry, 47 double spaced pages,
9 figure
Hierarchical formation of bulgeless galaxies II: Redistribution of angular momentum via galactic fountains
Within a fully cosmological hydrodynamical simulation, we form a galaxy which
rotates at 140 km/s, and is characterised by two loose spiral arms and a bar,
indicative of a Hubble Type SBc/d galaxy. We show that our simulated galaxy has
no classical bulge, with a pure disc profile at z=1, well after the major
merging activity has ended. A long-lived bar subsequently forms, resulting in
the formation of a secularly-formed "pseudo" bulge, with the final
bulge-to-total light ratio B/T=0.21. We show that the majority of gas which
loses angular momentum and falls to the central region of the galaxy during the
merging epoch is blown back into the hot halo, with much of it returning later
to form stars in the disc. We propose that this mechanism of redistribution of
angular momentum via a galactic fountain, when coupled with the results from
our previous study which showed why gas outflows are biased to have low angular
momentum, can solve the angular momentum/bulgeless disc problem of the cold
dark matter paradigm.Comment: 9 Pages, 10 Figures, accepted MNRAS version. Comments welcom
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