36,768 research outputs found

    On Dwarf Galaxies as the Source of Intracluster Gas

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    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

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    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

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    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

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    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

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    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

    Opening up the black box of a Gateway to Medicine programme: a realist evaluation

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    OBJECTIVES: A Gateway to Medicine programme, developed in partnership between a further and higher education setting and implemented to increase the socioeconomic diversity of medicine, was examined to identify precisely what works within the programme and why. DESIGN: This study employed realist evaluation principles and was undertaken in three phases: document analysis and qualitative focus groups with widening access (WA) programme architects; focus groups and interviews with staff and students; generation of an idea of what works. SETTING: Participants were recruited from a further/higher education setting and were either enrolled or involved in the delivery of a Gateway to Medicine programme. PARTICIPANTS: Twelve staff were interviewed either individually (n=3) or in one of three group interviews. Nine focus groups (ranging from 5 to 18 participants in each focus group) were carried out with Gateway students from three consecutive cohorts at 2-3 points in their Gateway programme year. RESULTS: Data were generated to determine what 'works' in the Gateway programme. Turning a realist lens on the data identified six inter-relating mechanisms which helped students see medicine as attainable and achievable and prepared them for the transition to medical school. These were academic confidence (M1); developing professional identity (M2); financial support/security (M3); supportive relationships with staff (M4) and peers (M5); and establishing a sense of belonging as a university student (M6). CONCLUSIONS: By unpacking the 'black box' of a Gateway programme through realist evaluation, we have shown that such programmes are not solely about providing knowledge and skills but are rather much more complex in respect to how they work. Further work is needed to further test the mechanisms identified in our study in other contexts for theory development and to identify predictors of effectiveness in terms of students' preparedness to transition
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