328 research outputs found

    OPTICON: EC Optical Infrared Coordination Network for Astronomy

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    OPTICON, the ICN Optical Infrared Coordination Network for Astronomy, brings together for the first time the operators of all Europe's medium to large optical-infrared telescopes, the largest corresponding data archives, and several user representatives. The OPTICON partners work with their communities to identify those major challenges for the future development of European optical-infrared astronomy which require Europe-wide collaboration. OPTICON sponsors and coordinates developments towards these goals, involving the entire astronomical community through workshops and meetings targeted towards these agreed common goals of general importance.Comment: to appear in Organizations and Strategies in Astronomy II, Ed. A. Heck, Kluwer Acad. Pub

    The Science of Galaxy Formation

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    Our knowledge of the Universe remains discovery-led: in the absence of adequate physics-based theory, interpretation of new results requires a scientific methodology. Commonly, scientific progress in astrophysics is motivated by the empirical success of the "Copernican Principle", that the simplest and most objective analysis of observation leads to progress. A complementary approach tests the prediction of models against observation. In practise, astrophysics has few real theories, and has little control over what we can observe. Compromise is unavoidable. Advances in understanding complex non-linear situations, such as galaxy formation, require that models attempt to isolate key physical properties, rather than trying to reproduce complexity. A specific example is discussed, where substantial progress in fundamental physics could be made with an ambitious approach to modelling: simulating the spectrum of perturbations on small scales.Comment: paper at IAU256, The Galaxy Disk in Cosmological Context, Copenhagen, 2008 eds J. Andersen, J. Bland-Hawthorn & B. Nordstro

    Near infrared star counts as a test of Galactic bar structure

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    We present survey data in the narrow-band L filter (nbL), taken at UKIRT, for a total area of 277 square arcmin, roughly equally divided between four regions at zero Galactic latitude and longitudes +-4.3 deg and +-2.3 deg. The 80% completeness level for these observations is at roughly magnitude 11.0. This magnitude limit, owing to the low coefficient for interstellar extinction at this wavelength (A(nbL)=0.047 A(V)), allows us to observe bulge giants. We match the nbL-magnitudes with DENIS survey K magnitudes, and find 95 per cent of nbL sources are matched to K sources. Constructing colour-magnitude diagrams, we deredden the magnitudes and find evidence for a longitude dependent asymmetry in the source counts. We find that there are ~15% and ~5% more sources at the negative longitude than at the corresponding positive longitude, for the fields at +-4.3 deg and +-2.3 deg respectively. This is compared with the predictions of some Galactic bar models. We find an asymmetry in the expected sense, which favours gas dynamical models and the recent deconvolution of surface photometry data (Binney et al. 1991; Binney, Gerhard & Spergel 1997), over earlier treatments of photometric data (e.g. Dwek et al. 1995).Comment: 10 pages, Latex, MNRAS accepte

    The Origin of the Gaussian Initial Mass Function of Old Globular Cluster Systems

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    [Abridged] Evidence favouring a Gaussian initial globular cluster mass function has accumulated over recent years. We show that an approximately Gaussian mass function is naturally generated from a power-law mass distribution of protoglobular clouds by expulsion from the protocluster of star forming gas due to supernova activity, provided that the power-law mass distribution shows a lower-mass limit. As a result of gas loss, the gravitational potential of the protocluster gets weaker and only a fraction of the newly formed stars is retained. The mass fraction of bound stars ranges from zero to unity, depending on the local star formation efficiency ϵ\epsilon. Assuming that ϵ\epsilon is independent of the protoglobular cloud mass, we investigate how such variations affect the mapping of a protoglobular cloud mass function to the resulting globular cluster initial mass function. A truncated power-law cloud mass spectrum generates bell-shaped cluster initial mass functions, with a turnover location mostly sensitive to the lower limit of the cloud mass range. We also show that a Gaussian mass function for the protoglobular clouds with a mean logmG6.16.2{\rm log}m_G \simeq 6.1-6.2 and a standard deviation σ0.4\sigma \lesssim 0.4 provides results very similar to those resulting from a truncated power-law cloud mass spectrum, that is, the distribution function of masses of protoglobular clouds influences only weakly the shape of the resulting globular star cluster initial mass function. The gas removal process and the protoglobular cloud mass-scale dominate the relevant physics. Moreover, gas removal during star formation in massive clouds is likely the prime cause of the predominance of field stars in the Galactic halo.Comment: 24 pages, accepted for publication in MNRA

    Dynamical friction in dwarf galaxies

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    We present a simplified analytic approach to the problem of the spiraling of a massive body orbiting within the dark halo of a dwarf galaxy. This dark halo is treated as the core region of a King distribution of dark matter particles, in consistency with the observational result of dwarf galaxies having solid body rotation curves. Thus we derive a simple formula which provides a reliable and general first order solution to the problem, totally analogous to the one corresponding to the dynamical friction problem in an isothermal halo. This analytic approach allows a clear handling and a transparent understanding of the physics and the scaling of the problem. A comparison with the isothermal case shows that in the core regions of a King sphere, dynamical friction proceeds at a different rate, and is sensitive to the total core radius. Thus, in principle, observable consequences may result. In order to illustrate the possible effects, we apply this formula to the spiraling of globular cluster orbits in dwarf galaxies, and show how present day globular cluster systems could in principle be used to derive better limits on the structure of dark halos around dwarf galaxies, when the observational situation improves. As a second application, we study the way a massive black hole population forming a fraction of these dark halos would gradually concentrate towards the centre, with the consequent deformation of an originally solid body rotation curve. This effect allows us to set limits on the fraction/mass of any massive black hole minority component of the dark halos of dwarf galaxies. In essence, we take advantage of the way the global matter distribution fixes the local distribution function for the dark matter particles, which in turn determines the dynamical friction problem.Comment: 8 pages, 1 figure, Accepted in MNRA
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