Extraplanar Dust in the Edge-On Spiral NGC 891

We present high-resolution (<0.65") optical broad-band images of the edge-on Sb galaxy NGC 891 obtained with the WIYN 3.5-m telescope. These BVR images reveal a complex network of hundreds of dust absorbing structures far from the mid-plane of the galaxy. The dust structures have a wide range of morphologies and are clearly visible to |z|<1.5 kpc from the mid-plane. In this paper we discuss the general characteristics of the population of absorbing structures, as well as physical properties of 12 individual features. These 12 structures are characterised by N_H >10^21 cm^-2, with masses estimated to be more than 2x10^5 - 5x10^6 solar masses, assuming Galactic gas-to-dust relationships. The gravitational potential energies of the individual dust structures, given their observed heights and derived masses, lie in the range of 20-200x10^51 ergs. Rough number counts of extraplanar dust features suggest the mass of high-z gas associated with extraplanar dust in NGC 891 likely exceeds 2x10^8 solar masses, or ~2% of the total neutral ISM mass of the galaxy. We discuss several mechanisms which may produce high-z dust structures such as those seen in the images presented here. It is not yet known which of these mechanisms are primarily responsible for the extensive extraplanar dust structures seen in our images. The data presented are part of a larger program to search for and characterize off-plane dust structures in edge-on systems. (Abstract Abridged)Comment: To appear in the Astronomical Journal: 37 pages, Latex; 9 separate figures; the paper and high-resolution figures are also available at http://www.astro.wisc.edu/~howk/Papers/papers.htm

A Method for Deriving Accurate Gas-Phase Abundances for the Multiphase Interstellar Galactic Halo

We describe a new method for determining total gas-phase abundances for the Galactic ISM with minimal ionization uncertainties. For sight lines toward globular clusters containing both UV-bright stars and radio pulsars, one can measure column densities of HI and several metal ions using UV absorption measurements and of H II using radio dispersion measurements, thereby minimizing ionization uncertainties. We apply this method to the globular cluster Messier 3 sight line using FUSE and HST ultraviolet spectroscopy of the post-asymptotic giant branch star von Zeipel 1128 and radio observations by Ransom et al. of millisecond pulsars. Ionized hydrogen is 45+/-5% of the total along this sight line, the highest measured fraction along a high-latitude pulsar sight line. We derive total gas-phase abundances log N(S)/N(H) = -4.87+/-0.03 and log N(Fe)/N(H) = -5.27+/-0.05. Our derived sulfur abundance is in excellent agreement with recent solar system determinations of Asplund, Grevesse, & Sauval, but -0.14 dex below the solar system abundance typically adopted in studies of the ISM. The iron abundance is ~-0.7 dex below the solar system abundance, consistent with significant depletion. Abundance estimates derived by simply comparing S II and Fe II to H I are +0.17 and +0.11 dex higher, respectively, than our measurements. Ionization corrections to the gas-phase abundances measured in the standard way are, therefore, significant compared with the measurement uncertainties along this sight line. The systematic uncertainties associated with the uncertain contribution to the electron column density from ionized helium could raise these abundances by <+0.03 dex (+7%). [Abridged]Comment: To appear in the ApJ. 25 pages, including figures and tex

XENON10/100 dark matter constraints in comparison with CoGeNT and DAMA: examining the Leff dependence

We consider the compatibility of DAMA/LIBRA, CoGeNT, XENON10 and XENON100 results for spin-independent (SI) dark matter Weakly Interacting Massive Particles (WIMPs), particularly at low masses (~ 10 GeV), assuming a standard dark matter halo. The XENON bounds depend on the scintillation efficiency factor Leff for which there is considerable uncertainty. Thus we consider various extrapolations for Leff at low energy. With the Leff measurements we consider, XENON100 results are found to be insensitive to the low energy extrapolation. We find the strongest bounds are from XENON10, rather than XENON100, due to the lower energy threshold. For reasonable choices of Leff and for the case of SI elastic scattering, XENON10 is incompatible with the DAMA/LIBRA 3$\sigma$ region and severely constrains the 7-12 GeV WIMP mass region of interest published by the CoGeNT collaboration.Comment: 23 pages, 9 figures. Version 2: more careful treatment of XENON10 efficiencies, expanded discussion. A response to arXiv:1006.2031 is found in the Appendi

Developing Walvis Bay Port into a logistics gateway for southern Africa: Issues, challenges and the potential implications for Namibia’s future

Many developing countries wish to become the ‘gateway’ to a region or part of a continent.One strategy involves encouraging logistics cluster development. These hubs support global supply chains and may enable the economic growth of the host country through the resulting trade, as well as providing direct and indirect employment opportunities during the build and subsequent operation of the hub. Namibia intends to develop the Port of Walvis Bay to be come the preferred gateway to southern Africa and the Southern African Development Community region. This article builds on research on Caribbean cluster potential and Namibian logistics to identify the potential benefits and impact on development, as well as the drawbacks and risks of such a strategy