On Measuring Accurate 21-cm Line Profiles with the Robert C. Byrd Green Bank Telescope

We use observational data to show that 21 cm line profiles measured with the Green Bank Telescope (GBT) are subject to significant inaccuracy. These include ~10% errors in the calibrated gain and significant contribution from distant sidelobes. In addition, there are ~60% variations between the GBT and Leiden/Argentine/Bonn 21 cm line profile intensities, which probably occur because of the high main-beam efficiency of the GBT. Stokes V profiles from the GBT contain inaccuracies that are related to the distant sidelobes. We illustrate these problems, define physically motivated components for the sidelobes, and provide numerical results showing the inaccuracies. We provide a correction scheme for Stokes I 21 cm line profiles that is fairly successful and provide some rule-of-thumb comments concerning the accuracy of Stokes V profiles.Comment: 39 pages, 20 figures, accepted for publication in PAS

Polarimetry in the Visible and Infrared: Application to CMB Polarimetry

Interstellar polarization from aligned dust grains can be measured both in transmission at visible and near-infrared wavelengths and in emission at far-infrared and sub-mm wavelengths. These observations can help predict the behavior of foreground contamination of CMB polarimetry by dust in the Milky Way. Fractional polarization in emission from aligned dust grains will be at the higher range of currently observed values of 4-10%. Away from the galactic plane, fluctuations in Q and U will be dominated by fluctuations in intensity, and less influenced by fluctuations in fractional polarization and position angle.Comment: To be published in the proceedings of "The Cosmic Microwave Background and its Polarization", New Astronomy Reviews, (eds. S. Hanany and K.A. Olive

Star Formation and Gas Dynamics in Galactic Disks: Physical Processes and Numerical Models

Star formation depends on the available gaseous "fuel" as well as galactic environment, with higher specific star formation rates where gas is predominantly molecular and where stellar (and dark matter) densities are higher. The partition of gas into different thermal components must itself depend on the star formation rate, since a steady state distribution requires a balance between heating (largely from stellar UV for the atomic component) and cooling. In this presentation, I discuss a simple thermal and dynamical equilibrium model for the star formation rate in disk galaxies, where the basic inputs are the total surface density of gas and the volume density of stars and dark matter, averaged over ~kpc scales. Galactic environment is important because the vertical gravity of the stars and dark matter compress gas toward the midplane, helping to establish the pressure, and hence the cooling rate. In equilibrium, the star formation rate must evolve until the gas heating rate is high enough to balance this cooling rate and maintain the pressure imposed by the local gravitational field. In addition to discussing the formulation of this equilibrium model, I review the current status of numerical simulations of multiphase disks, focusing on measurements of quantities that characterize the mean properties of the diffuse ISM. Based on simulations, turbulence levels in the diffuse ISM appear relatively insensitive to local disk conditions and energetic driving rates, consistent with observations. It remains to be determined, both from observations and simulations, how mass exchange processes control the ratio of cold-to-warm gas in the atomic ISM.Comment: 8 pages, 1 figure; to appear in "IAU Symposium 270: Computational Star formation", Eds. J. Alves, B. Elmegreen, J. Girart, V. Trimbl

Sortes in Latin and German. One Date, one Place, two Manuscript Cultures?

Poster presentation

Proseminar Fachliteratur. Bibliographische Hinweise

Bibliographische Hinweise zur deutschsprachigen Fachliteratur des Mittelalters

A New Technique for Heterodyne Spectroscopy: Least-Squares Frequency Switching (LSFS)

We describe a new technique for heterodyne spectroscopy, which we call Least-Squares Frequency Switching, or LSFS. This technique avoids the need for a traditional reference spectrum, which--when combined with the on-source spectrum--introduces both noise and systematic artifacts such as ``baseline wiggles''. In contrast, LSFS derives the spectrum directly, and in addition the instrumental gain profile. The resulting spectrum retains nearly the full theoretical sensitivity and introduces no systematic artifacts. Here we discuss mathematical details of the technique and use numerical experiments to explore optimum observing schemas. We outline a modification suitable for computationally difficult cases as the number of spectral channels grows beyond several thousand. We illustrate the method with three real-life examples. In one of practical interest, we created a large contiguous bandwidth aligning three smaller bandwidths end-to-end; radio astronomers are often faced with the need for a larger contiguous bandwidth than is provided with the available correlator.Comment: 37 pages, 8 figure