List of Past Astronomy Colloquia : 01-Jan-1999 to 01-Jun-1999

Date:   Wednesday 03-Feb-99
Speaker:   Dr. Prasenjit Saha (Oxford University)
Title:  Reconstructing gravitational lenses: some theory, some mass maps, and some inferences about H0

Date:   Monday 08-Feb-99 (Note Special Day)
Speaker:   Dr. Kathryn Johnston (Institute for Advanced Study)
Title:  The Milky Way as a Cannibal: What was for breakfast? What is for lunch? What will be for dinner?

Date:   Wednesday 10-Feb-99
Speaker:   Dr. Christopher Reynolds (University of Colorado)
Title:  Probing supermassive black holes with X-ray spectroscopy

Date:   Wednesday 17-Feb-99
Speaker:   Dr. Daniel Eisenstein (Institute for Advanced Study)
Title:  Combining CMB Experiments and Large Redshift Surveys

Date:   Wednesday 24-Feb-99
Speaker:   Dr. Max Tegmark (Institute for Advanced Study)
Title:  Precision Cosmology

I discuss how upcoming cosmic microwave background experiments, galaxy surveys, gravitational lensing and supernova studies can be combined to accurately measure the contents and geometry of the universe. I also present constraints on cosmological models from the latest measurements and discuss recent progress on real-world headaches such as CMB foregrounds and galaxy bias.

Date:   Monday 01-Mar-99 (Note Special Day)
Speaker:   Dr. Dimitrios Psaltis (Harvard-Smithsonian CfA)
Title:  Neutron Stars and Black Holes: Probing Strong-Field Gravity

Date:   Wednesday 03-Mar-99
Speaker:   Dr. John Dubinski (CITA, University of Toronto)
Title:  The Dynamical Evolution of Galaxies in Clusters

Clusters are unique laboratories for studying the dynamics and evolution of galaxies. Hierarchical merging and tidal interactions drive the dynamical evolution of spiral galaxies and their morphological transformation into S0 and ellipticals. A galaxy cluster is the place where cosmology meets the complexity of galaxy dynamics. I'll present results from high-resolution, parallel, N-body simulations from an ongoing simulation survey of clusters with resolved disk galaxies in different cosmological models. The simulations provide new insights into the morphology of high redshift galaxies, the origin of the brightest cluster galaxies and ellipticals in clusters through merging, tidal heating and stripping of spirals, and the distribution of intergalactic tidal debris.

Date:   Wednesday 10-Mar-99
Speaker:   Dr. Michael Fall (STScI)
Title:  Cosmic History Since z = 5

Date:   Wednesday 17-Mar-99
Speaker:   Dr. M. Coleman Miller (University of Chicago)
Title:  Taking the Pulse of a Neutron Star

Date:   Wednesday 31-Mar-99
Speaker:   Dr. Alan Boss (DTM, Carnegie Institution of Washington)
Title:  Giant Planet Formation: Gas Accretion or Disk Instability?

Date:   Wednesday 07-Apr-99
Speaker:   Dr. Patrick Shopbell and Dr. Neal Turner (U. Maryland)
Title:  "The Search for Extra-Galactic Ionized Gas" and "The role of magnetic instabilities in driving outflows from young stars"

Date:   Wednesday 14-Apr-99
Speaker:   Dr. Alexander Nindos and Dr. Friedrich Wyrowski (U. Maryland)
Title:  "Transient Energetic Phenomena on the Sun" and "Hot molecular cores in high mass star forming regions"

Date:   Wednesday 21-Apr-99
Speaker:   Dr. Philip Mannheim (U. Connecticutt)
Title:  How Good is Newton's Law of Gravity?

While there is extremely good evidence for the validity of Newton's Law of Gravity for systems of size up to that of the solar system, the situation for much larger astronomical systems such as galaxies is not at all as clear. Specifically, the motions of galactic stars and gas far from galactic centers do not show the familiar and expected Keplerian fall off. Rather, the orbital rotational velocity curves appear to be flat in structure, leading to the suggestion that galaxies be composed predominantly of large amounts of non-luminous faint or dark matter; with the very recent Hubble Space Telescope and gravitational microlensing searches for such matter within the Milky Way Galaxy having produced results which are both tantalizing and equivocal. We review all of these developments as well as a recent proposal by the speaker to modify gravity in a way that would eliminate the need for dark matter altogether. Further, we show that these same galactic rotation curves possess an explicit imprint due to the global cosmological Hubble flow, an imprint which is characteristic of this modified theory, and identify it as a possible manifestation of Mach's principle. Additionally, we show that this same modified theory naturally provides for the apparently recently detected cosmic repulsion which is causing the universe to speed up. ***** Dr. Mannheim will also give a talk in the Physics department Thursday 22 April titled "Cosmic Acceleration and a Natural Solution to the Cosmological Constant Problem" (gr-qc/9903005).

Date:   Wednesday 28-Apr-99
Speaker:   Dr. Ethan Vishniac (Johns Hopkins)
Title:  A Route to Fast Reconnection in Astrophysical Plasmas: Why Magnetic Fields (Sometimes) Change Their Stripes

In ideal MHD the topology of a magnetic field cannot change, even if the fluid containing the field undergoes strong turbulent motions. Allowing for a small, but nonzero, resistivity changes this conclusion, but not by much. Simple models of magnetic reconnection suggest that in any real astrophysical system, changes in topology take a very long time (roughly proportional to the resistivity to the -1/2 power). This is inconsistent with our observations of astrophysical systems, like the Sun, which show reconnection on dynamical time scales. This is true both for cases in which reconnection can be seen (almost) directly, for example the solar corona and chromosphere, and for cases in which reconnection is merely a strong inference, for example the reversal of the solar dipole moment every 11 years. We examine the effect of weak, small scale magnetic field structure on the rate of reconnection in a strongly magnetized plasma. This affects the rate of reconnection by reducing the transverse scale for reconnection flows, and by allowing many independent flux reconnection events to occur simultaneously. We show that this leads directly to reconnection speeds which are independent of the level of resistivity, although strongly dependent on the level of magnetic field line stochasticity. The bulk of the energy liberated in this kind of reconnection goes into large scale gas motions; a small fraction goes into high frequency Alfven waves; and a very small fraction goes into direct heating of the electrons. The presence of a substantial neutral component in the cold phases of the ISM may actually inhibit reconnection.

Date:   Wednesday 05-May-99
Speaker:   Dr. Marco Spaans (Harvard-Smithsonian CfA)
Title:  Dusty Lyman Alpha Emitters at High Redshift

Date:   Wednesday 12-May-99
Speaker:   Dr. Bradley Schaefer (Yale)
Title:  Cosmology from Supernovae

Supernovae are excellent standard candles visible across much of the Universe, and so they can map out distances and curvature of space. With eight Type Ia supernovae calibrated with Cepheids, the distance scale gives a Hubble Constant of 60+-3 km/s/Mpc. When very distant events (0.4

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