List of Past Astronomy Colloquia : 01-Sep-2007 to 31-Dec-2007

Date:   Wednesday 05-September-2007
Speaker:   Dr. Linda Sparke (U. of Wisconsin)
Title:  "Bars in bars and rings round stars"

High-resolution optical and near-infrared images of the centers of barred galaxies often show smaller, secondary bars within the main bar. A recent survey of 38 barred S0 and Sa galaxies shows bars-inside-bars to be surprisingly common: at least one quarter of the sample is double-barred. We see no significant link between secondary bars and nuclear activity. Some galaxies also show kiloparsec-scale inner disks, and nuclear rings. These central stellar structures suggest that the inner regions of early-type barred galaxies can be dynamically cool and disklike.

The inner bars appear randomly oriented with respect to the main outer bar, suggesting that the two rotate independently. How would the stars and gas orbit in a periodically-changing gravitational potential? I will discuss how one can characterize orbits in the double-bar system, and also in the circumstellar disk of a star in an eccentric binary system.

Date:   Wednesday 12-September-2007
Speaker:   Dr. Ed Shaya (U. of Maryland)
Title:  "Micro-Arcsecond Astrometry with the Space Interferometry Mission (SIM) and the Proper Motion of Galaxies"

The SIM Planetquest Mission will be the first space-based long-baseline Michelson interferometer and will deliver 4 microarcsec parallax on targets as faint as 20th magnitude and 0.6 microarcsec differential positions on bright sources. This represents 100-1000 times improvement over previous astrometric measurements, providing answers to a large range of science questions. It will be able to find earth massed planets at the habitable zones around nearby stars and the frequency of planets in young or old stars. There are also a number of fundamental astrophysical issues that can be addressed by having precision distances anywhere in the galaxy and precision proper motions out to neighboring groups of galaxies. This talk will summarize some of the key projects planned for this mission.

Date:   Wednesday 19-September-2007
Speaker:   Dr. John Debes (DTM, Carnegie Institute of Washington)
Title:  "Red, Grey, or Blue? Going Beyond Simple Colors for the Scattered Light of Resolved Circumstellar Disks"

As more spatially resolved young circumstellar disks are imaged in more wavelengths, astronomers can construct crude measures of the efficiency with which dust in a disk scatters light. These measures can directly probe the composition and dominant grain sizes of the observed dust. Most disks have only been imaged at one or two wavelengths and analysis of these objects is limited. I present recent HST NICMOS coronagraphic imaging of a handful of disks that provide the first step toward creating spatially resolved reflectance spectra of disks that may be in the process of forming planets.

Date:   Wednesday 26-September-2007
Speaker:   Dr. Rachel Somerville (Max-Planck-Institut fuer Astronomie, Heidelberg)
Title:  "The Assembly of Galaxies and their Black Holes: the Role of AGN Feedback in Galaxy Formation"

Over the past decade, it has become increasingly clear that 'standard' models of galaxy formation set within the hierarchical Cold Dark Matter paradigm fail to reproduce observed galaxy properties in several important ways. I will review these failures, which include the "overcooling" and "cooling flow" problems, the failure to reproduce the color-magnitude relation and observed strongly bi-modal distributions of galaxy properties, and the failure to reproduce "downsizing" (earlier quenching of star formation in more massive objects). Recently, it has been proposed that the energy produced during the growth of supermassive black holes could solve all of these problems. I will discuss the physical mechanisms whereby AGN are thought to be able to couple with their surroundings, and assess the feasibility of this proposal from the standpoint of empirical energy constraints from observations. Then, I will discuss new models of galaxy formation in which the co-evolution of black holes and their galaxies are followed self-consistently, along with the effects of the the energy released by growing BH on the host galaxy as well as the feeding of the AGN itself. I will present a status report on how successfully these new models can reproduce key observations of galaxies and AGN at low and high redshift.

Date:   Wednesday 03-October-2007
Speaker:   Dr. Henry Throop (South Western Research Institute)
Title:  "Planet Formation in Dense Dense Star Clusters"

Our Solar System's closest stellar neighbors are several light years away, and most models of the Solar System's formation assume that we have always been isolated from other stars. But a growing body of evidence suggests that most stars and planets form in star clusters far more dense, where tens of thousands of young stars are initially packed into the same volume of space as between us and our closest stellar neighbors. The Orion region is one such nearby dense stellar cluster, where UV radiation, clouds of gas, and encounters with other stars can shape the future of planetary systems. I will discuss recent observations and modeling of the formation of stars, disks, and planets in dense star clusters such as Orion.

Date:   Wednesday 10-October-2007
Speaker:   Dr. Marta Volonteri (University of Michigan)
Title:  "Massive Black Holes: an Evolutionary Path"

I will discuss models for the hierarchical growth of supermassive black holes, feeding pregalactic black hole seeds. Accretion properties, black hole mergers and dynamical interactions, as well as their implications and observational signatures, will be critically addressed.

Date:   Wednesday 17-October-2007
Speaker:   Dr. Michael Blanton (New York University)
Title:  "Cosmology with low luminosity galaxies"

The major features of the cold dark matter cosmological model are now well-established, with the age of the Universe and the contributions of dark matter and dark energy, all well known based on observations of the Universe on large scales. This picture also makes a series of predictions about what happens in the Universe on much smaller scales, for example the scales of dwarf galaxies. I will discuss a method of testing cold dark matter theory using the dynamics and number density of isolated dwarf galaxies (galaxies in halos circular velocities around 50 km/s). Such isolated dwarf galaxies also present an interesting laboratory for studying galaxy formation, and I will present preliminary results on the formation history of such galaxies and how they differ from dwarf galaxies that have interacted with a larger neighbor.

Date:   Wednesday 24-October-2007
Speaker:   Dr. K. Papadopoulos (University of Maryland)
Title:  "Space as an open laboratory for MHD and plasma turbulence"

Date:   Wednesday 31-October-2007
Speaker:   Dr. Anton M. Koekemoer (Space Telescope Science Institute)
Title:  "Populations of Obscured Active Galaxies at High Redshift"

Searches for new populations of obscured active galactic nuclei (AGN) at high redshifts have recently expanded significantly with the advent of deep, wide multiband surveys with Chandra, HST, Spitzer and large ground-based telescopes (such as GOODS, COSMOS and similar projects). In this talk I will describe current work on searches for candidate high-redshift AGN, including `EXO's (extreme X-ray / optical sources) which are faint or undetected at optical wavelengths but well detected in the IR. Together with deep spectroscopy programs, the X-ray and optical/IR fluxes help constrain the spectral energy distributions of the sources and hence the properties of their central black holes. The results are used to examine the evolution of the AGN luminosity function up to high redshift, with corresponding implications for the co-evolution of galaxies and their central black holes.

Date:   Wednesday 07-November-2007
Speaker:   Dr. Kartik Sheth (Caltech/Spitzer Science Center)
Title:  "The Redshift Evolution of Galactic Structures (Bars, Bulges & Disks) at z < 1 from COSMOS: Quantifying the Assembly of the Hubble Sequence"

We have analyzed the redshift-dependent fraction of galactic bars over 0.2

Date:   Wednesday 14-November-2007
Speaker:   Dr. Jerry Sellwood (Rutgers University)
Title:  "New Developments in Spiral Structure Theory"

Progress in the theory of spiral structure has stalled for many years largely because no observational data existed that could discriminate between the various mechanisms proposed to account for the phenomenon. This situation changed dramatically with the publication of the Geneva-Copenhagen Survey, which provides full phase-space information for a large, and unbiased, sample of nearby F & G stars. I will present an extensive review of the current state of spiral structure theory before showing how data from the GCS impinge on the generating mechanism for spirals.

Date:   Wednesday 21-November-2007
Speaker:   No Colloquium Scheduled
Title:  Happy Thanksgiving!

Date:   Wednesday 28-November-2007
Speaker:   CANCELLED

Date:   Wednesday 05-December-2007
Speaker:   Dr. Matt Tiscareno (Cornell University)
Title:  "Knots and Ripples in the Fabric of Saturn's Rings"

Saturn's ring system is the only massive astrophysical disk that we can visit and observe at close range, shedding light on the processes that shape protoplanetary disks and other systems that are distant in space and/or time. Far from being a uniform and tranquil disk, Saturn ring system is perturbed in a number of ways. At close range, ring particles constantly clump together and shear apart under their mutual gravity and Saturn's tides, while embedded moonlets carve out propeller-shaped disturbances. At longer range, massive perturbers exchange angular momentum with the disk, raising spiral waves and complex scalloped gap edges. Cassini Imaging data has shed new light, and raised new questions, on all of these processes.

Date:   Wednesday 12-December-2007
Speaker:   Dr. Margaret Meixner (STScI)
Title:  "Spitzer Survey of the Large Magellanic Cloud, Surveying the Agents of a Galaxy's Evolution (SAGE)"

The recycling of matter between the interstellar medium (ISM) and stars are key evolutionary drivers of a galaxy's visible matter. The SAGE team is performing a Spitzer Legacy imaging survey of the Large Magellanic Cloud (LMC), using the IRAC (3.6, 4.5, 5.8 and 8 microns) and MIPS (24, 70, and 160 microns) instruments on board Spitzer. The Spitzer wavelengths provide a sensitive probe of circumstellar and interstellar dust and hence, allows us to study the physical processes of the ISM, the formation of new stars and the injection of mass by evolved stars and their relationships on the galaxy-wide scale of the LMC. Due to its proximity, favorable viewing angle, multi-wavelength information, and measured tidal interactions with the Milky Way (MW) and Small Magellanic Cloud (SMC), the LMC is uniquely suited for surveying the agents of a galaxy's evolution (SAGE), the ISM and stars. Our uniform and unbiased survey of the LMC (7x7 degrees) will have much better wavelength coverage, up to ~1000 times better point source sensitivity and ~11 times better angular resolution than previous IR surveys. SAGE will reveal over 6 million sources including ~150,000 evolved stars, ~50,000 young stellar objects and the diffuse ISM with column densities >1.2x10^{21} H/cm^2. The diffuse IR emission in the LMC can be associated with individual gas/dust clouds, thereby permitting unique studies of dust processes in the ISM. SAGE's complete census of newly formed stars with masses >1-3 Msun will reveal whether tidally-triggered star formation events in the LMC are sustained or short-lived. SAGE's complete census of evolved stars with mass loss rates >1x10^{-8} Msun/yr will quantitatively measure the rate at which evolved stars inject mass into the ISM. In this talk, I will present an overview of the SAGE survey including preliminary results on ISM, star formation and evolved stars.

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