Astrophysics Colloquium
Spring 2008 - Coordinated by P. Woodward
Held in Physics 210 at 3:00pm on Fridays
(unless otherwise noted)
| February 1 | Dr. Richard Massey, Caltech |
| Title: Viewing Dark Matter with Weak Gravitational Lensing | |
| Abstract: Ordinary baryonic particles (such as protons and neutrons) account for only one-sixth of the total matter in the Universe. The remainderis a mysterious "dark matter" component, which does not interact via the electromagnetic force and thus neither emits nor reflects light. However, it has the usual gravitational signature, and gravitational evidence for it is mounting. The past seven years have seen dramatic progress in measurements of weak gravitational lensing, the slight deflection of light from distant sources due to the curvature of intervening space. Recent observations from the Hubble Space Telescopehave provided direct proof for, and large-scale maps of dark matter in the Universe. I review the current state of the art and prospects/challenges for the future of the field. Indeed, gravitational lensing provides one of the most promising routes to fulfil the astrophysical end of the deal in a larger quest to determine the nature of dark matter. | |
| Host: L. Williams | |
| February 8 | Dr. David Yuen, U. Minnesota, Geology |
| Title: Tsunami Waves Along the Chinese Coast from Earthquakes Along Nearby Subducting Zones | |
| Abstract: In this talk we will discuss the nature of tsunami waves and the way probability of tsunami hazards can be estimated from using basic earthquake physics and hydrodynamics. We have used the earthquake statistics and the nonlinear shallow water equations to model the wave height which can appear on the shorelines along the Chinese coast. | |
| Host: T.W. Jones | |
| February 15 | Dr. Juri Toomre, U. Colorado, Boulder |
| Title: Unfolding the Sources of Solar Magnetism with Helioseismology and Simulations | |
| Abstract: There exist major challenges to understand how the Sun builds the large-scale and intense magnetic fields that we observe at its surface and how these fields evolve in time. The origin of these magnetic fields must rest with dynamo processes occurring deep within the star. Many complex dynamical elements are involved in the operation of the solar global dynamo. These include the differential rotation of the convection zone and the tachocline at its base, turbulent production and transport of the magnetic fields by convection, shear amplification of the fields, and magnetic buoyancy that leads to the eventual field eruption onto the photosphere. Major advances in supercomputing are allowing us to improve the fidelity with which we can model these intensely turbulent processes. These efforts are aided by continuing guidance provided by helioseismology in probing dynamics in the solar interior. We discuss the close interplay between helioseismology and recent 3-D simulations in studying the solar global dynamo. | |
| Host: P. Woodward | |
| February 22 | Dr. Annick Pouquet, High Altitude Obs., Boulder, CO |
| Title: Interactions of Eddies and Waves in MHD Turbulence | |
| Abstract: Magnetohydrodynamic (MHD) turbulence is found e.g. in the solar wind, in the solar convection zone or in the interstellar medium. In that context, direct numerical simulations of three-dimensional MHD at a Taylor Reynolds number of 1700 on a grid of 1536^3 points are reported. The flow is incompressible and decaying in time, and the initial condition is a superposition of large scale ABC Beltrami flows for wavenumbers k<= 4 and random noise at small scales with a k^-3 spectrum, with negligible correlation between the velocity and the magnetic field (ρ_C ~ 10^-4 ) and equal kinetic and magnetic energies; finally, no uniform magnetic field is imposed and the magnetic Prandtl number is equal to unity. Parallel current and vorticity sheets form at the same spatial locations, and further destabilize and fold or roll-up after an initial exponential phase. A self-similar evolution of the current and vorticity maxima is found, in which they grow as a cubic power of time; the flow then reaches a finite dissipation rate independent of Reynolds number. A Kelvin-Helmoltz instability of current and vorticity sheets is seen only at the highest Reynolds number, reminiscent of recent observations in the magnetosphere. At peak of dissipation, the total energy spectrum is a combination of two components, each moderately resolved. Isotropy obtains in the large scales, with a spectrum compatible with the Iroshnikov-Kraichnan theory stemming from the weakening of nonlinear interactions due to Alfven waves and leading to a ~k^3/2 law; scaling of structure functions confirms the non-Kolmogorovian nature of the flow in this range. At small scales, weak turbulence emerges with a k_perp^-2 spectrum, the perpendicular direction referring to the local quasi-uniform magnetic field. Finally, local directional alignment of the velocity and magnetic field fluctuations occurs rapidly, both observed in direct numerical simulations and in solar wind data. This relaxation process leads to a local weakening of the nonlinear terms in the small scale vorticity and current structures where alignment takes place. Whether such results are universal is not clear, and several parameters may play a role, such as ρ_C or the amount of magnetic helicity in the flow. Thus, high-resolution parametric studies are needed in order to understand in detail the interactions of turbulent eddies and Alfven waves and the dynamics of reconnection events. In order to reach higher Reynolds numbers, several possibilities will be evoked if time permits. |
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| Host: P. Woodward | |
| February 29 | Dr. David Brain, U.C. Berkeley |
| Title: Aurora in the Lumpy Magnetic Fields of Mars | |
| Abstract: Magnetometer observations from the Mars Global Surveyor spacecraft demonstrated that it has been billions of years since Mars possessed a significant global dynamo magnetic field. As a result, the planet's extended atmosphere interacts directly with the solar wind, similar to the plasma interaction at Venus or comets. At Mars, especially, many independent lines of evidence suggest that this interaction has contributed substantially to the evolution of the climate. Interestingly, strongly magnetized regions of the crust substantially perturb the interaction on both local and global scales. The presence of crustal fields has many interesting consequences, including the creation of localized pockets of protected atmosphere, atmospheric 'escape hatches' for particle deposition and escape, and auroral emission near crustal fields. I will give an overview of the interaction of the solar wind with the Martian atmosphere, including a discussion of three big picture science questions addressed by the study of this interaction. I will then discuss in more detail two examples of the effects of crustal fields: a complex, variable magnetic field topology more similar to the Sun than to any other solar system object, and the observations and consequences of auroral processes operating near Mars. | |
| Host: C. Cattell | |
| March 7 | Dr. Mordecai-Mark Mac Low, American Museum of Natural History |
| Title: Control of Star Formation by Gravitational Instability and Supersonic Turbulence | |
| Abstract: I show how star formation depends on the nonlinear development of gravitational instability in galaxies, using high-resolution numerical models of star cluster formation in isolated and merging disk galaxies. Three main assumptions of these models are an isothermal equation of state as an implicit model of feedback, quick production of molecular gas during gravitational collapse, and neglect of magnetic field. I examine and justify each of these assumptions in turn using extensive local models. Despite the simplicity of our assumptions, the global models quantitatively reproduce not only observed global and local relationships between gas surface density and star formation rate, but also observed star formation thresholds in disk galaxies, properties of globular cluster systems in spiral and elliptical galaxies, and even the observed relation between central massive object and bulge masses. Our results suggest that the dominant physical mechanism determining the star formation rate is the quantitative strength of gravitational instability. | |
| Host: P. Woodward | |
| March 14 | Dr. John Monnier, U. Michigan |
| Title: Young Stellar Objects: The Inner AU | |
| Abstract: Newly-commissioned long-baseline interferometers can scrutinize galactic (and extragalactic) objects with nano-radian angular resolution. This incredible resolution allows us a look into the inner accretion disks of young stellar objects (YSOs) in nearby star forming regions. The inner AU of these "protoplanetary" disks hosts a variety of important and fascinating phenomena: it is the site of magnetospheric accretion lifting material off the disk, the disk material here mediates the orbital migration of newly formed "Hot Jupiter" exoplanets, and, of course, terrestrial planet building occurs in the the inner AU. Here, I will give an overview of recent work in this field, results which have directly challenged existing paradigms of the inner accretion disk and the star-disk connection. Soon, we will have the capability to directly image the inner disk using the CHARA interferometer and I will show some exciting new results imaging the surfaces of rapid rotators and interacting binaries. | |
| Host: R.M. Humphreys | |
| March 21 | No Colloquium - Spring Break |
| March 28 | Dr. Brad Peterson, Ohio State U |
| Title: Masses of Black Holes in Active Galactic Nuclei | |
| Abstract: Supermassive black holes, long suspected of driving nuclear activity in quasar-like objects, reside at the centers of most, if not all, massive galaxies. Methods by which their masses are measured, such as modeling of stellar or gas dynamics on resolvable scales, are difficult or impossible to use when there is an active nucleus present. However, in the case of active galaxies, the central mass can be measured by the process of reverberation mapping. The time-delayed response of the broad emission lines to continuum flux variations allows us to determine the size of the line-emitting region, and the mass of the central object is obtained by combining the size with the Doppler-broadened line width. In this talk, I will outline the fundamentals of reverberation mapping and black hole mass measurement and show how these anchor simple scaling relationships that allow us to easily estimate black hole masses for large samples of quasars. | |
| Host: R.M. Humphreys | |
| April 4 | Dr. Pauline Barmby, U. Western Ontario |
| Title: Big and Little Pictures of M31 | |
| Abstract: The Andromeda galaxy, M31, has been of key importance to astronomy ever since Hubble used it to show that spiral nebulae were external galaxies. In this talk I'll present two views of M31: the first is a large mid-infrared mosaic made with the IRAC instrument on the Spitzer Space Telescope. We've used this image to measure M31's stellar mass and star formation rate, and to discover an inner ring which we believe to be the result of a head-on collision with M32. The second view of M31 is a close-up look at its most massive globular clusters with the Hubble Space Telescope. Fitting structural models to these images allows us to determine their place on the "globular cluster fundamental plane", which for M31 clusters extends over nearly four decades in luminosity. I'll discuss the implications of the very tight fundamental plane correlations for both cluster and galaxy formation. | |
| Host: M. Boyer | |
| April 11 | Dr. Alex Lazarian, U. Wisconsin, Madison |
| Title: Quantitative Theory of Grain Alignment and its Implications | |
| Abstract: Alignment is one of the most fundamental properties of cosmic dust. Grain alignment is believed usually believed to happen in respect to magnetic field and thus provides a way of tracing magnetic fields with extinction and emission polarimetry. Potentially, this is also a great way to use polarization measurements to test the dust properties, e.g. magnetic properties, and dust environment. This, however, requires a quantitative understanding of grain alignment, which has been missing for nearly 60 years. I shall show that the textbook solutions of the alignment problem are not tenable for most of the interstellar grains. Instead, I shall identify grain helicity, which stems from their irregularity, as the most important component of the alignment theory. I shall demonstrate a simple analytical model of a helical grain that is able to explain the existing observational data (including the cases when the alignment fails) and thus allows reliable studies of magnetic fields. I s hall discuss observations that can determine whether aligned grains contain superparamagnetic particles, as well as situations when we expect aligned grains to fail tracing of magnetic fields. | |
| Host: T.J. Jones | |
| April 18 | Dr. Henric Krawcyznski, Washington U., St. Louis, MO |
| Title: Gamma-ray Astronomy with the VERITAS Experiment | |
| Host: T.W. Jones | |
| April 25 | Dr. Ben Davies, Rochester Institute of Technology |
| Title: The Scutum Red-Supergiant Clusters | |
| Abstract: I present the recent discoveries of two Galactic massive young clusters, which together contain 40 Red Supergiants - 20% of all those known in the Galaxy, and as many in the entire Large Magellanic Cloud. From observations and evolutionary synthesis models, we argue that the cluster masses are comparable to the other Galactic 'Super Star Clusters' such as Westerlund 1 and the Arches Cluster. The two clusters are located at the base of the Scutum-Crux spiral arm, and appear to lie at the tip of the Galactic Bar. Therefore, these objects can be used as a probe to study the star-formation history and evolution of the Galaxy. Further, the distinctly different ages of the clusters, uniform metallicity, and large number of RSGs, mean that these objects now offer an unprecedented opportunity to study the pre-supernova evolution of massive stars. | |
| Host: R.M. Humphreys | |
| May 2 | Dr. Ben Williams, U. Washington |
| Title: The Chandra View of M31 and M33 | |
| Abstract: Chandra X-ray imaging of M31 and M33 is providing a wealth of detailed information about both the individual sources and whole populations within these galaxies. High spatial resolution imaging has probed the galactic nuclei, the morphology of supernova remnants, and the X-ray binary luminosity functions of crowded regions. High-precision astrometry, along with available optical data, supplies insights into the multiwavelength nature of the sources. Furthermore, the ability of Chandra to observe M31 and M33 at many different times throughout the year has allowed the discovery of many transient X-ray sources that would otherwise have been missed. Such results supply new tools for constraining the formation and evolution of both X-ray sources and galaxies. | |
| Host: D. Weisz | |
| May 9 | Dr. John Moustakas, New York U. |
| Title: Ten Billion Years of Chemical Evolution in Star-Forming Galaxies | |
| Abstract: Local star-forming galaxies obey a striking correlation between stellar mass and metallicity spanning five orders of magnitude in mass and a factor of 300 in metallicity. The gas-phase metallicity of a galaxy reflects its integrated star formation history, modulo inflows and outflows of gas and metals; therefore, the mass-metallicity relation, and its evolution with redshift, serves as a powerful metric of galaxy evolution. We have measured the mass-metallicity relation between z of 0.01 and 0.8 using spectrophotometry of ~5000 I-band selected galaxies observed as part of the AGN and Galaxy Evolution Survey. We find that the mean metallicity of luminous star-forming galaxies has increased by less than 15% since z~0.8, indicating that the bulk of their metals (and therefore stars) were synthesized at z greater than 1. However, we have identified an intriguing population of luminous/massive metal-poor galaxies at z~0.7 that must trace a much more rapid chemical enrichment history to reach the present-day mass-metallicity relation. Finally, I compare these results to observations of star-forming galaxies at z=1-2, and discuss the challenges of extending metallicity measurements to lower mass/luminosity and higher redshift. | |
| Host: E. Skillman |