Department of Astronomy

Abstracts for Spring 2008 Colloquia (most recent update: Apr. 22, 2008)

January 15, 2008	Jenny E. Greene, Princeton University "Local Galaxies with Radiating Black Holes" The evolution of supermassive black holes and galaxies are apparently inextricably linked, and we might hope to gain unique insight into the nature of their connection by studying galaxies with currently accreting black holes. The study of active host galaxies has a long history; I focus here on three innovative experiments that exploit both the recent availability of large spectroscopic surveys such as the SDSS as well as improved techniques to estimate black hole masses in active galactic nuclei (AGNs). The first uses HI observations of local AGNs to measure their circular velocities and disturbance levels. The second focuses on luminous narrow-line AGNs, where we have a direct view of the host galaxy, to look for direct evidence of large-scale outflows, possibly driven by the nuclear activity. Finally, I examine the host galaxies of AGNs selected to have the lowest known black hole masses known, a regime in which the correlations between black holes and bulges must begin to break down. I discuss the implications for AGN feedback, and the possibility of observing continuing evolution in the relations between black hole mass and bulge properties at the present epoch.
January 22, 2008	Ramesh Narayan, Harvard University Special Event: Beatrice M. Tinsley Visiting Professor in Astronomy "Measuring Black Hole Spin" An astrophysical black hole is completely described with just two parameters: its mass and its dimensionless spin. A few dozen black holes have mass estimates, but until recently none had a reliable spin estimate. The first spins have now been measured for black holes in X-ray binaries. The talk will summarize the current state of the field and will discuss prospects for the future.
January 29, 2008	Todd Thompson, Ohio State University "Feedback in Starburst Galaxies" I will describe a set of projects aimed at developing a physical picture of the feedback and regulation processes in the ISM of starburst and ultra-luminous infrared galaxies. The UV radiation produced by massive stars in the ISM of starbursts is absorbed, scattered, and reprocessed into the IR by dust grains. The associated radiation pressure may provide the dominant vertical support against gravity. I will discuss the implications of this important feedback process. Because the disk radiates at its Eddington limit (for dust), starbursts supported by by radiation pressure achieve a characteristic radiative flux and their "Schmidt Law" for star formation changes qualitatively with respect to normal star-forming galaxies. I will also describe observational and theoretical constraints on the magnetic field strength in starbursts, with direct implications for the origin of the FIR-Radio correlation, the shape of the radio spectra of starbursts, and their and their observability at GeV energies by GLAST. Lastly, I will discuss the starburst contribution to the diffuse extragalactic gamma ray and radio backgrounds.
February 5, 2008	No talk scheduled.
February 12, 2008	Ivan Hubeny, University of Arizona Special Event: Beatrice M. Tinsley Visiting Professor in Astronomy (visit: Feb. 11-21) "From Stars (Planets) to Quasars: Modeling Radiation-Dominated Objects in Astrophysics"
February 13, 2008 Wednesday	Klaus M. Pontoppidan, California Institute of Technology (Caltech) Special Colloquium (visit: Feb 13-14) "Straddling the Snow Line: Water Vapor and Carbon-Containing Molecules in Protoplanetary Disks at 1 AU" The planet-forming zones (radii of 0.1 - 10 AU) of disks around young solar-type stars is an active environment characterized by a rich chemistry and a complex dynamical synergy between gas, dust ad potentially proto-planets. I will discuss how how very high resolution infrared spectroscopy of warm molecular gas can provide unique tests for models of the evolution of such disks. New results include the detection of water vapor and OH in a large fraction of protoplanetary disks, showing that the water molecule is abundant, not only beyond the snow line, but also in the inner warm regions of the disk, suggesting that it is being replenished by inward migration of icy bodies. I will also present the first results imaging molecular gas in planet-forming zones at milli-arcsed resolution (or ~0.1 AU) using spectro-astrometry. The technique is used to image the distribution and kinematics of gas inside the inner dust gaps of so-called 'transition' disks, some of which have been suggested to harbor newly formed planets. I will discuss how spectro-astrometry of molecular gas can be used to distinguish between various gap formation mechanisms.
February 19, 2008	Nick Kaiser, University of Hawaii "The Pan-STARRS Wide Field Survey Project The Institute for Astronomy at the University of Hawaii is developing a large optical/near-IR survey telescope system; the Panoramic Survey Telescope and Rapid Response System. Pan-STARRS will employ 1.8m optical imagers, very large (7 square degree) field of view and revolutionary 1.4 billion pixel CCD cameras with low noise and rapid read-out to provide broad band imaging from400-1000nm wavelength. The project is proceeding in two phases: PS1 is a single aperture system that has been deployed onHaleakala on Maui and will start a 3.5 year survey in 2008. Support for operations is being provided by the PS1 Science Consortium, which includes scientists in the US, the UK and in Germany. The full 4-aperture system PS4 will be sited on Mauna Kea and will become operational following the end of the PS1 mission. The data from Pan-STARRS will be reduced in near real time to produce both a cumulative image of the staticsky and difference images, from which transient, moving and variable objects can be detected. Pan-STARRS will be able to sca the entire visible sky to approximately 24th magnitude in less than a week, and this unique combination of sensitivity and cadence will open up many new possibilities in time domain astronomy. A major goal for the project is to survey potentially dangerous asteroids, where Pan-STARRS will be able to detect most objects down to 300m size, much smaller than the km size objects accesssible to existing search programs. In addition, the Pan-STARRS data will provide a dramatic leap in data quality and extent over existing wide-field image survey data that will be used to advance our understanding of the formation of the Solar System, the Galaxy and the Cosmos at large. In this talk I will describe the science drivers for the project, review the technical design, the various scientific goals, and give an update on the progress that has been made in commissioning the first telescope system.
February 20, 2008 Wednesday	Ivan Hubeny, University of Arizona Special Event: Beatrice M. Tinsley Visiting Professor in Astronomy (visit: Feb. 11-21) "Transiting Extrasolar Planets: A Key to Understanding their Physics Chemistry, and even Meteorology"
February 21, 2008 Thursday	Seth L. Redfield, University of Texas at Austin Special Colloquium (visit: Feb. 21-22) "The Age of Exometeorology: Observing the Atmospheres of Transiting Exoplanets" High signal-to-noise ratio (S/N) and high spectral resolution observations of transiting exoplanets provide an opportunity to measure the properties of exoplanet atmospheres through transmission spectroscopy. I present the results of a large-scale program which led to the first ground-based detection of absorption due to an exoplanetary atmosphere. Observations were taken with the Hobby-Eberly Telescope (HET), which is well suited for this type of program given its large mirror, high resolution spectrograph, and queue scheduling. We observe significant additional absorption in the cores of the neutral sodium doublet relative to the continuum, when in-transit observations are compared to out-of-transit observations. Two tests are performed to confirm the detection: (1) analysis of a strong control line that is predicted to show no absorption, and (2) an empirical Monte Carlo analysis to quantify the impact of systematic errors. Physical properties of the exoplanet atmosphere, such as atomic and molecular composition, cloud altitudes, and wind speeds may be derived from the transmission spectrum. I will discuss the richness of the acquired datasets and describe several complimentary research projects. Observations of several other bright transiting systems are ongoing with the HET, while future large telescopes will push research toward observations of smaller atmospheres (e.g., terrestrial exoplanets), weaker spectral features (e.g., biomarkers), and temporal variability (e.g., weather). A growing sample of transiting exoplanets coupled with continued observational achievements and capabilities are just beginning to make direct comparisons of physical characteristics of exoplanet atmospheres possible, ventually leading to comparative exometeorology.
February 26, 2008	Yoram Lithwick, Canadian Institute for Theoretical Astrophysics (CITA) Special Colloquium (visit: Feb. 26-27) "Dynamics of Protoplanetary Disks: Vortices and Turbulence" Protoplanetary disks are gaseous accretion disks around young stars. I will show how, in hydrodynamical accretion disks such as these, vortices form out of a nonlinear instability. I will also show that a vortex's ultimate fate in 3D depends on whether it is weak or strong. Weak vortices live essentially forever. Strong vortices decay into turbulence via a 3D instability. One interesting implication is that in protoplanetary disks dust can be trapped by weak vortices, triggering planet formation. A second implication is that strong vortices can be responsible for the turbulence that makes these accretion disks accrete.
February 28, 2008 Thursday	Sarah E. Robinson, University of California / Lick Observatory Special Colloquium (visit: Feb. 28-29) "Chemistry of Giant Planet Formation: Future Directions in Theory and Observations" What role do ices play in giant planet formation? New chemical models of the solar nebula show that in addition to water ice, Saturn's feeding zone contains solid ammonia, methane, and hydrogen cyanide. These simulations cann account for both Saturn's large core (<= 22 Earth masses) and its formation at a large distance from the sun, where solid core growth is slow. Organic ices incorporated into planetesimals help boost Saturn's core mass and growth rate so that the planet forms in only 3.4 Myr, similar to the observed median protostellar disk lifetime. Models of planet formation in ice-rich regions can be extended to other planets with heavy cores such as HD 149026 b, Neptune and Uranus. Core accretion simulations based on a a disk with twice solar metallicity allow HD 149026 b to form in only 1.6 Myr, leaving ample time for migration into the current short-period, transiting configuration.
March 4, 2008	Nicole P. Vogt, New Mexico State University (NMSU) "Nine Billion Years of Galaxy Evolution: Disentangling Recent Evolution and Selection Biases in Disk Galaxies We review the status of current observations of the fundamental parameters of intermediate redshift (z < 1.3) disk galaxies. Modern instrumentation enables detailed measurements of galaxy luminosity, morphology, kinematics and mass, in both optical and the infrared passbands. By studying well known star formation indicators, the internal velocity structure and star formation rates of galaxies can be traced though this entire redshift regime. The combination of throughput and optimum seeing conditions yields spectra which can be combined with high resolution multiband imaging to explore the evolution of galaxies of various morphologies, and to place constraints on current models of galaxy formation and star formation histories. Out to redshifts of unity, these data form a high redshift Tully-Fisher relation that spans four magnitudes and extends to well below L*, with no obvious change in shape or slope with respect to the local relation. A comparison of disk surface brightness betwen local and high redshift samples yields an offset in accordance with distance-dependent surface brightness selection effects, as can the apparent change in disk size with redshift for disks of a given mass. The effects of imaging and spectral selection are shown to be significant, dependent not only upon the broad-band luminosity and surface brightness of targets but also a strong function of emission line strength and spectral flux distributions. These results provide further evidence for modest increases in luminosity with lookback time for the bulk of the observed field spiral galaxy population.
March 5, 2008 Wednesday	Michael P. Muno, California Institute of Technology (Caltech) Special Colloquium (visit: Mar. 5-6) "Identifying Black Holes and Neutron Stars in Order to Understand How They Formed (or, Making Dead Stars Talk)" I will describe my observational programs to assemble samples of black holes and neutron stars. I am using these samples to address several open questions, including: Which stars from black holes? What fraction of neutron stars ar born with extremely bestrong magnetic fields (B > 10^14G)? What is the distribution of masses for stellar black holes, and why have we found so few with masses just larger than the maximum mass of a neutron star? I will report the progress I am making by combining X-ray images from the Chandra Observatory with ground-based infrared follow-up, and briefly describe the potential of planned X-ray observatories, such as the Nuclear Spectroscopic Telescope Array (NuSTAR).
March 18, 2008	Harold "Hal" F. Levison, Southwest Research Institute/Dept. of Space Studies "The Early Dynamical Evolution of the Outer Solar System: A Nice Story" The orbits of the giant planets in our Solar System have changed significantly since the planets formed. This is made clear by the complex and excited dynamical state of the small body reservoirs. In this talk, I will discuss a new model for the early evolution of the outer Solar System. In this scenario, the giant planets start in a very compact configuration that went violently unstable some 600 Myr after the Solar System formed. This model explains, for the first time, many of the observed characteristics, including (1) the orbits of the giant planets, particularly the eccentricities of Jupiter and Saturn, (2) the structure of the trans-Neptunian region, (3) the origin of the Trojan and Hilda asteroids, and (4) the late heavy bombardment of the Moon.
March 25, 2008	Eiichiro Komatsu, University of Texas at Austin "The 5-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Interpretation" We have just announced the results from 5 years of observations of the Wilkinson Microwave Anisotropy Probe (WMAP) on March 5, 2008. In this talk I will give a summary of significant new findings, as well as the cosmological interpretation of the WMAP 5-year data.
April 1, 2008	No talk scheduled.
April 4, 2008 Friday	Jane Arthur, Centro de Radioastronomia y Astrofisica, UNAM Special Event: Beatrice M. Tinsley Visiting Scholar in Astronomy (visit: Apr. 2-5) "HII Regions for the 21st Century" Textbook spherical HII regions do not bear much resemblance to reality, e.g. observed HII regions such as the Orion Nebula. Recent modeling work has gone a long way towards including the physical processes and ambient conditions necessary to enable meaningful comparisons, both morphological and spectral, to be made. In this talk, I describe these endeavours and discuss improvements to the models that could be made in the future.
April 8, 2008	Phil Hopkins, Harvard-Smithsonian Center for Astrophysics Special Event: Beatrice M. Tinsley Visiting Scholar in Astronomy (visit: Apr.6-18) "Dissipation in the Formation of Elliptical Galaxies: Putting Our Understanding of Spheroid Formation to the Test" We generally believe that galaxy spheroids are formed from the merger of (initially) disk-dominated systems, but for ~30 years this idea has been challenged on the basis that ellipticals are observed to be much more dense than spirals. Dissipation -- the formation of new stars in a gas-rich starburst -- has generally been invoked as the means to solve this dilemma, but simulations are only just reaching the point where they can include this in a self-consistent manner and make objects that look like "real" ellipticals. Meanwhile, although observations are probing nearby galaxies with incredible detail, there is no well-established way to actually estimate the quantity desired to test these models and measure the most important element of bulge formation history: how much gas dissipation was involved in their formation. By combining new generations of numerical simulations with exceptional space and ground-based observations, we show not only that can we reproduce the properties of observed ellipticals, but we can use the simulations to understand how the profiles of ellipticals are shaped by dissipation and merger history. This allows us to construct robust, empirical decompositions, separating the observed systems into violently relaxed stars and those formed dissipationally. The dissipational-dissipationless breakdown has dramatic consequences for galaxy properties, and allows us to test the key elements of any model for the origin of ellipticals and the fundamental plane scalings of mergers.
April 10, 2008 Thursday	Phil Hopkins, Harvard-Smithsonian Center for Astrophysics Special Event: Beatrice M. Tinsley Visiting Scholar in Astronomy (visit: Apr.6-18) "Quasars and Galaxy Mergers: Fueling, Feedback, and Evolution" Most models of galaxy formation now invoke (or even require) some form of feedback and interplay between supermassive black holes and galactic gas, and observations of quasars increasingly probe their host galaxy properties and fueling mechanisms. But we know disturbingly little about the details and implications of the 'overlap' between these areas. I'll discuss work we've done to explore evolutionary models for starbursts, quasars, and speroidal galaxies in which supermassive black holes play a dominant role. In this picture, mergers between gas-rich galaxies drive nuclear inflows of gas, producing intense starbursts and feeding the growth of supermassive black holes. During this phase, the black hole is heavily obscured, but feedback energy from its growth expels the gas, rendering the black hole briefly visible as a bright, optical source, and eventually halting accretion. This class of models provides a natural explanation for correlations between black hole bass and host galaxy properties, but there arise a number of other interesting, and in some ways unexpected implications for the nature of quasar lifetimes and lifecurves, the impact of quasar feedback on the host galaxy, and the interpretation of the quasar luminosity function. Moreover, this is almost certainly no the only way to make an AGN: I'll discuss alternative models of quasar fueling, where and when they are expected to be important, and how observers might be able to tell the difference.
April 14, 2008 Monday	Fred K. Y. Lo, Director, National Radio Astronomy Observatory (NRAO) "Mega-masers, Dark Energy and the Hubble Constant Powerful water maser emission (water mega-masers) can be found in accretion disks in the nuclei of some galaxies. Besides providing a measure of the mass at the nucleus, such mega-masers can be used to determine the angular diameter distance to the host galaxy, based on a simple model. We will explain the importance of determining the Hubble Constant to a high accuracy for constraining the equation of state of Dark Energy and describe the Mega-maser Cosmology Project that has the goal of determining the Hubble Constant to ~3%. We will also present the scientific capabilities of the current and future NRAO facilities: ALMA, EVLA, VLBA and GBT, for addressing key astrophysical problems.
April 22, 2008	Ehud Nakar, California Institute of Technology (Caltech) (visit: Apr. 21-25) "The Search for the Origin of Short Gamma-Ray Bursts" Two types of Gamma-Ray Bursts (GRBs) are observed: short duration and long duration. While it is known for several years now that long GRBs are the emission of cosmic ultra-relativistic outflows that are launched following the collapse of massive stars, the origin of short GRBs remained a complete mystery until recently. The breakthrough came on the summer of 2005 with the first detection of short GRB afterglows, long wavelength emission that follows the burst of gamma-rays. These observations established that short GRBs are cosmological relativistic explosions as well, but unlike their long relatives they do not originate from massive stars. Instead, observations suggest that double neutron star mergers may be the progenitors of short GRB, in which case they are the electromagnetic counterparts of a strong gravitational-wave signal. The search for the progenitors of short GRBs, following the recent discoveries, is reviewed.
April 29, 2008	Jonathan McKinney, Harvard-Smithsonian Center for Astrophysics Special Event: Beatrice M. Tinsley Visiting Scholar in Astronomy (visit: Apr.20 - May 4) "Magnetically-Driven Ultrarelativistic Jets from Rotating Black Holes" We now understand the basic mechanisms for astrophysical systems, such as accreting black holes, to generate moderately relativistic outflows from x-ray binaries and active galactic nuclei. An unsolved theoretical problem is how gamma-ray burst (GRB) engines are able to produce Lorentz factors of roughly 400 and opening angles of a few degrees. I will review the mechanisms for how jets are produced, discuss recent advances from general relativistic magnetohydrodynamical simulations of accreting rotating black holes, and describe recent theory and simulations that show how the Lorentz factors, opening angles, and jet structures might naturally be determined for GRBs.
May 6, 2008	Travis Barman, Lowell Observatory, Flagstaff, Arizona Special Event: Beatrice M. Tinsley Visiting Scholar in Astronomy (visit: May 5-16) "Exploring the Atmospheres of Extrasolar Planets" Planets are now routinely found around other stars and the flood of discoveries shows no signs of abating. A growing number of planetary systems are currently being discovered in very accommodating orbits, opening up new possibilities for more detailed study of distant atmospheres. Cleverness and new technology have been combined to allow us to "see" the atmospheres of distant worlds for the first time. This talk will summarize some of the recent discoveries and describe how theoretical models are being used to extract information about atmospheric chemical composition, thermal structure, and global circulations.
May 20, 2008	Rene Goosmann, Astronomical Institute, Academy of Sciences, Czech Republic Special Event: Beatrice M. Tinsley Visiting Scholar in Astronomy (visit: May 10-31) "Exploring the Innermost Accretion Regions of Supermassive Black Holes" The regions closest to the supermassive black holes (SMBHs) in AGNs must be explored in the X-ray range. The innermost part of the accretion disk reaches down to the last marginally stable orbit of the SMBH. Above the disk, co-orbiting magnetic flares are expected to occur. I investigate the spectral appearance of such flares and the effect of these on the radiation that is reprocessed by the disk. I present the results of modeling involving detailed radiative transfer simulations and ray-tracing to account for general relativistic and Doppler effects. I show how this modeling can be applied to current and future AGN spectra and variability data. It delivers important constraints on the irradiation pattern of the accretion disk, and on the mass and spin of the SMBH.
May 27, 2008	Rene Goosmann, Astronomical Institute, Academy of Sciences, Czech Republic Special Event: Beatrice M. Tinsley Visiting Scholar in Astronomy (visit: May 10-31) "Using Polarization to Probe Unresolved Structure of AGNs and Other Objects" The structure of many astrophysical objects (AGNs, supernovae, X-ray binaries, and gamma-ray bursts) cannot be directly resolved yet. I discuss various mechanisms by which scattered line and continuum radiation is polarized, and show how polarization measurements allow profound insights to be gained into the geometrical structure and the dynamics of AGNs and other objects. Using the Monte Carlo radiative transfer code STOKES, the polarization signatures of various reprocessing regions around compact objects can be modeled. The radiative coupling between individual scattering regions can be complex but is correctle modeled with STOKES because it automatically includes multiple scattering effects. Recently we introduced the new technique of polarization reverberation mapping. This method exploits the time-dependence of the polarization signal and allows us to measure distances between radiation sources and polarizing mirrors.

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