List of Past Astronomy Colloquia : 01-Sep-2012 to 31-Dec-2012

Date:   Wednesday 05-Sep-2012
Speaker:   Prof. Marc Kamionkowski (Johns Hopkins University)
Title:  New signals from the early Universe

Over the past several decades we have obtained increasingly precise data on the distribution of galaxies in the Universe and on the distribution of primordial perturbations via CMB measurements. This trend is likely to continue for the foreseeable future. In this talk I will discuss some new things to do with data from the CMB, galaxy surveys, and future 21-cm surveys look for new physics in the early Universe, and in the not-so-early Universe.

Date:   Wednesday 12-Sep-2012
Speaker:   Dr. Martin Elvis (Harvard-Smithsonian)
Title:  "Quasars are Simple"

To a remarkably good approximation, all quasars look alike. The highest redshift quasar at z=7.1 has an optical emission ine spectrum *identical* to the typical z=1-2 quasar measured by SDSS. Cosmic time seems irrelevant to quasar properties as does luminosity spanning 6 decades, and black hole mass over 3 decades. This means that the physics of quasars must be highly robust, yet complex enough to produce the many atomic emission and absorption features in their X-ray through infrared spectra. The quasar "standard model" of a supermassive black hole, an accretion disk and a relativisitic jet makes no predictions for these features. The best contender is the physics of radiation pressure via electron scattering, UV absorption and dust absorption, both where these succeed in driving a wind off the accretion disk, and where they fail to do so. I will show how this works.

Date:   Wednesday 19-Sep-2012
Speaker:   Dr. Brian Metzger (Princeton)
Title:  "Observational Signatures of Coalescing Compact Binaries"

"A fraction of binary neutron stars are sufficiently compact that they will completely merge due to the emission of gravitational waves within a Hubble time. Such mergers are among the most promising sources for the direct detection of gravitational waves with ground based interferometers such as Advanced LIGO. Maximizing the scientific return of such a detection will, however, require identifying a coincident electromagnetic (EM) counterpart. One possible counterpart is a short duration gamma ray burst (GRB), powered by the accretion of material that remains in a rotationally supported torus around the central black hole. Although observations of short GRBs are largely consistent with the merger model, the puzzling discovery has been made that many of them are followed by late ray flaring, which does not fit current theory and may require considering alternative progenitor models. Another source of EM emission is a supernova like optical transient, powered by the radioactive decay of heavy elements synthesized in neutron rich ejecta from the merger. I will present the first calculations of the radioactively powered transients from mergers that include realistic nuclear physics and radiative transport, and I will discuss the prospects for detecting and identifying such events following a gravitational wave trigger. In the second part of my talk I will describe a model for accretion following the tidal disruption of a white dwarf by a neutron star. I will demonstrate that densities and temperatures in the disk are sufficiently high to burn the white dwarf material into increasingly heavier elements at sequentially smaller radii. Because the energy released by nuclear reactions is comparable to that released gravitationally, I introduce the concept of a "nuclear dominated accretion flow". Outflows from the disk power a ~ week long transient, which may be related to some classes of recently discovered subluminous supernovae."

Date:   Wednesday 26-Sep-2012
Speaker:   Dr. John Wise (Georgia Tech)
Title:  "The Imprint of Pop III Stars on the First Galaxies"

I present results from a suite of cosmological radiation hydrodynamics simulations that focus on the transition from Population III star formation to the first galaxies. Each simulation captures the radiative and chemical feedback from ~300 Population III stars, leading to the formation of a 10^9 solar mass galaxy at redshift 7. We have investigated the variations in galaxy properties when changing the Pop III characteristic mass, critical metallicity, UV backgrounds, metal cooling, and feedback from radiation pressure. One constant result from all the simulations is a metallicity floor between [Z/H] = -3 to -4. We show that momentum transfer from ionizing radiation plays an important role in providing turbulent support and mixing metals, preventing the overproduction of stars and metals. This results in a stellar population with a tight metallicity distribution function centered at [Z/H] = -2, agreeing with the observed luminosity-metallicity relation in dwarf galaxies.

Date:   Wednesday 03-Oct-2012
Speaker:   Dr. Ehud Behar (Technion-Isreal Inst. of Tech)
Title:  X-Ray Absorption of High-Redshift Quasars and Gamma Ray Bursts

Soft X-ray photoelectric absorption of high-z quasars has been known for more than a decade, but its physical origin, whether intrinsic to the quasar, or in the intergalactic medium (IGM) has mostly remained a mystery. We have constructed the largest sample to date of more than 60 high redshift quasars (z > 0.5) selected from the XMM-Newton archive based on high photon count criteria. We analyze the measured optical depth t at 0.5 keV for the sample, in an attempt to find which physical parameters of the quasars drive the observed absorption, e.g., redshift, radio luminosity, radio loudness, or the X-ray luminosity. Clearly, high-z quasars are more absorbed than low-z quasars, and radio-loud quasars are more absorbed than radio quiet ones. In fact, z < 2 and radio quiet objects show very little X-ray absorption, and for the most part provide only upper limits for the optical depth. Taken at face value, optical depth does seem to steeply increase with redshift as t ~ (1+z)^2.5. We confront the quasar absorption dependence on redshift with a comparison sample of gamma ray bursts (GRBs). The mean 0.5 keV optical depths of both the quasars and the GRBs tend at high redshift to a value of t ~ 0.4. Interestingly, such behavior is expected if the absorption arises from the diffuse IGM. However, quasar X-ray opacity at intermediate redshift (0.5 < z < 2) is generally too low, deeming the diffuse IGM scenario unlikely.

Date:   Wednesday 10-Oct-2012
Speaker:   Prof. Andrew West (Boston University)
Title:  "What can Large Samples of Low-Mass Stars tell us about the Galaxy, the Habitability of Exoplanets and the Evolution of Stellar Dynamos?"

My primary goal of this colloquium is to demonstrate that we can do big science with little stars. M and L dwarfs are the smallest, coolest and least massive stars in the Galaxy. Yet despite their diminutive physical properties, low-mass stars make up ~70% of all of the stars in the Milky Way galaxy and have lifetimes that exceed trillions of years. Their dominance in the Galaxy make low-mass stars excellent tracers of both the structure and evolution of the local Milky Way. In addition, low-mass stars have intense stellar flares and strong magnetic fields that allow us to probe their interiors and may have important consequences for their space weather environments and the habitability of planets that orbit them. I will present results from the largest samples of low-mass stars ever assembled. The advent of large surveys such as the Sloan Digital Sky Survey (SDSS) has yielded photometric and spectroscopic catalogs of more than 100 million and 70,000 stars respectively. Specifically, I will highlight work that has used the unprecedented statistical power of the SDSS to examine the structure and kinematics of low-mass stars in the Milky Way, as well as the nature of their magnetic fields (and subsequent "magnetic activity") and what this may tell us about the ages of stars. In addition, I will s hare some resent results from a survey that cataloged some of the widest binaries in the Milky Way and demonstrate how a large sample of M dwarfs has helped us map the three-dimensional distribution of dust in the local Galaxy.

Date:   Wednesday 17-Oct-2012
Speaker:   Dr. Daisuke Nagai (Yale University)
Title:  A New Era of Cosmology and Astrophysics with Galaxy Clusters

Recent years have witnessed the emergence of galaxy clusters as powerful laboratories for cosmology and astrophysics. Being the largest and most magnificent structures in the Universe, clusters of galaxies serve as excellent tracers of the growth of cosmic structures. The current generation of X-ray and Sunyaev-Zel'dovich cluster surveys have provided independent confirmation of the cosmic acceleration and significantly tighten constraints on the nature of mysterious dark energy and dark matter as well as new insights into how massive galaxies and black holes form and grow in the Universe. A number of new surveys and large supercomputer simulations are underway to test our understanding of the structure formation and fundamental physics of the cosmos. However, the use of galaxy clusters as sensitive and robust cosmological probes requires solid understanding of the cluster astrophysics. In this talk, I will review recent advances and future challenges in this rapidly growing area of cosmology and astrophysics, with highlights on new insights provided by modern cosmological hydrodynamical simulations.

Date:   Wednesday 24-Oct-2012

Date:   Wednesday 31-Oct-2012
Speaker:   Dr. Selma de Mink (StSci)
Title:  The Evolution of Massive Stars towards their Death: Rotation, Binarity and Mergers

Although they are rare and short-lived, massive stars play a major role in Universe. With their large luminosities, strong stellar winds and spectacular explosions they act as cosmic engines, heating and enriching their surroundings, where the next generation of stars are forming. The latest stellar evolutionary models show that rotation can have drastic effects, which has been suggested as a evolutionary path for the progenitors of long gamma-ray bursts. I will discuss the recent efforts of theorists and observers to understand the effects of rotation including some highlights of the "VLT-FLAMES Tarantula Survey of Massive Stars". A further challenge arises from the preference of massive stars to come in close pairs. Interaction with a companion leads to spectacular phenomena such as runaways, X-ray binaries and stellar mergers. I will present new results on the true close binary fraction for massive stars, which imply that only a minority evolve undisturbed towards their death.

Date:   Wednesday 07-Nov-2012
Speaker:   Dr. Andreea Petric (Caltech)
Title:  Mid-Infrared and Millimeter Spectroscopic Diagnostics of Local Luminous Infrared Galaxies

I will present an analysis of of 248 luminous infrared galaxies (LIRGs) nuclei which comprise the Great Observatories All-sky LIRG Survey (GOALS) observed with the Infrared Spectrograph on Spitzer in the rest-frame wavelength range between 5 and 38 microns. I will present and compare several diagnostics effective at isolating the Active Galactic Nuclei (AGN) contribution to the Mid-infrared (MIR) (fAGN) emission using [NeV], [OIV], [NeII], the 6.2 micron PAH EQW and the shape of the MIR continuum. These diagnostics suggest that between 10% and 13% of local LIRGs are AGN dominated in the MIR and that AGNs contribute 12% of the total bolometric luminosity of the entire sample. Warm H2 is detected in at least one transition in 40% of sources. I find that the H2 scales with aromatic band emission and [Si II[. The range of total H2 to IR ratios in LIRGs is wider than in ULIRGs but similar to normal galaxies. We detect H2 resolved emission and compare it with similar findings in radio galaxies and nearby mergers. I will also present 16 CO (1-0) high resolution maps taken with the Combined Array for Research in Millimeter-Wave Astronomy of 16 LIRGs. We find a median mass of H2 of 2 x 1e10 Solar Masses, with median molecular gas surface densities of about 200 Solar Masses/pc2 as well as correlations between the interaction stage and the average gas surface density and cold gas depletion times. I will compare their morphologies to those of normal galaxies and ULIRGs.

Date:   Wednesday 14-Nov-2012
Speaker:   Dr. Brian Jackson (Carnegie Institution of Washington)
Title:  Ellipsoidal Variation Analysis of Kepler Observations using the EVIL-MC Model

The Kepler mission has revolutionized planetary astronomy by revealing thousands of planetary candidates. The mission has also opened up vast new astronomical vistas by providing unprecedented photometric sensitivities that allow incredibly minute signals to be detected. Too small to have been observed before, photometric signals of the tidal distortion of a star by a short-period planet, planet-induced ellipsoidal variations (EVs), can now be measured. Impressively, the tidal bulges that give rise to this signal are ~ 10 km in height. As for binary stars, the EV amplitudes can provide constraints on several key system parameters, including the ratio of the orbiting companionís mass to its host starís. Thus, EVs can help confirm or refute the planetary nature of a transiting companion, without the need for additional, often costly follow-up observations. EVs may be observed even when a planet does not transit its host star, so searching for EVs in the Kepler data may reveal the presence of planets that would otherwise go undetected. For systems in which EVs can be observed, typically the planetary eclipse depth (when the star occults the planet) can also be measured (or at least constrained), and analysis of the eclipse depth provides information on the planetís atmosphere. However, since both the atmospheric signal and EVs are present, both must be considered to provide accurate constraints. In this talk, I will discuss development of a new model for Ellipsoidal Variations Induced by a Low-Mass Companion, the EVIL-MC model, and its application to select Kepler targets. I will also discuss strategies and prospects for discovering non-transiting, close-in exoplanets hiding in the Kepler dataset and for leveraging the model to learn about planetary atmospheres.

Date:   Wednesday 21-Nov-2012

Date:   Tuesday 27-Nov-2012
Speaker:   Dr. Adam Riess (STScI)
Title:  Precision Measurements of the Hubble Constant and PASS

The Hubble constant remains one of the most important parameters in the cosmological model, setting the size and age scales of the Universe. Present uncertainties in the cosmological model including the nature of dark energy, the properties of neutrinos and the scale of departures from flat geometry can be constrained by measurements of the Hubble constant made to higher precision than was possible with the first generations of Hubble Telescope instruments. Streamlined distances ladders constructed from infrared observations of Cepheids and type Ia supernovae with ruthless attention paid to systematics now provide 3.5% precision and offer the means to do much better. While WFC3 has helped open this new route, its full exploitation can come from a new technique, Parallel Astrometric Spatial Scanning (PASS), to measure parallax distances beyond a kiloparsec. I will review recent and expected progress.

Date:   Wednesday 05-Dec-2012
Speaker:   Dr. Samaya Nissanke (Caltech)
Title:  Seeing and hearing the violent universe

The strongest gravitational waves (GWs) in the universe are expected to arise from violent events such as the mergers of neutron star or black hole binaries. Decades of theoretical and experimental efforts could result in the first direct detection of GWs. Subsequent measurements should offer us an unprecedented view of strong-field gravity in action. A subset of neutron star binary mergers could be associated with a transient electromagnetic (EM) counterpart, and EM follow-up using radio, optical and high-energy facilities of these events are critical for improving our understanding of the physics underlying strong-field gravity astrophysics, and compact objects. I will discuss the wealth of information which a combined EM+GW detection would immediately bring and the challenges that lie ahead in pinpointing neutron star binary merger using networks of GW interferometers and multiwavelength EM telescopes and arrays.

Date:   Tuesday 11-Dec-2012
Speaker:   Dr. Steve Kahn (Stanford University)
Title:  The Large Synoptic Survey Telescope (LSST)

LSST is a large-aperture, wide-field, ground-based telescope designed to provide an imaging survey of half the sky in six optical colors every few nights. As such, it will enable a diverse array of distinct scientific investigations, ranging from compiling a census of moving objects in the solar system to charting the formation and structure of the Milky Way galaxy. Of particular interest for cosmology and fundamental physics, LSST will provide tight constraints on the expansion history of the universe via statistical measurements of the shapes and distributions of billions of galaxies out to moderate-to-high redshifts. In view of these capabilities, LSST was ranked as the highest priority major new ground-based facility by the 2010 decadal survey in astronomy and astrophysics commissioned by the National Research Council. I will review the key aspects of the design of this facility and highlight some of its scientific potential, with particular emphasis on the power of LSST to constrain the properties of dark energy.

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