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Staff Research
2013 Spring Series

All talks are held on Wednesdays in the STScI John N. Bahcall Auditorium at 3:30 p.m. preceded by tea at 3:15 p.m.

Please direct questions or comments to the colloquium committee. The 2012-13 committee members are Janice Lee, Selma de Mink and William Sparks.

STScI presents live and archived webcasting of talks and Colloquium Series.

Date Speaker/Title
Feb. 6 Nima Arkani-Hamed (IAS/JHU)
Don't Modify Gravity--Understand It!
Host: Mario Livio
Feb. 13 Lucianne Walkowicz (Princeton University)
Understanding Exoplanets in Light of their Host Stars: Stellar Astrophysics and Kepler
Abstract: Although the primary purpose of NASA's Kepler mission is the discovery of exoplanets, its high precision photometry also reveals the effects of stellar magnetic activity in exquisite detail. These phenomena present on a variety of timescales, from spots that modulate the stellar brightness from days to months, to dramatic flares that evolve over a few hours. The Kepler data enable new investigations into the fundamental nature of stellar magnetism by furthering our understanding of the stellar rotation and differential rotation that generate the field, and the photometric variability caused by the surface manifestations of the field. In the case of stars with planetary candidates, these data also offer synergy between studies of stars and planets. The stellar rotation acts as a proxy for age, allowing us to place a time stamp on the planetary system, while the spot characteristics and flare rate constrain the high energy radiation input at the top of the planetary atmosphere. The Kepler data offer a new chance to not only understand the stars themselves, but to understand the range of circumstellar habitats in which planets exist. In this talk, I will discuss our ongoing work to characterize the variability due to starspots, stellar rotation and flares in the Kepler planet host stars and the larger sample of targets as a whole.
Host: Rachel Osten
Feb. 20 Rachel Mandelbaum (Carnegie Mellon University)
The Next Decade of Weak Lensing Science
Abstract: Gravitational lensing is a highly useful way to observe the total matter content of the universe, including dark matter. Given the strong observational support for the claim that most of the matter in the universe is dark, weak lensing is a critical part of current and future observational efforts in cosmology and galaxy formation/evolution. In this talk, I will begin with a review of some key ways that lensing has already addressed questions in these areas. Next, I will discuss some important lessons learned in the past 5 years, focusing particularly on the synergy between weak lensing measurements and other observations such as redshift surveys; this will include some new results using SDSS data. I will conclude with a summary of what we expect to learn with near-future surveys such as DES, HSC, KIDS, and Pan-STARRS, on our way to longer-term projects such as LSST, WFIRST, and Euclid.
Host: Dan Coe
Feb. 27 No Colloquium due to HST Phase I Proposal Deadline on 3/1.
Mar. 6 Karl Gebhardt (University of Texas, Austin)
Black Holes and Dark Matter in the Local Universe
Abstract: A galaxy's black hole mass and dark matter profile provide significant clues as to its evolution and formation, namely the mass accretion history. These essential components are difficult to measure robustly however. I will give a general overview of the observational results for measuring the masses of black holes and the dark matter profile in nearby galaxies, including discussion of their effects on the stellar orbital distribution. I will focus the talk on the two extreme mass ranges, from whether globular clusters contain black holes up to black holes in brightest cluster galaxies. The present results suggests that the black hole correlations span over seven orders of mass. I will show results from both new instrumentation and computational approaches that demonstrate we are obtaining robust measures of both the black hole mass and dark matter profile.
Host: Preethi Nair
Mar. 13 Charlie Conroy (University of California, Santa Cruz)
Early-Type Galaxy Archeology
Abstract: One of the main avenues for understanding the formation and evolution of early-type galaxies is through studying their present day stellar populations. A new generation of population synthesis tools that we have been developing are now capable of extracting an unprecedented amount of information from high quality spectra of galaxies. In this talk I will present results from an ongoing program aimed at measuring the stellar initial mass function and detailed elemental abundance pattern of early-type galaxies. Current data suggest that the IMF varies systematically across the galaxy population, with implications both for star formation theory and the inferred dark matter content in the central regions of galaxies. In addition, constraints on the abundances of the alpha elements, iron peak elements, and neutron capture elements offer the promise of reconstructing the detailed star formation histories of these now dormant galaxies.
Host: Harry Ferguson/Paul Goudfrooij
Mar. 20 Ranga Chary (California Institute of Technology/IPAC)
The Feeding of Galaxies and Reionization of the Universe
Abstract: The discovery of an unusual population of galaxies with extremely strong nebular line emission has come as a panacea to several problems in galaxy evolution. These objects, classified as Halpha emitters, were first detected in the z>5 Universe in deep Spitzer imaging, where they dominate the spectroscopically confirmed star-forming galaxy population. Since then, they have been re-discovered in the local Universe in the Sloan Digital Sky Survey where they constitute 0.04% of the spectroscopic sample and at intermediate redshifts using Hubble/WFC3 grism surveys. Their ubiquity in the distant Universe and the unusual properties of their stellar population make them strong candidates for being progenitors of massive galaxies at z~2 and for being the primary sources responsible for reionization. I will present our work in identifying these galaxies, insights into the mechanism for fueling star-formation in these objects and their role in reionization. The merits of using multiwavelength observations in obtaining an improved understanding of high redshift galaxy populations will in particular be highlighted.
Host: Christine Chen
Mar. 27 Aaron Evans (NRAO/UVA) -- Caroline Herschel Fellow
GOALS: The Great Observatories All-sky LIRG Survey
Abstract: Luminous Infrared Galaxies (LIRGs) are observed primarily to be interacting and merging galaxies. They are the sites of rampant star formation and active galactic nuclei (AGN), which are fed by abundant supplies of molecular gas. However, the very property that led to their initial discovery by IRAS as a significant galaxy population - their high infrared luminosity - also makes them difficult to study; the majority of the UV and optical light from young, massive stars and AGN is absorbed by obscuring dust and re-emitted in the infrared. The Great Observatories All-sky LIRGs Survey thus makes use of the diversity in wavelength coverage of the present space-based telescopes to probe the activity in a large (~ 100 - 200), flux-limited sample of LIRGs from the Revised Bright Galaxy Sample (RBGS). The majority of the talk will be devoted to discussing the survey as a whole. The latter part of the talk will be focused specifically on a GOALS analysis of NGC 2623.
Host: Rachel Osten
Apr. 3 Judd Bowman (Arizona State University)
Reionization and Galactic Science with the Murchison Widefield Array
Abstract: The Murchison Widefield Array (MWA) is the first radio telescope in the Southern Hemisphere designed specifically to explore the astronomical sky between 80 and 300 MHz with arcminute angular resolution and high survey efficiency. The telescope has entered commissioning and will soon enable new advances along four key science themes, including searching for redshifted 21 cm emission from neutral hydrogen in the IGM during the epoch of reionization; large Galactic surveys well suited for probing turbulence in the ISM and detecting new SNRs; time-domain astrophysics; and solar, heliospheric, and ionospheric science and space weather. Full science observations will begin in July 2013. Here, I will present initial results from commissioning observations and their implications.
Host: Jens Chluba
Apr. 10 Andrea Ghez (UCLA) -- Bahcall Lecture
The Galactic Center: Unveiling the Heart of our Galaxy
Abstract: The proximity of the center of our Galaxy has presented us with a unique opportunity to study a galactic nucleus with orders of magnitude higher spatial resolution than can be brought to bear on any other galaxy. This advantage, along with the recent advances in high angular resolution imaging technologies, has allowed the first observations of individual stars at the very heart of a galaxy. After more than a decade, such observations have transformed the case for a supermassive black hole at the Galactic center from a possibility to a certainty, thanks to measurements of individual stellar orbits. The rapidity with which these stars move on small-scale orbits indicates that 4 million times the mass of the sun resides within a region comparable to the size of our solar system and provides the best evidence yet that supermassive black holes, which confront and challenge our knowledge of fundamental physics, do exist in the Universe. Subsequent high-resolution imaging studies of the Galactic center have shown that the stellar population near our Galaxy’s supermassive back hole is quite different from the predications of theoretical models for the interaction between central black holes and their environs ( an essential input into models for the growth of nuclear black holes). In particularly, the observations have revealed an abundance of young stars in a region that is inhospitable to star formation and, conversely, a dearth of old stars where as a stellar cusp is expected. Further improvements in measurement precision should enable tests of Einstein’s theory of General Relativity in the extreme environment near a supermassive black hole.
Host: Bob Williams
Apr. 17 Jean-Paul Kneib (Ecole Polytechnique Fédérale de Lausanne)
The Magic of Cluster Lensing
Abstract: Observing the dark night sky far from our big cities is something magical as we know -- what astronomer has not dreamt about seeing the full depth of our Universe? A quarter of a century ago, giant luminous arcs and arclets revealed the gravitational lensing power of clusters. Since then, theoretical and observational work have unveiled the magical power of cluster lensing which allows us to study: the dark matter distribution with unprecedented accuracy; the distant universe by enabling the identification and analysis of the (most) distant normal galaxies via magnification; galaxy evolution at the earliest times; and cosmography in a complementary way to other cosmological probes. The Hubble Space Telescope has been a key instrument in making these discoveries possible, and with the new Hubble Frontier Field program it will continue to lead this legacy onward to new discoveries by revealing to us the otherwise invisible.
Host: Andy Fruchter
Apr. 24 Simon Portegies Zwart (Leiden University)
Studying the Formation and Evolution of Star Clusters using the Astrophysics Multipurpose Software Environment
Abstract: I will introduce a general-purpose framework for interconnecting scientific simulation programs using a homogeneous, unified interface. Our framework is intrinsically parallel, and conveniently separates all component numerical modules in memory. This strict separation allows automatic unit conversion, distributed execution of modules on different cores within a cluster or grid, and orderly recovery from errors. The framework can be efficiently implemented and incurs an acceptable overhead. In practice, we measure the time spent in the framework to be less than 1% of the wall-clock time. Due to the unified structure of the interface, incorporating multiple modules addressing the same physics in different ways is relatively straightforward. Different modules may be advanced serially or in parallel. Despite initial concerns, we have encountered relatively few problems with this strict separation between modules, and the results of our simulations are consistent with earlier results using more traditional monolithic approaches. This framework provides a platform to combine existing simulation codes or develop new physical solver codes within a rich "ecosystem" of interchangeable modules. I will apply the framework to simulating the star cluster formation process. I will present simulation results of the earliest star formation process, from the turbulent gaseous phase to the formation of the first stars, and the subsequent phase in which the stars blow out the residual gas by mean of mechanical feedback.
Host: Paul Goudfrooij
May 1 Scott Gaudi (The Ohio State University)
Exoplanet Demographics with WFIRST
Abstract: Measurements of the demographics of exoplanets over a broad range of planet and host star properties provide fundamental empirical constraints on theories of planet formation and evolution. Because of its unique sensitivity to low-mass, long-period, and free-floating planets, microlensing is an essential complement to our arsenal of planet detection methods. I motivate microlensing surveys for exoplanets, and in particular describe how they can be used to test the currently-favored paradigm for planet formation, as well as inform our understanding of the frequency and potential habitability of low mass planets located in the habitable zones of their host stars. I explain why a space-based mission such as WFIRST is necessary to realize the full potential of microlensing, and outline the expected returns of such surveys. When combined with the results from complementary surveys such as Kepler, WFIRST will yield a nearly complete picture of the demographics of planetary systems throughout the Galaxy.
Host: Peter McCullough
May 8 Mark Krumholz (University of California Observatories)
The Origin of the Initial Mass Function
Abstract: Star-forming environments vary by orders of magnitude in density, pressure, metallicity, and other properties, yet the characteristic mass distribution of the stars they produce remains stubbornly unchanged. Explaining the origin and universality of this distribution is one of the oldest problems in theoretical astrophysics, but in the last few years advances in understanding how gas fragments, together with algorithmic developments that have allowed simulations to include improved physics such as radiative transfer, have produced significant progress. I describe these advances, and lay out the beginnings of a theoretical model capable of explaining why stars have the masses they do. This model suggests that stellar masses are nearly but not perfectly universal, and the subtle variations that it admits provide avenues for future observational tests.
Host: Marshall Perrin
May 15 Tori Hoehler (NASA Ames)
Physical and Chemical Toeholds for Exoplanet Bioastronomy
Abstract: If a search for exoplanet life were mounted today, the likely focus would be to detect oxygen (or ozone) in the atmosphere of a water-bearing rocky planet orbiting roughly 1AU from a G-type star. This appropriately conservative and practical default is necessary in large part because biological input on the question of where and how to look for life has progressed little beyond a purely empirical reliance on the example of terrestrial biology. However, the apparent and expected diversity of exoplanetary environments that may be accessible to observation provides motivation to reevaluate habitability and biosignatures in broader terms. Consideration of the core properties of life in reference to fundamental chemical and physical principles may serve to significantly constrain the search space for exoplanet bioastronomy, without purely empirical reference the specifics of terrestrial life. Potential physical and chemical “toeholds” for consideration of the liquid water + oxygen paradigm will be suggested, as specific examples of this approach.
Host: Peter McCullough
May 22 John Johnson (California Institute of Technology)
Hot on the Trail of Warm Planets Orbiting Cool Stars
Abstract: Just three years ago the prospect of finding temperate, rocky worlds around other stars was still the subject of science fiction: none had been found and reasonable estimates put us years or decades away from such a momentous discovery. All of that has changed very recently on the heels of the extraordinarily successful NASA Kepler mission. By searching for the tiny diminutions of starlight indicative of an eclipsing planet, Kepler has produced thousands of new planet candidates orbiting distant stars. Careful statistical analyses have shown that the majority of these candidates are bona fide planets, and the number of planets increases sharply toward Earth-sized bodies. Even more remarkably, many of these planets are orbiting right “next door,” around tiny red dwarf stars. I will describe our multi-telescope campaign to validate and characterize these tiny planetary systems, and present some early, exciting results that point the way to the first detection of the first Earth-sized planet in the habitable zone of a star.
Host: David Soderblom
May 29 Jennifer Sokoloski (Columbia University)
Novae: New Findings from Radio to Gamma-Rays
Abstract: With three to five nova eruptions per month in our Galaxy alone, novae are the most common stellar explosions. Described by ancient peoples as 'new stars' in the sky, we still primarily rely on amateurs to find them. Although it is well known that novae are triggered by a thermonuclear runaway on the surface of an accreting white dwarf (WD), major questions about novae remain. For example: 1) How much mass do they expel? 2) What shapes their ejecta? 3) Do novae prevent accreting WDs from becoming type Ia supernovae? 4) How do some novae produce gamma-rays? I will describe how radio through X-ray observations, including recent HST images of light-echos around the recurrent nova T Pyxidis, suggest that the answers to these questions are all related.
Host: Bob Williams