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Staff Research
Colloquia - Fall 2012

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.

STScI presents live and archived webcasting of talks and Colloquia.

Date Speaker/Title
Sept. 5 Lisa Kewley (Institute for Astronomy, University of Hawaii)
Galaxy Formation and Evolution Through Metals
Abstract: Chemical abundances in galaxies provide a fossil record of previous generations of star formation, modulated by galactic-scale gas flows. I will present the latest results from our investigation into the chemical evolution of galaxies, both locally, and at high redshift. Theory predicts that as a merger progresses, galaxy disks become disrupted by tidal effects, causing large radial gas flows toward the central regions where kpc-scale starbursts and AGN may be fueled. Isolated disk galaxies have strong chemical abundance gradients that may become disrupted during a merger. We have conducted the first investigation into chemical abundance gradients across the merger sequence. We show that abundance gradients and the presence of shocks evolve dramatically with merger progress, providing a smoking gun for galactic-scale gas flows in merging galaxies. Moving out in redshift, I present recent results from our investigation into the cosmic chemical history of galaxies. We have measured the chemical abundances for an unprecedented number of galaxies to z~1, finding little evolution between z~1 and the present day. At higher redshift, we exploit the power of gravitational lensing to investigate the chemical evolution in galaxies between 1 < z < 3. We have measured the first metallicity gradient in a normal spiral galaxy at z~2, showing a steeper gradient than observed in local spiral galaxies. I will discuss the implications of this result and the future of this field with the next generation of telescopes.
Hosts: Claus Leitherer and Aida Wofford
Sept. 12 Ivan Ramirez (Department of Astronomy, University of Texas, Austin)
Finding Planets with Stellar Chemical Abundances
Abstract: The process of planet formation has likely left detectable signatures in the chemical composition of the host stars, as exemplified by the now well-established connection between high stellar metallicity and presence of giant planets. It has been claimed that a planet connection exists also for the lithium abundances, but age and metallicity effects might be producing spurious results. Highly precise analyses of several other elements in Sun-like stars have revealed interesting correlations between elemental abundance and dust condensation temperature, which can be explained as a signature of the formation of terrestrial planets alone. In this talk, I will discuss the proposed connection between host star chemical abundances and planet formation, and how it could be used to find or confirm the presence of both terrestrial and gas giant exoplanets using relatively simple spectroscopic analysis of their host stars.
Host: David Soderblom
Sept. 19 Paul A. Crowther (Department of Physics & Astronomy, University of Sheffield, UK)
The Most Massive Stars in the Local Universe
Abstract: The lower stellar mass limit is well defined, but the question of an upper mass limit has proven more elusive. Theoretically, the Eddington limit depends upon the mass-luminosity relation for massive stars while pulsational stability limits vary widely in the literature. Observationally, as recently as one decade ago, no empirical upper limit had been identified. However, statistical studies of young, high mass clusters established a limit close to 150 solar masses which have rapidly gained widespread acceptance (e.g. Figer 2005). Since then, this limit has been challenged via (a) candidate pair-instability SNe requiring progenitor masses in excess of this limit (SN 2007bi, Gal-Yam et al. 2009); (b) Analysis of VLT/SINFONI+MAD observations of the brightest members of the R136 star cluster indicating progenitor masses of up 300 solar masses (Crowther et al. 2010). In this talk I shall set out evidence both for and against the currently accepted stellar mass limit, plus connections between the upper mass limit and the formation mechanisms for high mass stars, early cluster evolution and the death of the most massive stars.
Host: Danny Lennon
Sept. 26 No Colloquium
Oct. 3 Warren Brown (Smithsonian Astrophysical Observatory)
Hypervelocity Stars
Abstract: A massive black hole sits in the heart of the Milky Way. One consequence of the black hole is that it ejects "hypervelocity stars" from the Milky Way at ~1000 km/s velocities. We discovered the first hypervelocity star in 2005, and since then our targeted survey has discovered 20 unbound stars and a comparable number of possibly bound hypervelocity stars. Recent results include new constraints on their origin in the Milky Way, a surprising anisotropic spatial distribution over the sky, and Hubble Space Telescope proper motion measurements that may allow us constrain the shape and orientation of the Galactic potential.
Hosts: Danny Lennon and Andrea Bellini
Oct. 10 Edwin Bergin (Astronomy Department, University of Michigan)
Exploring New Spectral Windows with the Herschel Space Observatory
Abstract: The Herschel Space Observatory, an ESA cornerstone mission with NASA participation, has been in operation for over two years. I will briefly outline the overall capabilities of Herschel which has both photometric and spectroscopic coverage from 63 to 610 microns. Herschel offers unprecedented sensitivity as well as continuous spectral coverage across the gaps imposed by the atmosphere, opening up a largely unexplored wavelength regime to high resolution spectroscopy. This talk will broadly address results from several observational programs. In this talk I will first describe a selection of Herschel's efforts to explore the origins of chemical complexity in space. I will summarize the power of Herschel where we have detected over 40,000 molecular lines in a single spectrum. This information directly exposes the molecules tracing cold regions far from the star and the water/organic factory in close proximity toward the young star. I will argue that these latter organics are potentially important as they will provide the foundation for the development of complexity in the planet-forming disk. I will also discuss several efforts that use deuterium-bearing isotopes of hydrogen and water to trace the origin of Earth's oceans and set new constraints on the uncertain mass available to form planetary systems.
Host: Klaus Pontoppidan
Oct. 17 Jessica Lu (Institute for Astronomy, University of Hawaii)
Extremophiles: Stars Born around Black Holes and in Dense Clusters
Abstract: The formation of stars is a fundamental astrophysical process; and yet we still debate whether it varies with environment. Milky Way young star clusters range in mass over four orders of magnitude; but, the best-studied star forming regions, such as Taurus and Orion, represent only a small range of initial conditions at the low mass end. Young star clusters with masses greater than 10,000 solar masses are promising targets for determining whether the initial mass function (IMF) that results from the star formation process is universal or depends on environment. Such clusters are challenging observational targets as they require high spatial resolution at infrared wavelengths and are heavily contaminated by field stars. I present results from a Keck adaptive optics and HST study of several massive young star clusters in the Milky Way, including around the supermassive black hole at the Galactic Center. IMFs are constructed by using high-precision astrometry and spectroscopy to distinguish individual cluster stars. I will discuss whether the measured IMFs differ for massive clusters at a range of Galactocentric radii and how they compare to the "universal" IMF established locally.
Host: Marshall Perrin
Oct. 24 David Charbonneau (Department of Astronomy, Harvard University)
How to Find the Best Transiting Exoplanets Before JWST Launches
Abstract: The James Webb Space Telescope may grant us the opportunity to study the atmospheres of potentially habitable worlds orbiting nearby stars. There is only one problem: We don't yet know at which stars we should point it. Using data from the NASA Kepler Mission, we can deduce the distance within which we can expect a few such worlds, and hence the observational effort that will be required to characterize them. It appears extremely likely that the nearest systems will orbit low-mass M-dwarf stars, which is fortunate as the small physical sizes and low-luminosities of the stars permit novel ground-based detection schemes that are not feasible for Sun-like stars. The MEarth Project is an array of automated telescopes that survey several thousand of the closest and least massive stars to search for transiting planets. In pursuit of this quest, MEarth has informed our understanding of the physics governing the structure, evolution, and rotation of the most common stars in the galaxy.
Host: Myron Smith
Oct. 31 Annika Peter (Department of Physics & Astronomy, University of California, Irvine)
The Dark Side of Dark Matter
Abstract: The nature of dark matter is one of the major "known unknowns" of physics of the universe. From astronomical observations, we know that dark matter exists, makes up 23% of the mass budget of the universe, clusters strongly to form the load-bearing frame of structure for galaxy formation, and hardly interacts with ordinary matter. Although most of what we know about dark matter comes from astronomy, most of the effort now to characterize dark matter is focused on non-gravitational interactions between dark matter and standard-model particles. However, dark matter may live in a new, dark sector of physics with only tenuous connections to the standard model. The challenge is to learn about the physics of this new sector without relying on measurable non-gravitational interactions with the standard model. In this talk, I argue that astronomical searches can elucidate some of the dark physics of dark matter. I will show how the physics within a new dark sector may imprint itself on the formation and growth of cosmological structure in the universe, which may then be probed using the gravitational interactions between dark matter and standard-model particles. I will outline which currently existing data sets can provide interesting constraints already, and what kinds of theoretical and observational work should be done in the future for even better constraints on the physics of dark matter.
Host: Dan Coe
Nov. 7 Samir Salim (Indiana University)
Continued Star Formation in Early Type Galaxies - Insights from the Ultraviolet
Abstract: Early-type galaxies are generally considered to be old and passive systems, having no current star formation (SF). In this talk I show how UV observations have revealed that a significant fraction of field galaxies on the red sequence exhibit moderate levels of galaxy-scale SF, up to a solar mass per year. What is the origin of the gas that gives rise to SF? Is the SF episodic, related to merging events, or continuous? Our HST and ground-based observations of a sample of UV-excess early types shows that the extended SF is only found in S0s, but not in true ellipticals, regardless of the galaxy mass. The dichotomy suggests that the feedback mechanisms have different effectiveness in the two Hubble types. Regular morphology of young stars in our sample is indicative of long-lived low-level accretion of gas from the IGM. Minor gas-rich mergers do not appear to be the dominant drivers of SF in early types. These new observations point towards a picture in which significant fraction of field S0s continue to grow their outer disks, while ellipticals remain truly passive.
Host: Preethi Nair
Nov. 14 Joe Silk (Department of Physics, University of Oxford)
Unleashing Feedback: Galaxies and AGN
Abstract: Progress has been made in understanding the origin of spiral galaxies, but elliptical galaxy formation continues to pose unresolved problems. In both cases, feedback seems to be a central issue. I will review our understanding of feedback, on scales from stars to galaxies. I will focus on massive spheroids, where one invariably finds evidence for the presence of supermassive black holes. These are often visible, especially at early epochs, as active galactic nuclei. Negative feedback, due to the outflow from the central black hole during its accretion phase, quenches the gas supply to the spheroid. Is there an earlier period of feedback during protogalaxy formation that could result in an efficient phase of early black hole growth and star formation? Are there observational implications of AGN triggering of star formation? I will discuss some of these issues in this talk.
Host: Andy Fruchter
Nov. 21 No Colloquium
Nov. 28 Pieter van Dokkum (Department of Astronomy, Yale University)
The Formation and Evolution of Massive Galaxies
Abstract: Owing to large surveys with the Hubble Space Telescope and ground-based telescopes we now have high resolution snapshots of the Universe dating back to only a few hundred million years after the Big Bang. The challenge now is to use this wealth of data to figure out how galaxies assembled, and what physical processes drove their evolution. The talk will highlight recent results on the evolution of the most massive galaxies, as these galaxies are in some ways the easiest to study out to high redshifts. We now have a self-consistent description of the evolution of these galaxies from compact, star forming ancestors at z~3 to their giant passively evolving descendants today. Interestingly, we do not yet understand why these galaxies followed this particular path; spectroscopic surveys such as the 3D-HST project may shed light on the physics behind the observed events.
Hosts: Paul Goudfrooij and Maria Peņa-Guerrero
Dec. 5 Emily Levesque (Department of Astrophysical & Planetary Sciences, University of Colorado, Boulder)
Stellar Rotation and its Impact on Ionizing Spectra
Abstract: Models of stellar evolution play a key role in a wide variety of research areas within astrophysics, including galaxy evolution, the properties of the first stars, supernova and GRB progenitor studies, and the hosts of extrasolar planets. Among their many applications, stellar evolutionary models are a key ingredient in stellar population synthesis codes, providing the key framework that is used to assemble a complete stellar population and track its temporal evolution. Ultimately, such codes are used to generate synthetic ionizing spectra, which can be directly compared to existing observations or used as inputs in photoionization codes to model the emission line spectra of HII regions and star-forming galaxies. Recently, a new grid of stellar models has been released that includes the first detailed implementation of stellar rotation effects. We have found that the inclusion of rotation has a substantial impact on the main-sequence lifetimes, luminosities, abundances, and effective temperatures of stars, producing a larger and hotter population of massive stars and strongly affecting the ionizing spectrum produced by such a population. By examining ionizing spectra generated with these rotating stellar evolutionary tracks and the Starburst99 stellar population synthesis code, we can evaluate the new stellar models and consider the strong influence that individual components of stellar physics can have on synthetic ionizing spectra and their applications.
Host: Claus Leitherer
Dec. 12 Christopher Martin (California Institute of Techonology)
The IGM Project: Searching For IGM Emission Over 0<z<7
Abstract: I will discuss several experimental projects underway or proposed designed to discover and map emission from the IGM. The Cosmic Web Imager (CWI) is a ground-based high resolution spectrometer designed to detect low surface brightness emission from redshifted Lyman alpha, OVI and CIV at Palomar and Keck Observatories, over 2<z<7. FIREBALL is a balloon-borne telescope/spectrometer that is searching for IGM emission in the 0.3<z<1.0 range in the UV balloon window at 200 nm. ISTOS is a proposed mission to discover and map baryons in the IGM in the space UV. I will report on preliminary results from FIREBALL and CWI. This work is supported by NASA and NSF.
Hosts: Knox Long and Andrew Fox
Dec. 19 Kevin Hand (Jet Propulsion Laboratory, California Institute of Techonology)
Ocean Worlds of the Outer Solar System
Abstract: At least five moons in the outer solar system may harbor liquid water oceans. These oceans have likely persisted for much of the history of the solar system and as a result they are highly compelling targets in our search for life beyond Earth. Dr. Hand will explain the science behind why we think we know these oceans exist and what we know about the physical and chemical conditions that likely persist on these worlds. He will focus on the surface chemistry of Jupiter’s moon Europa and connect laboratory spectroscopic measurements to ground and space-based observations of Europa’s surface.
Host: John Debes