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Research
Colloquium 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 2015-16 committee members are Peter McCullough (chair), Jennifer Lotz, and Laura Watkins.

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

Date Speaker/Title
Feb. 03 (No Colloquium: JWST Meeting)
Feb. 10 Julianne Dalcanton (University of Washington)
Title: The Panchromatic Hubble Andromeda Treasury: Science from 117 Million Stars
Abstract: The Panchromatic Hubble Andromeda Treasury is an HST multicycle program to image the north east quadrant of M31 to deep limits in the UV, optical, and near-IR. The HST imaging has resolved the galaxy into over 150 million stars (comparable to ~1/2 the number of stars in SDSS), all with common distances and foreground extinctions. As its legacy, this survey adds M31 to the Milky Way and Magellanic Clouds as a fundamental calibrator of stellar evolution and star-formation processes for understanding the stellar populations of distant galaxies. I will briefly describe the survey strategy, data reduction, and key data products. I will then highlight new work using the NIR stellar populations to constrain the large scale properties of the cold ISM, with 25 pc resolution. These new maps offer the highest resolution available in M31, and point to surprising challenges facing models of dust emission.
Host: Erik Tollerud
Feb. 17 Mark Voit (Michigan State University)
Title: Waterfalls or Rain: How Does Gas Get Into Galaxies?
Abstract: Galaxies are commonly thought to acquire much of the gas that fuels star formation through streams of cold gas that flow along filaments of larger-scale structure: waterfalls. However, the universe\'s largest galaxies appear to have a different gas supply: precipitation of cold clouds out of hot circumgalactic gas via thermal instability. I will present both observational and theoretical support for the precipitation mode in large galaxies and show how the precipitation model can be applied to galaxies of all masses. One of the attractive features of the precipitation model is that it makes observationally testable predictions about the state of the circumgalactic medium, if most star-forming galaxies are indeed in a precipitating state.
Host: Brad Whitmore
Feb. 24 Catherine Pilachowski (Indiana University)
Title: Odd Elements in the Milky Way: What can they tell us about nucleosynthesis in stars?
Abstract: The spectra of cool stars provide access to the (nearly) full pattern of light element and isotopic abundances from C through Ti in stellar populations. The observed abundance patterns can be compared with nucleosynthesis and chemical evolution models to identify the specific contributors to light element abundances in three stellar populations (thin disk, thick disk, and Bulge); to constrain the relative contributions of different types of supernovae, massive stars and AGB stars; and to explore exotic nucleosynthesis processes that may occur during explosive nucleosynthesis. Diagnostics include not only specific element ratios, but matching the full pattern of abundances, including the amplitude of the odd-even effect and the slope of abundance vs. atomic number for both even and odd elements. Key to understanding the origin of the light elements is the determination of the abundances of the rarer odd light elements, including fluorine, phosphorus, and chlorine, and stable rare isotopes from carbon through titanium. Such an analysis is the next step in precision needed to understand the chemical evolution of Galactic stellar populations, using not just [element/Fe] ratios but a more holistic approach to examine the patterns of element abundances in stellar populations.
Host: Paul Goudfrooij
Mar. 02 (No Colloquium: JWST Meeting)
Mar. 09 Eddie Schlafly (LBNL)
Title: The Optical-Infrared Extinction Curve and its Variation in the Milky Way
Abstract: The dust extinction curve is a critical component of many observational programs and an important diagnostic of the physics of the interstellar medium. In this talk, I will present new measurements of the dust extinction curve and its variation towards tens of thousands of stars, a hundred-fold larger sample than in existing detailed studies. We use data from the APOGEE spectroscopic survey in combination with ten-band photometry from Pan-STARRS1, 2MASS, and WISE. Our data and analysis have revealed two new aspects of Galactic extinction: first, we find significant, wide-area variations in the shape of the extinction curve throughout the Galactic plane. These variations are on scales much larger than individual molecular clouds, posing a challenge to existing paradigms of dust formation and processing: the extinction curve must be tracing much more than just grain growth in dense molecular regions. Indeed, we find no correlation between the extinction curve shape and and dust column density up to E(B-V) ~ 2. Second, we discover a strong relationship between shape of the extinction curve and the far-infrared dust emissivity, an important new constraint on models of dust physics.
Host: Joshua Peek
Mar. 16 Hiranya Peiris (University College -London)
Title: Towards Fundamental Physics from Cosmological Surveys
Abstract: Surveys of the cosmic microwave background and large galaxy surveys of the next decade carry immense promise for measurements of new physics beyond the Standard Models of cosmology and particle physics. However, these observations are complicated by multiple sources of systematics, either intrinsic, observational, or instrumental, which must be carefully controlled in order to make reliable inferences from the data about fundamental physics. In this talk I will show how some of these real-world effects impact the data. I will present an example of how spatially-varying observing conditions impact measurements of fundamental physics (such as primordial non-Gaussianity) from galaxy surveys, and discuss techniques that can be used to control these systematics. I will present a comprehensive survey of the capabilities of future CMB experiments, taking account of Galactic foregrounds and the effect of lensing by intervening large-scale structure. Incorporating these effects, I will present forecasts for the constraining power of these experiments in terms of inflationary physics, the neutrino sector, and dark energy parameters.
Host: Armin Rest
Mar. 23 Diana Buchwald (California Institute of Technology)
Title: Living on Paper: Einstein's Manuscripts 100 Years after GR
Abstract: The anniversary of the theory of general relativity, completed by Albert Einstein (1879-1955) in late 1915, provides an opportunity to reflect not only on the significant changes in theoretical physics and astronomy over the past century, but also on the changes in our understanding of the historical development of modern science. Einstein’s massive written legacy reveals how Einstein and his generation traversed theory and experiment on the path from classical physics to relativity and quantum theory, and also how they responded and were shaped by the social, political, and institutional transformations that have led to our current scientific culture.
Host: Van Dixon
Mar. 30 Ewine van Dishoeck (Leiden Observatory); Bahcall Lecturer
Title: Zooming In On the Planet-Forming Zones of Disks: Sweet Results from ALMA
Abstract: Protoplanetary disks are the birthplaces of planets, and have been beautifully revealed by iconic HST images. However, the spatial resolution at long wavelengths has so far been insufficient to resolve the critical 5-30 AU region and probe the bulk of the highly obscured disk material. ALMA now allows us to zoom in to nearby disks and determine the physical and chemical structure associated with planet formation. This talk will provide an overview of recent work on observations and models of protoplanetary disks around young stars in various stages of evolution. Early ALMA results include evidence for rotationally supported disks in the deeply embedded stage, the detection of organic molecules (including sugar) and water in forming disks, and the first images of the CO snowline in mature disks.

Special attention will be given to transitional disks, which are a subset of disks with evidence for sharp-rimmed cavities (gaps or holes) in their inner part but with otherwise normal outer disks. These disks are called 'transitional' because they are thought to represent the evolutionary phase from the gas-rich protoplanetary disk to the gas-poor debris disk stage. They are the best candidate sources for harboring just-formed giant planets. ALMA allows imaging of both the gas and dust in these disks, with gas cavities found to be significantly smaller than those of the dust, providing constraints on the properties of the young planets. The surprising discovery of huge asymmetric dust traps ('planetesimal or Kuiper-Belt factory') will be highlighted. Future prospects for JWST will be discussed.
Host: I. Neill Reid
April 06 (No Colloquium: HST Phase I Deadline Week)
April 13 Tobias Marriage (JHU)
Title: Probing AGN Feedback with the SZ Effect
Abstract: Energy injected into the circumgalactic medium (CGM) by active galactic nuclei (AGN) is invoked to explain several aspects of galaxy formation, including the decrease in cosmic star formation since z=2 and the correlation between black hole and galaxy bulge mass. Progress towards observing this energy feedback has been made through optical and X-ray observations. However, probing the diffuse hot CGM presents significant observational challenges, especially at redshifts z>=2 corresponding to the rise and peak in cosmic star formation and central black hole growth. The Sunyaev-Zel'dovich (SZ) effect, a spectral distortion in the CMB due to interaction with the hot CGM, provides a unique probe of the energetics of the CGM to high redshift. I will discuss two recent studies that use the SZ to probe AGN feedback.
Host: N/A
April 13 Ron Allen (STScI)
Title: Dark Molecular Gas in the Galaxy - An Update
Abstract: In the last decade it has become increasingly clear that a major fraction of the molecular gas in the ISM is not revealed in observations made with the "classical" tracer, the 1-0 rotational line of the CO molecule. I will present a brief summary of what we presently know about the location and physical state of this dark molecular gas in the Galaxy, and explain why the rotational lines of the OH molecule at 18-cm wavelength are a better tracer for it. The promise of this new tracer will be illustrated with selected results from recent and ongoing high-sensitivity observing programs in the 18-cm OH lines using the 100-m Green Bank Telescope.
Host: N/A
April 20 Klaus Pontoppidan (Space Telescope Science Institute)
Title:Toward a Comparative Inventory of Volatile Molecules in Protoplanetary Disks, Comets and Primordial Ice
Abstract: Volatile molecules are the main condensible carriers of the elements essential for life, such as carbon, hydrogen and nitrogen. Their chemical evolution during star and planet formation determines the compositions of planets, including those destined to orbit in a habitable zone. Yet, the path from the ISM to planets is one fraught with complexity and twists, making it difficult to derive precise theoretical predictions for planetary chemistry. Indeed, planet-forming disks are no longer considered uniform well-mixed structures; rather, they are complex worlds with many different heterogenous environments, most of which play some part in determining the composition of planetesimals and planets. Direct observations of atomic and molecular abundances on all size scales are therefore needed for understanding planet formation at a very fundamental level, and for answering the question of how chemically common the Earth is among exoplanets. In the past years, great progress has been made in observing protoplanetary chemistry, in particular in measuring the molecular composition in protoplanetary disks across the planet-forming regions from 1 to 10s of AU. I will present recent observational work that demonstrates strong chemical evolution between the interstellar medium, planet-forming material and our own solar system, and discuss the implications our understanding of exoplanet composition and habitability.
Host: N/A
April 27 (No Colloquium: Spring Symposium)
May 04 Nicholas Suntzeff (Texas A&M University)
Title: From Calán/Tololo to HZT to the Carnegie Supernova Project: Thirty Years of Watching Stars Explode
Abstract: Starting in 1986, Mark Phillips, Mario Hamuy, and I began the study of the properties of nearby supernovae, and were the first to produce a light curve based on CCD data. With Jose Maza, in 1989, we began the concentrated study of nearby supernovae called the Calan/Tololo Survey, which led to discoveries including the establishment of Type Ia supernovae as standardizable candles, the deeper understanding of reddening and temperature effects in light curves and spectra, and with the HST calibration of distances to nearby host galaxies of these SNe, the modern value of the Hubble constant based on the quiet Hubble flow defined by supernovae. In 1994, Brian Schmidt and I founded the High-Z Supernova Team utilizing the Calán/Tololo results and MLCS techniques developed by Riess et al. The image subtraction software was developed by Schmidt and later Tonry. These techniques underlie the discovery by both the HZT and the Supernova Cosmology Project of Saul Perlmutter et al (who developed independent software) of the apparent accelerated expansion of the Universe. All these discoveries rest on the rickety photometric system astronomers have organically developed over the last 70 years. With the improvement in the fundamental calibration system led by HST astronomers, and a reanalysis of astronomical photometric techniques by Stubbs and Tonry, we now see the results of supernova cosmology are limited by the systematic errors in how we do photometry. We founded the Carnegie Supernova Project to create a new and precisely calibrated set of nearby supernovae to dig into these systematic effects and to anchor the acceleration results. In this talk, I will present our recently completed ten year survey of exploding stars in the nearby Universe.
Host: D. Soderblom and Adam Riess
May 11 Tracy Webb (McGill University)
Title: The Growth of the Most Massive Galaxies in the Highest Density Regions: Evidence for In-Situ Star Formation in SpARCS Brightest Cluster Galaxies
Abstract: The most massive galaxies in the local universe reside at the centres of galaxy clusters. Often called Brightest Cluster Galaxies (BCGs), they exhibit, as a class, highly uniform properties and are distinct from the general galaxy population. This suggests formation processes which are themselves distinct from those which dominate in massive galaxies outside of cluster cores. The mass growth of BCGs is likely linked to the overall physics of hierarchical structure formation on galaxy cluster scales, including the fundamental processes of gas cooling, star formation, energy feedback and galaxy mergers, at the centers of giant dark matter halos. In this talk I will present new results from the largest study of high-redshift BCGs conducted to date, drawn from the SpARCS optical/NIR cluster survey. Using archival infrared data we show the star formation rate within BCGs increases to z~2, and can add as much mass to the BCG population as the previous standard model of growth by dry mergers. At low redshifts, and in X-ray/SZ selected clusters, the rare examples of star forming BCGs appear to be fed by large-scale cooling flows. However, the first of the SpARCS systems we have studied in detail, SpARCS1049, has revealed a very different phenomenon - a train-wreck of a galaxy merger at the center of the cluster. This is the first example of such a process in high-redshift cluster cores and may represent a new phase of BCG evolution, previously unaccounted for.
Host: Jennifer Lotz
May 18 (No Colloquium: JWST Meeting)
May 25 J. J. Eldridge (University of Auckland)
Title: What's the Link Between Superluminous Supernovae and Gamma-Ray Bursts?
Abstract: TBS
Host: Andy Fruchter