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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
Sept. 02 Casey Papovich (Texas A&M)
Title: Witnessing Galaxy Formation from Modern Infrared Surveys
Abstract: One of the great triumphs of astrophysics is that we are able to predict the growth of dark-matter structures in our Universe with high accuracy. I will discuss how we use observations of galaxies to trace these dark matter structures, and how we use observations of some of the most distant galaxies to measure directly the properties of galaxies as they evolve. I will focus on results from two modern surveys using near-Infrared (IR) imaging (beyond 1 micron): the FourStar Galaxy Evolution (ZFOURGE) survey and the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS), which use very deep imaging from the Magellan Observatory in Chile and observations from the Hubble Space Telescope. Combined with observations at far-IR wavelengths from the Spitzer Space Telescope and the Herschel Observatory, I will show the typical (median) star-formation history, stellar-mass assembly history, and structural formation history of galaxies like the Milky Way and Andromeda galaxies over the past 10 billion years. I will also discuss evidence that galaxies depart widely from this "typical" evolution, and that the Milky Way and Andromeda themselves may be examples of such "atypical" evolution for galaxies of their size. I will discuss ongoing and future research to understand the formation epochs of galaxies, where the ultimate goal is to form a coherent physical theory for galaxy formation.
Host: Harry Ferguson
Sept. 09 Mark Reid (Harvard–Smithsonian Center for Astrophysics)
Title: Mapping the Milky Way: The BeSSeL Survey
Abstract: Over 2000 years ago, Hipparcus measured the distance to the Moon by triangulating from two locations across the Mediterranean Sea. However, determining distances to stars proved much more difficult. Many of the best scientists of the 16th through 18th centuries attempted to measure stellar parallax, not only to determine the scale of the cosmos but also to test Heliocentric cosmologies. While these efforts failed, along the way they lead to many discoveries, including atmospheric refraction, precession, and aberration of light. It was not until the 19th century that Bessel measured the first stellar parallax.
Distance measurement in astronomy remained a difficult problem even into the early 20th century, when the nature of "spiral nebulae" was still debated. While we now know the distances of galaxies at the edge of the Universe, we have only just begun to measure distances accurately throughout the Milky Way. Using the Very Long Baseline Array, we now can achieve parallax accuracies of 10 micro-arcseconds! I will present new results on parallaxes and motions of star forming regions from the BeSSeL Survey. These measurements address the nature of the spiral structure, size, rotation speed, and mass of the Milky Way.
Host: Johannes Sahlmann
Sept. 16 Zeljko Ivezic (University of Washington)
Title: What Did We Learn about the Milky Way during the Last Decade, and What Shall We Learn Using Gaia and LSST?
Abstract: Studies of stellar populations, understood to mean collections of stars with common spatial, kinematic, chemical, and/or age distributions, have been reinvigorated during the last decade by the advent of large-area sky surveys such as SDSS, 2MASS, RAVE, and others. These data, together with theoretical and modeling advances, are revolutionizing our understanding of the nature of the Milky Way, and galaxy formation and evolution in general. I will briefly review the progress over the last decade, including the mapping of stellar counts, metallicity and kinematics distributions, interstellar dust using extinction of stars, and dark matter halo using Jeans equations. I will discuss in more detail new breakthroughs expected from Gaia and LSST surveys, which will improve measurement precision manyfold, and comprise billions of individual stars.
Host: Annalisa Calamida
Sept. 23 (No Colloquium)
Sept. 30 Frank van den Bosch (Yale University)
Title: The Substructure of Dark Matter Halos Revisited
Abstract: The Cold Dark Matter paradigm predicts that dark matter haloes are abound with substructure. A detailed characterization of dark matter subhalos is important for understanding the evolution of satellite galaxies, for modeling time-delays and flux-ratio anomalies in gravitational lensing, for boosting the dark matter annihilation signal and for the survivability of fragile structures such as streams and disks. I discuss the abundance and evolution of dark matter subhalos based on a new analytical model, use the model to revisit the Too-Big-To-Fail problem, demonstrate important shortcomings in the studies of substructure based on numerical simulations, and present a comprehensive analysis of the segregation of dark matter subhalos within their host halos.
Host: Dan Coe and Armin Rest
Oct. 07 Remy Indebetouw (University of Viriginia/NRAO)
Title: Star Formation and Feedback in the LMC: ALMA and HST Analysis of the Evolution and Destruction of GMCs
Abstract: The Magellanic Clouds offer the opportunity to study star formation at reduced metallicity with no distance ambiguity and minimal line-of-sight confusion. The Magellanic Clouds are arguably the easiest location to observationally analyze entire galaxies simultaneously with critical subparsec physics, especially using ALMA and HST (1"~0.25pc). We can dissect giant molecular clouds and understand the details of their evolution and life cycle. I will discuss the molecular clouds in 4 different LMC regions: N11B and 30-Doradus-10 are evolving under the strong external influence of neighboring massive star clusters. By contrast, N55 appears to be mostly destroying itself with internal star formation. The younger N159 region shows complex internal structure. Resolving parsec-sized (~500Msun) molecular clumps with ALMA shows that while radiation can photodissociate diffuse gas in giant molecular clouds, the dense cluster-forming clumps may not be strongly affected, even within 50pc of R136. In clouds with less external radiation, molecular structures are more filamentary, with more massive clumps forming more massive stars. HST uniquely allows one to study the population of pre-main-sequence stars less than 3 solar masses in these clouds. Furthermore, that population reveals not just current star formation, but its progression over tens of Myrs. Thus, the combination of HST and ALMA reveals the progression of star formation within each cloud, and constrains timescales for cloud evolution and destruction.
Host: Margaret Meixner
Oct. 14 Marc Pinsonneault (Ohio State University)
Title: Asteroseismology and the Time Domain Revolution in Stellar Astrophysics
Abstract: Large time domain and spectroscopic surveys are revolutionizing stellar astrophysics. In this talk I will focus on the APOKASC project, involving a combination of asteroseismic and rotational data from the Kepler satellite and high-resolution H-band spectroscopy from APOGEE. Asteroseismology gives precise surface gravities and evolutionary state measurements, and the frequency pattern combined with spectroscopic data can also be used to independently measure mass, radius, and age. I critically assess the performance of asteroseismology against independent measurements, and also discuss the prospects for detailed tests of stellar physics (helium, convection zone depth, core mass and rotation.) Insights into stellar populations, such as an unexpectedly large fraction of young alpha-rich stars, will also be covered. Future and ongoing missions will be reviewed, and synergies with Gaia will be discussed.
Host: Molly Peeples
Oct. 21 Caryl Gronwall (Pennsylvania State University)
Title: HETDEX and Star-Forming Galaxies of the z ~ 2 Universe
Abstract: Next spring, the Hobby Eberly Telescope Dark Energy Experiment will begin obtaining redshifts for roughly a million Ly-alpha emitting galaxies (LAEs) between 1.9 < z < 3.5. While the main purpose of the project is to study the evolution of Dark Energy, the project will provide an incredible data base for studies of galaxy evolution. In preparation for this, we have been investigating the physical and chemical properties of emission-line galaxies in the z ~ 2 universe, using LAEs discovered from the ground and samples of [O III]-emitting objects identified from space. We show that LAEs are not “low mass, dust-poor galaxies caught in the act of formation”, but instead span the entire range of stellar masses, from at least 7.5 < log M/Msun < 10.5. We then use our galaxy samples to explore issues such as the (non)-Fundamental Metallicity Relation, the systematics of star-formation rate indicators, the behavior of dust attenuation laws versus stellar mass, and the question of what makes an LAE and LAE.
Host: Stefano Casertano
Oct. 28 Meg Urry (Yale University); Caroline Hershel Speaker
Title: Supermassive Black Hole Growth and Galaxy Evolution
Abstract: Using multi-wavelength surveys like GOODS, COSMOS, and Stripe 82X, we measure the growth of supermassive black holes at the centers of galaxies over the last ~12 billion years. Most actively growing black holes are heavily obscured and thus are not seen in large optical surveys, like the Sloan Digital Sky Survey; at the same time, the deep multi-wavelength surveys are too small to find rare objects like luminous SDSS quasars. So completing the census of black hole growth will require a large-volume X-ray survey, to explore obscured growth at high luminosity and/or high redshift. Theorists have suggested that mergers trigger Active Galactic Nuclei (AGN), whose radiation and/or outflows may quench star formation and strongly affect galaxy evolution (“feedback”). But our morphological analyses showed that most moderate luminosity AGN at redshifts z ~1-2 have not undergone a recent major merger. Finally, using morphological classifications in the local universe from Galaxy Zoo, we identify two distinct modes of galaxy evolution in the local universe, with mergers and AGN feedback affecting only a minority.
Host: Susan Kassin, Gerard Kriss, and Marco Chiaberge
Nov. 04 Veselin Kostov (Goddard Space Flight Center)
Title: Planets with Two Suns
Abstract: Planets with two suns have long fascinated our imagination yet it was only recently that we were able to provide direct evidence of their existence. It was not until 2011 and the launch of NASA's Kepler mission that such circumbinary planets were unambiguously detected, for the first time, through their transits. I will review the current status of the field, then focus on the detection methods and analysis tools required for the characterization of transiting circumbinary planets, discuss the theoretical implications of their discoveries, and touch on the future prospects in the field.
Host: Peter McCullough
Nov. 11 Christopher Johns-Krull (Rice University)
Title: Searching for Planets Around the Youngest Stars
Abstract: The past two decades have seen the discovery of over 5000 extra-solar planets and planet candidates. With these exciting discoveries has come new interests and developments in the theory of planet formation. Much of that interest has been focussed on the timescale for planet formation and the role of planetary migration. Along with the discovery of many extra-solar planets, we have also mapped out the brown dwarf desert: a surprising lack of brown dwarf companions within a few AU of solar-like stars. It is not yet clear whether this desert is the result of an inability to form such companions in the first place, or whether the desert is the result of a process such as migration removing such companions once they form. To address these issues it is desirable to search for planets around very young stars, including those still surrounded by active accretion disks. Due to accretion and stellar activity, such systems are quite challenging for traditional planet search techniques; nevertheless, progress is being made. In this talk I will describe a radial velocity search for brown dwarfs and giant planets around low mass pre-main sequence stars. I'll detail some of the pitfalls that have been encountered and present observations indicating the discovery of 2 Jupiter mass planets in short period orbits around ~2-3 Myr old T Tauri stars; one an accreting classical T Tauri star and the other a non-accreting weak-lined T Tauri star.
Host: Andrea Banzatti
Nov 18 Mercedes Lopez-Morales (Harvard–Smithsonian Center for Astrophysics)
Title: Exoplanet Atmosphere Observations Spinoffs
Abstract: TBD.
Host: Marie Ygouf
Nov. 25 (No Colloquium)
Dec 02 Karin Sandstrom (Arizona/UCSD)
Title: Dust and Gas in Nearby Galaxies
Abstract: TBD.
Host: Karl Gordon
Dec 09 Daryl Haggard (McGill University)
Title: Interpreting Sgr A*'s Most Luminous X-ray Flares
Abstract: Sagittarius A* is the closest example of a supermassive black hole (SMBH) buried within a dense, massive stellar cluster. Sgr A* is more than 100 times closer than any other SMBH, and our proximity allows us to detect emission from its accretion flow in the radio, submillimeter (submm), near infrared (NIR), and X-ray regimes. These rich multiwavelength, time-resolved data have the power to probe the physical processes that underlie rapid flares originating near the black hole's event horizon. During ambitious Chandra and VLA monitoring campaigns over the last several years, we have detected the brightest-ever X-ray flares from Sgr A*. However, despite years of observational and theoretical study, we do not have a complete, unique model to explain these high-energy flares, or their relationship to variability at other wavelengths. Viable models range from the tidal disruption of asteroids to gravitational lensing to magnetic reconnection, motivating observers to place tighter constraints on the timing and multiwavelength properties of these outbursts. X-ray flares may also help us relate SgrA* to weakly accreting black holes across the mass spectrum. In this colloquium, I will discuss the possible origins and continuing mysteries surrounding Sgr A*'s high-energy flares and give a brief update on the Sgr A*/G2 interaction.
Host: Laura Watkins
Dec 15

Change of Date
Robert Mathieu (University of Wisconsin-Madison)
Title: Alternative Pathways of Normal Stellar Evolution
Abstract: TBD.
Host: David Soderblom