STScI Logo

Staff Research
2016 Fall 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 2016-17 committee members are Brad Whitmore (chair), Gabe Brammer, Andy Fruchter (ex officio), Olivia Jones and Nora Luetzgendorf.

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

Date Speaker/Title
Sep. 07 Tim Heckman (Johns Hopkins University)
Title: Feedback from Massive Stars: Implications for the Evolution of Galaxies and the Inter-Galactic Medium
Abstract: I will begin by summarizing why global outflows driven from intensely star-forming galaxies are believed to play a significant role in the evolution of galaxies and the IGM. I will then review the physical mechanisms believed to be responsible for driving these outflows, before giving a “guided tour” of M 82 (the prototypical starburst-driven outflow). I will summarize how the basic properties of outflows are measured, describe how these properties scale with those of the galaxy driving the outflow, and compare these relations to expectations from simple models and with the “sub-grid” prescriptions used in numerical simulations of galaxy evolution. Finally, I will circle back to discuss the implications of these outflows, based on our current understanding.
Host: None
Sep. 14 Alycia Weinberger (Carnegie Institution of Washington)
Title: Circumstellar Disks: Tracing the Formation of Planetary Systems
Abstract: I will show observations of circumstellar disks that elucidate the timescales for planetary formation, the compositions and structures found at various stages of disk evolution, and observations of young stars that clarify their timescales for disk dissipation.
Host: TBS
Sep. 21 Jennifer Lotz (Space Telescope Science Institute)
Title: Galaxy Assembly and Mergers through Cosmic Time
Abstract: Deep HST observations have revealed galaxies fainter than ever seen before, at look-back times when the universe was less than a billion years old. These first galaxies grow throughout cosmic time via the accretion of gas and dark matter, and via mergers with other galaxies. The detailed structures of galaxies provide direct insight into their most recent assembly events. From SDSS and HST imaging surveys, we now have a broad-brush picture of how galaxy shapes and sizes have evolved over the past 10 billion years. But the role of galaxy mergers in galaxy evolution is poorly understood, particularly at early times. More subtle morphological tracers are needed to track the complex processes responsible for the transformation of galaxies. I use new machine learning classifications of galaxy morphology at 0 < z < 3 to identify galaxy mergers and galaxies transitioning to today's Hubble types. Numerical simulations are used to inform the interpretation of these morphological classes, and to constrain the precise merger states of local systems. I track the evolution of galaxies as a function these new measures, and discuss the role of mergers in the size growth of galaxies. Finally, I discuss the prospects for studying galaxy assembly with JWST, LSST, and WFIRST in the coming decade.
Host: None
Sep. 28 Brad Peterson (Ohio State University/Space Telescope Science Institute)
Title: Exploring the Inner Structure of Active Galactic Nuclei by Reverberation
Abstract: The innermost structure of active galactic nuclei (AGNs) consists of an accretion disk surrounding a supermassive black hole and, on somewhat larger scales, rapidly moving diffuse gas. The ultraviolet through near IR spectrum of AGNs is dominated by thermal continuum emission from the accretion disk and broad emission lines and absorption features from the diffuse gas. The continuum flux from the accretion disk varies with time, and the emission lines also change in brightness, or “reverberate,” in response to these variations, with a delay due to the light-travel time across the line-emitting region. Measurement of the emission-line time delay yields the size of the line-emitting region and by combining this with the emission-line Doppler width, the central black hole mass can be inferred. I will discuss results from recent “reverberation mapping” experiments, including a 179-orbit HST Cycle 21 program, that have been designed to explore the dynamics of the emission-line gas and are yielding a wealth of new and quite surprising information about AGN structure.
Host: TBS
Oct. 05 Charles Steidel (California Institute of Technology)
Title: Reconciling the Massive Stars and Nebular Emission in High Redshift Galaxies
Abstract: Massive stars produce the most readily-observed signatures of forming galaxies at high redshift -- the FUV stellar continuum and the nebular/recombination emission excited by stellar EUV radiation field. Encoded in these spectra, which can be observed from the ground in the observed-frame optical and near-IR, are details of the stellar and gas-phase chemical abundances, the physical conditions in the galaxy ISM, and the nature of the massive stellar populations. Most of the efforts to understand the rapidly-improving observations of high-redshift star-forming galaxies have used low-redshift samples as both statistical baseline and to establish calibrations for extracting physical insight from comparatively crude data such as that available at high redshift. I will discuss why this type of approach is dangerous, and may lead to incorrect conclusions for galaxies observed at high redshift. Using data obtained as part of the Keck Baryonic Structure Survey (KBSS), I will show that the simultaneous analysis of FUV stellar and FUV/optical nebular spectra of the same galaxies can be used successfully to ``close the loop'' with internally consistent models of high redshift galaxies -- without reference to low-redshift, where the conditions that once prevailed at high redshift have become extremely rare. I will discuss what one can hope to measure reliably, and what will remain challenging, using current and future surveys based on rest-frame UV/optical spectra of high redshift galaxies.
Host: TBS
Oct. 12 (No Colloquium)
Oct. 19 Andrew Fox (Space Telescope Science Institute)
Title: The Gaseous Halo of the Milky Way: A Close-Up View of Galactic Inflow and Outflow
Abstract: Gaseous inflows and outflows are key components of the life cycle of galaxies. Inflows bring in fuel for future star formation, while outflows take away the ashes from past star formation. In the Milky Way, we have a front-row seat to observe these flows and study their impact on galaxy evolution. They can be seen in the form of the gaseous high-velocity clouds (HVCs), visible both in 21 cm H I emission and in UV metal-line absorption. I will give an overview of recent research on HVCs using HST and radio spectroscopy. This will include examples of inflow (the Magellanic Stream and its Leading Arm), and outflow (the Galactic Center wind). These observations allow us to quantify the mass flow rates of gas circulating in the Galactic halo, and relate them to the Galactic star formation rate.
Host: None
Oct. 26 (No Colloquium)
Nov. 02 (No Colloquium)
Nov. 09 Nadine Neumayer (Max Planck Institute for Astronomy)
Title: Nuclear Star Clusters and Black Holes
Abstract: The centers of massive galaxies are special in many ways, not least because all of them are believed to host supermassive black holes. Since the discovery of a number of relations linking the mass of this central black hole to the large scale properties of the surrounding galaxy bulge it has been suspected that the growth of the central black hole is intimately connected to the evolution of its host galaxy. However, at lower masses, and especially for bulgeless galaxies, the situation is much less clear. Interestingly, these galaxies often host massive star clusters at their centers, and unlike black holes, these nuclear star clusters provide a visible record of the accretion of stars and gas into the nucleus.

I will present our ongoing observing programme of the nearest nuclear star clusters, including the one in our Milky Way. These observations provide important information on the formation mechanism of nuclear star clusters. They allow us to measure potential black hole masses and might give a clue on how black holes get to the centres of galaxies.
Host: TBS
Nov. 16 Anne Marie Lagrange (Observatoire de Grenoble)
Title: Direct imaging of Exoplanets: Current Status and Prospects
Abstract: More than 3500 extrasolar planets, with masses ranging between a few Earth masses and several Jupiter masses have been found since 20 years, revealing an unexpected diversity that lead to broaden our view of planet formation and evolution processes. Yet, we do not have today a complete view of planetary systems architectures, of planets properties and formation mechanisms. While the close environments of stars have already been well explored with radial velocity and transit technics, and the 2-5 au environment will be well explored with GAIA, planets on more distant orbits (> 5 au) are still largely to be detected. Direct imaging offers in principle the opportunity to detect and characterise such planets. However, direct imaging is very demanding in terms of image quality and only the most recent instruments coupling adaptive optics (when observing from the ground), coronagraphs, and sophisticated data reduction algorithms allow separating the faint planetary signals from the bright stellar ones and detecting Jupiter mass planets.

Even though few planets have been imaged so far, each of the few planets imaged has brought new insights and has triggered new questions about planet formation and dynamical evolution. I will describe the results obtained so far, in particular with the recent Extreme-Adaptive Optics instruments, and discuss the results expected in the forthcoming decade, with JWST and the future Extremely Large Telescopes.
Host: TBS
Nov. 23 (No Colloquium)
Nov. 30 Gurtina Besla (University of Arizona)
Title: Environment and the Evolution of Dwarf Galaxies
Abstract: Low mass, dwarf galaxies (M* = 1e8 - 5e9 Msun) are typically gas rich, unless found in proximity to a massive host. This suggests that the evolution of dwarfs is dominated solely by their host environment, through processes such as tidal and ram pressure stripping. Turning the argument around, this also implies that dwarfs are ideal probes of the circumgalactic medium (CGM) and dark matter potentials of their hosts. However, dwarf galaxies are often found with companions, invoking an additional environmental pathway for their evolution. Recent results from the TiNy Titans Survey illustrate that dwarf-dwarf galaxy interactions can be an important mode of gas removal, causing these galaxies to possess large envelopes and streams of gas. Such extended structures can be efficiently stripped upon entering a more massive environment, highlighting an overlooked mode of gas supply to the CGM of galaxies like our Milky Way.
Host: TBS
Dec. 07 (No Colloquium)
Dec. 14 Beth Willman (Large Synoptic Survey Telescope)
Title: Triumphs and Tribulations of Near-field Cosmology with Wide-field Surveys
Abstract: Over the last 15 years, large sky surveys have revolutionized our view of the stellar halos and dwarf satellite populations orbiting galaxies, such as the Milky Way. Much of this observational progress has been motivated by a series of apparent “crises” for the cold dark matter cosmological model. However, these challenges have effectively functioned as flashlights pointing us to interesting galaxy formation physics. I will present recent observational progress in our understanding of galaxy formation using observations of stellar halos and dwarf satellite populations in the Local Volume, along with the limiting impacts of observational and sample bias. This talk will include first results from the MADCASH (Magellanic Analog Dwarf Companions And Stellar Halos) survey, a Subaru/HSC observational program to map LMC stellar mass galaxies at distances of ~2-4 Mpc, and a look ahead to LSST.
Host: TBS