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Staff Research
2014 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 2014-15 committee members are Rongmon Bordoloi, Susana Deustua, Peter McCullough and Eileen Meyer.

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

Date Speaker/Title
Sept. 10 Claude-Andre Faucher-Giguere (Northwestern University)
Title: Feedback-Regulated Star Formation on Galactic and Cosmological Scales
Abstract: A central problem in galaxy formation is to understand why star formation is so inefficient. Within individual galaxies, gas is converted into stars at a rate two orders of magnitude slower than unimpeded gravitational collapse predicts, a fact embodied in the low normalization of the observed Kennicutt-Schmidt (K-S) relationship between star formation rate surface density and gas surface density. Star formation in galaxies is also globally inefficient in the sense that the stellar mass in dark matter halos is a small fraction of the universal baryon fraction. I will show that these two facts can be explained by the self-regulation of star formation by feedback from massive stars. Within galaxies, stellar feedback drives turbulence that supports the interstellar medium against collapse and the K-S law is set by the low strength of gravity relative to stellar feedback. The energy input from the same stellar feedback processes drive powerful galactic outflows that remove most of the gas accreted from the intergalactic medium before it has time to turn into stars. Using cosmological hydrodynamical simulations from our FIRE project ("Feedback In Realistic Environments"), I will show that gas removal by star formation-driven galactic winds successfully explains the observed galaxy stellar mass function, at least for galaxies less massive than the Milky Way. I will also discuss the observational signatures of circum-galactic gas flows, focusing on predictions for HI around the peak of the cosmic star formation history. Feedback from massive black holes may be required to quench galaxies much more massive than the Milky Way.
Host: Jason Tumlinson
Sept. 17 Ray Jayawardhana (York University)
Title: Characterizing Exoplanets from the Ground and from Space
Abstract: On-going searches for extrasolar planets have revealed a remarkable diversity of worlds, from close-in super-Earths to far-out super-Jupiters, and challenged our preconceptions many times over. Meanwhile, comparative studies of exoplanet physical properties have begun in earnest: planets caught in transit and those imaged directly are best suited for detailed characterization, especially of their atmospheres. I will discuss recent results and future prospects for ground-based and space observations, including the possibility of extending these techniques to lower-mass planets.
Host: Hussein Jirdeh
Sept. 24 Alberto Bolatto (University of Maryland – College Park)
Title: The ALMA View of NGC253, One of the Nearest Nuclear Starbursts
Abstract: In the context of galaxy evolution it is particularly interesting to understand better the mechanisms that regulate star formation activity in galaxies. ALMA observed the prototypical nearby starburst galaxy NGC 253 in cycles 0 and 1 in the 3mm band that contains the ground rotational transitions of several key molecules. In this talk I will present an analysis of the molecular ISM properties in the nuclear starburst region derived from the ALMA data. I will show that the molecular superwind is very significant at determining the duration of the starburst phase, and I will discuss the possible gas entraining mechanisms. These data also allow us to obtain for the first time a detailed view of giant molecular clouds in a starburst environment. I will show that the clouds we are able to identify have very large surface and volume densities, and consequently short free-fall and sound crossing times, which are likely connected to the large star formation efficiencies found in these environments. Finally, I will present and discuss some of the chemical complexity we see in the data. The extremely rich spectroscopy, a common feature in many ALMA datasets, opens new windows for the study of physical conditions in extragalactic systems.
Host: Eileen Meyer
Oct. 01 Michael Cooper (University of California – Irvine)
Title: Expanding the Low-Mass Galaxy Frontier
Abstract: While modern models of galaxy evolution have made significant strides in explaining the observed properties of massive galaxies over cosmic time, recent observational results have illustrated a fundamental problem with the ability of these same models to predict the evolution of low-mass systems. In particular, simulations of galaxy evolution tend to overpredict the number of passive (or "quenched") low-mass galaxies at low and intermediate redshift. Using data from the SDSS, I will present recent work to constrain the quenching of satellite galaxies in the local Universe, with a focus on identifying the host and satellite properties critical to the cessation of star formation at low stellar masses. This analysis, when combined with observations of the Local Group, points towards a characteristic mass scale for satellite quenching as well as strong limits to our understanding of dwarf galaxies. As time permits, I will present ongoing efforts to expand our knowledge of this low-mass galaxy frontier.
Host: Molly Peeples
Oct. 08 Felix Mirabel ( CEA-Saclay-France & CONICET-Argentina)
Title: Stellar Black Holes at the Dawn of the Universe
Abstract: The end of the dark age of the universe is one of the major frontiers in cosmology. Until recently most models assumed that the ultraviolet radiation from the first generations of massive stars was the main (and in some models the only) cause of re-ionization of the intergalactic medium. Based on recent observational and theoretical results I will show that during the first billon years, sources of X-rays, such as accreting stellar black holes, determined the early thermal history of the universe, as well as the re-ionization of the intergalactic medium over large volumes of space. X-rays from the fossils of massive stars had a direct impact on the properties of the faintest galaxies at high redshifts and on the smallest dwarf galaxies in the local universe. In this context, feedback from compact high mass X-ray binaries can provide additional clues to understand the tension between the number of dwarf galaxies observed in the local universe and the number of those galaxies predicted by the cold dark matter model (lCDM) of the universe.
Host: Andy Fruchter
Oct. 15 Michael Boylan- Kolchin (University of Maryland – College Park)
Title: The Local Group as a Cosmological Laboratory
Abstract: The Local Group affords us the opportunity to study the low-mass extremes of galaxy formation and cosmology. In this capacity, it presents some of the most enduring challenges to the very successful LCDM cosmology. I will discuss to what degree standard theoretical models of the local Universe - including the idea that photoionization heating from cosmic reionization is crucial to suppressing galaxy formation in low mass halos - match the growing volume and diversity of observations in the Local Group and beyond. Since reionization is expected to be a dominant process in determining the abundance of low-mass galaxies around the Milky Way, observations hold the promise to understand aspects of the high-redshift Universe by studying its descendants locally. I will argue that, even in the JWST era, the local Universe may be our best probe of low-mass, currently unobservable galaxies at high redshift that are expected to be crucial for reionization.
Host: Molly Peeples
Oct. 22 Amy Barger (University of Wisconsin - Madison)
Title: New Insights on Star Formation in the Distant Universe
Abstract: The most luminous galaxies in the universe are hidden by dust. I will present submillimeter and radio observations that allow us to map the evolution of these galaxies, to find their contributions to the overall star formation history, and to determine the maximum star formation rates that individual galaxies can have. A key question is whether the far-infrared and rest-frame UV selected populations are disjoint. I will discuss how this can be resolved with the Hubble Frontier Fields. I will also address the question of whether luminous AGNs suppress star formation in their host galaxies.
Host: Susana Deustua
Oct. 29 Martha Boyer (NASA/GSFC)
Title: DUSTiNGS Reveals Dust Production in Very Metal-Poor Environments
Abstract: The origin of interstellar dust in distant quasars is under debate. Infrared observations of nearby galaxies and the Milky Way show that there are two dominant (known) sources of dust: supernovae ejecta and the winds of evolved stars like asymptotic giant branch (AGB) stars. Progenitors of core-collapse supernovae produce the seeds necessary for dust production in their cores and are thus capable of producing dust at any metallicity. Dust production by AGB stars is not well understood, and it is unclear whether metal-poor AGB stars at high redshift can produce any dust. Part of the problem lies in the scarcity of examples of metal-poor dust-producing AGB stars. This scarcity motivated the survey of Dust in Nearby Galaxies with Spitzer (DUSTiNGS), which imaged 50 nearby dwarf galaxies (D < 1.5 Mpc) in the infrared to search for metal-poor dusty stars in galaxies with metallicities as low as [Fe/H] = -2.7 dex. We find more than 500 dusty AGB stars (sometimes called extreme AGB stars) in the DUSTiNGS sample that are likely in the superwind phase and thus dominate the current AGB dust input to the interstellar medium of their respective galaxies. I will describe how the properties of these newly discovered stars depend on metallicity and discuss the implications for the early Universe.
Host: Karl Gordon
Nov. 05 Sarah Burke-Spolaor (California Institute of Technology)
Title: Fast Radio Bursts: Ere One Can Say ‘It Lightens’
Abstract: The discovery of intense radio pulses---lasting only milliseconds--have been fuelling a heated debate as to the origin of these "Fast Radio Bursts" (FRBs). Until recently, it was hypothesized that such bursts were Earth-local, but a growing body of evidence is demonstrating that some are arriving from extragalactic sources at redshifts up to z~2. As an extragalactic population, these bursts would be unprecedented probes of extragalactic plasma and hitherto unidentified physical processes. Multiple teams worldwide are racing to provide real-time detection of FRBs, which can provide triggers to identify FRB precursors and counterparts. I will outline the ongoing excitement around FRB research: evidence for sub-populations that range from local to cosmic origins, the myriad of potential extragalactic progenitors, and highlights of recent advances in isolating the origin of these mysterious signals via real-time detection.
Host: Marc Postman
Nov. 12 Meredith Hughes (Wesleyan University)
Title: Planet Formation through Radio Eyes
Abstract: Circumstellar disks provide the raw material and initial conditions for planet formation. Millimeter-wavelength interferometry is a powerful tool for studying gas and dust in planet-forming regions, and it is undergoing a multiple-order-of-magnitude leap in sophistication with the advent of the ALMA interferometer that is now in the late stages of construction. I will discuss some ways in which millimeter-wavelength interferometry is being used to study the process of planet formation in circumstellar disks, with particular emphasis on the kinematics of turbulence in protoplanetary disks and the degree to which debris disk structure reflects the dynamics of embedded planetary systems.
Host: Susan Deutsua
Nov. 19 Richard Townsend (University of Wisconsin – Madison)
Title: Ghost Hunting at Five Hundred Parsecs: The Glowing Magnetospheres of Massive, Luminous Stars
Abstract: Massive stars were long thought to be non-magnetic, but we now know that around 10% of them harbor intense, ordered, stable magnetic fields. These fields interact strongly with the stars' hypersonic radiation-driven winds, channeling and confining the wind plasma into circumstellar magnetospheres which emit across the full electromagnetic spectrum, from X-rays through to radio.

With their host stars typically situated hundreds of parsecs away, there's little chance of spatially resolving these ghostly magnetospheres --- at least for the foreseeable future. Nevertheless, with techniques such as magnetic Doppler imaging we can leverage spectropolarimetric observations at multiple rotation phases into detailed maps of the stellar-surface magnetic fields. In turn, these maps allow us to extrapolate the circumstellar field topology and build detailed three-dimensional models for the distribution of trapped wind plasma throughout the magnetospheres.

In this talk I'll demonstrate how this process leads us to new models for the magnetospheres of a handful of well-studied magnetic B-stars: sigma Ori E, HD 37776 and tau Sco. I'll likewise explore how the synthetic observables predicted by these models stack up against archival observations; and discuss how discrepancies between the two are driving further refinements to the models. I'll finish with an update on the spin-down of sigma Ori E, the only main-sequence star for which magnetic braking has been directly and unambiguously measured.
Host: Nolan Walborn and Alex Fullerton
Nov. 26 No Colloquium;
Dec. 03 Ryan Hickox (Dartmouth College)
Title: The SF-AGN Connection: Do All Star-Forming Galaxies Host an AGN?
Abstract: Recent years have seen remarkable advances in our understanding of how supermassive black holes form and grow over cosmic time, and how energy released by active galactic nuclei (AGN) connects the growth of black holes to their host galaxies and large-scale structures. Still a basic and essential question has remained largely unanswered: Does the fueling of AGN activity occur along with star formation in galaxies, or does feedback from AGN suppress star formation? I will argue that both scenarios are correct, depending on the properties of the galaxy and its dark matter halo. Specifically, I will review recent results suggesting that the long-term rate of black hole accretion is closely tied to the rate of star formation, so that essentially *all* star-forming galaxies may be thought of as hosting an AGN after accounting for rapid stochastic variability. I will further discuss the strong evidence that in passive galaxies in massive halos, mechanical feedback from AGN serves to heat gaseous atmospheres and suppress further star formation. These results suggest that the connection between black holes and their host galaxies changes significantly as halos grow and galaxies evolve from star-forming to passive systems.
Host: Eileen Meyer
Dec. 10 Sally Dodson-Robinson (University of Delaware)
Title: From Snowflakes to Snowballs: Bypassing the Bouncing Barrier in Icy Planetesimal Growth
Abstract: Planet searches and debris-disk observations have demonstrated that icy planet formation is an inevitable outcome of Population I star formation. However, theorists have yet to converge on a physical description of the first stage of planet formation--the growth from millimeter-sized grains to pebbles. While micron-sized grains can easily grow into fluffy aggregates by electrostatic sticking, the surface area/mass ratio becomes too high for sticking when the particles reach roughly one millimeter in size. At the average collision speeds for icy regions of the solar nebula, large grains either bounce or fragment. Here we explore collision outcomes at the low and high ends of the collision speed distribution. At extremely low speeds, electrostatic attraction may overcome the rebound energy for colliding millimeter-sized particles. Even a few "lucky" particles that grow to centimeter sizes may provide the seeds for planetesimal formation. At extremely high speeds, the surface layers of icy particles may melt during collisions. If the melted layer can re-freeze within the collision timescale, the two particles may stick together. We have re-formulated the collisional fusion theory of Wettlaufer (2010), which describes icy particles sticking to a meter-sized boulder, to apply to smaller bodies of similar sizes. We demonstrate that both collisional fusion and the growth of "seeds" can provide a pathway toward planetesimal formation.
Host: Karoline Gilbert
Dec. 17 Remi Soummer (Space Telescope Science Institute)
Title: Direct Imaging of Exoplanets and Disks
Abstract: Direct imaging will enable finding and characterizing exo-Earth with a future space mission, and searching for life. How can we prepare for such an ambitious goal? What can be done already with existing and upcoming facilities in the mean time? With new-generation instrument coming online direct imaging has reached a "routine mode" and new science results are flowing. I will illustrate the progress that have led to these new results using both HST and ground-based instruments, and will discuss the current needs to prepare for a coronagraph on AFTA and on a future large UVOIR flagship.
Host: Neill Reid