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Northwestern University

CIERA Special Seminars 2017-2018

*Tech = Technological Institute (2145 Sheridan Road)
**Db = Dearborn Observatory (2131 Tech Drive)


Spring Quarter 2018

  Date/Time      Visitor Host
  May 11
  Tech F227A

Tarraneh Eftekhari
    Harvard University

Wen-fai Fong
  Jun. 27
  Tech F160

Laura Keating
     CITA; University of Toronto
    The Intergalactic Medium at the End of Reionization

Studying the luminous sources responsible for reionizing the Universe directly is difficult, as they are challenging to identify. An alternative method is to instead look for signatures of reionization in the diffuse gas between these sources. In this talk I will discuss my work using cosmological simulations to model different ways of studying the high-redshift intergalactic medium with quasar absorption lines. I will show that these absorption lines are a powerful tool for learning about the intergalactic medium and reionization, providing insight into the ionization state, metallicity and temperature of the gas out past redshift 7.

Claude-André Faucher-Giguère


Winter Quarter 2018

  Date/Time      Visitor Host
  Feb. 12
  Tech F160

David Chuss
    Villanova University
    The Cosmology Large Angular Scale Surveyor (CLASS)

Precise observations of the cosmic microwave background have played a leading role in the development of the Lambda-CDM model of cosmology, which has been successful in describing the universe’s energy content and evolution using a mere six parameters. With this progress have come hints that the universe underwent an inflationary epoch during its infancy. Cosmic inflation is predicted to produce a background of gravitational waves that would imprint a distinct polarized pattern on the cosmic microwave background (CMB). Measurement of this polarized signal would provide the first direct evidence for inflation and would provide a means to study physics at energy scales around the predicted GUT scale.

The Cosmology Large Angular Scale Surveyor (CLASS) is an array of telescopes located in the Atacama Desert in Chile that is designed to map the large-scale polarization of the cosmic microwave background to test the inflation paradigm. CLASS will measure 70% of the sky at four frequency bands that straddle the Galactic foreground minimum. CLASS’s sensitivity to the large-scale signal is unique to ground-based instruments. It enables CLASS to probe the inflationary signal on angular scales where its magnitude is above contamination due to gravitational lensing and provides a complement to the measurements from current and future high-resolution CMB experiments.

CLASS is enabled by two technology innovations. First, high-sensitivity focal planes have been developed to meet the sensitivity and systematic control requirements of this measurement. Second, measurement of the large-scale polarization requires fast polarization modulation to stabilize the signal. For this, large variable-delay polarization modulators (VPMs) have been developed.

In this talk, I will provide an overview of CLASS, including its targeted science goals, instrument design, and descriptions of its key technologies.

Giles Novak


Fall Quarter 2017

  Date/Time      Visitor Host
  Aug. 25
   Tech F160

Janet Chen
    Max Planck University
    Giant Explosions in Dwarf Hosts : “GREAT” Survey of Superluminous Supernovae

A new class of supernovae, superluminous supernovae, has been discovered in the past few years. They are 100 times brighter (with absolute mag ~ -21) than normal core-collapse supernovae. This means that the standard paradigm of iron-core collapse cannot account for the origin of superluminous supernovae. An alternative mechanism is needed to power such high luminosities, including magnetar spin down, pair-instability explosions and shell collisions. In this talk, I will present our work from discovery of superluminous supernovae with our "GREAT" (GRond-Epessto-ATlas) survey, which aim is to find superluminous supernovae at very first stage; to classification with the Public ESO Spectroscopic Survey for Transient Objects (PESSTO), and follow-up campaign with large facilities such as the Very Large Telescope. We found superluminous supernovae appear to occur exclusively in dwarf, metal-poor host galaxies, and a sub-solar metallicity seems required to produce superluminous supernovae. We also found a possible relation that if magnetar powering is the source of the extreme luminosity then the required initial spins appear to be correlated with metallicity of the host galaxy. Finally I will also focus on the diversity of superluminous supernovae and a challenge of metal rich environment has been found for SLSNe.

Giacomo Terreran
  Sep. 11
   Tech F210

Om Sharan Salafia
    University of Milano-Bicocca
    Where and When: How to Combine Information from the GW Signal and EM Counterpart
    Models to Inform and Optimize the EM Follow-up Strategy

The electromagnetic follow-up of a gravitational wave event requires astronomers to scan a wide sky region for the detection and identification of a transient whose features are very uncertain a priori. A sky position posterior probability map (“skymap”) is provided by the LIGO/Virgo collaboration (containing information on "where" to search), but no timing ("when") information is available, other than conjectures based on the expected properties of the candidate EM counterparts. I will describe a novel approach to schedule observations using information from GW parameter estimation, thus allowing for an event-specific optimization of the EM follow-up strategy. I will illustrate the result of applying the approach to an example NS-NS injection with an associated SGRB oprhan afterglow and dynamical ejecta macronova, showing that the follow-up is significantly improved. I will discuss the possible application of the method to future real GW triggers, including the case when only low-latency information is available.

Wen-fai Fong
  Sep. 20

Adrian Barker
    University of Leeds
    Tidal Flows in Extrasolar Planets

Tidal interactions between short-period planets and their host stars are thought to play an important role in the evolution of planetary orbits, and stellar and planetary spins. However, the mechanisms responsible for tidal dissipation are not well understood theoretically. I will present results from hydrodynamical (and magnetohydrodynamical) simulations of tidal flows in short-period gaseous planets and stars from first principles. I will discuss the outcome of two fluid instabilities that could be important for tidal dissipation (the elliptical and precessional instabilities). Finally, I will present some new results on the influence of semi-convective layers on tidally-excited waves and tidal dissipation in planets.

Yoram Lithwick
  Sep. 22

Suzanne Aigrain
    University of Oxford
    Taming the Stochastic: How Gaussian Processes are Transforming Exoplanet Studies

Almost all of the staggering progress in detecting and characterizing extra-solar planetary systems over the past 20 years or so has relied on time-series data. The planetary signals are typically buried in complex, correlated noise, such as stellar variability or instrumental systematics, which the "traditional" statistical toolbox of most astronomers was ill equipped to deal with. Over the past few years, Gaussian process (GP) regression has become increasingly popular as a means of modelling these "nuisance signals" explicitly, within a Bayesian framework, so that the resulting uncertainties can be propagated through to the final exoplanet parameters.

Adam Miller,
Ben Nelson
  Oct. 3

Ian Christie
    Purdue University
    Modeling Stellar Wind Interactions

Interactions between stellar winds and their surrounding medium are powerful tools for investigating local properties of the medium (e.g. gas density, temperature), whose features can be revealed through thermal and non-thermal radiation mechanisms. As the stellar wind collides with the medium, the ram pressure terminates the wind by a strong shock. The non-thermal emission from these interactions is produced by relativistic electron-positron pairs accelerated at the shock front, while the thermal emission is produced from the compression and heating of the shocked stellar wind. In this talk, I will present model predictions about the observational signatures in X-rays and/or radio wavelengths of three different systems: i) the S2-star interacting with the accretion flow surrounding Sgr A*, ii) a fiducial young and bright pulsar interacting with the Galactic Center environment, and iii) the pulsar/Be binary system PSR J2032+4127.

Sasha Tchekhovskoy
  Oct. 19

Payaswini Saikia
    Radboud University, Netherlands
    The Optical Fundamental Plane of Black Hole Activity

Black hole accretion disc and its associated jets form a coupled system, which is thought to scale globally across the entire black hole mass range - from the stellar mass X-ray Binaries to the supermassive Active Galactic Nuclei. Using a sample of 39 low-luminosity AGN selected from the Palomar Spectroscopic Survey and the 4 best-studied stellar mass X-ray binaries in the low/hard state, we report the discovery of a fundamental plane of black hole activity in the optical band, with the nuclear [OIII] emission line luminosity as a tracer of accretion rate.

We show that the fundamental plane can be used to provide insights on the underlying distributions of relativistic jet parameters (eg. opening angles, Lorentz factor distribution) in blazars. We also study a large sample of 10149 AGN on the optical fundamental plane, obtained by cross-correlating the AGN samples in SDSS survey and 1.4 GHz VLA FIRST catalogue and show that 1.4 GHz FIRST fluxes do not trace pure AGN nuclear activities, and is rather heavily contaminated by environmental and other non-nuclear factors.

Deanne Coppejans
  Nov. 8

Adrian Hamers
    IAS; Princeton
    Hot Jupiters Driven by High-eccentricity Migration in Globular Clusters

Hot Jupiters (HJs) are short-period giant planets that are observed around ~1% of solar-type field stars. One possible formation scenario for HJs is high-eccentricity (high-e) migration, in which the planet forms at much larger radii, is excited to high eccentricity by some mechanism, and migrates to its current orbit due to tidal dissipation occurring near periapsis. We consider high-e migration in dense stellar systems such as the cores of globular clusters (GCs), in which encounters with passing stars can excite planets to the high eccentricities needed to initiate migration. We study this process via Monte-Carlo simulations of encounters with a star+planet system including the effects of tidal dissipation, using an efficient regularized restricted three-body code. HJs are produced in our simulations over a significant range of the stellar number density n_*. Assuming the planet is initially on a low-eccentricity orbit with semimajor axis 1 AU, for n_* < 1e3 pc^{-3} the encounter rate is too low to induce orbital migration, whereas for n_* > 1e6 pc^{-3} HJ formation is suppressed because the planet is more likely ejected from its host star, tidally disrupted, or transferred to a perturbing star. The fraction of planets that are converted to HJs peaks at ≈2% for intermediate number densities of ≈4e4 pc^{-3}. Warm Jupiters, giant planets with periods between 10 and 100 days, are produced in our simulations with an efficiency of up to ≈0.5%. Our results suggest that HJs can form through high-e migration induced by stellar encounters in the centers of of dense GCs, but not in their outskirts where the densities are lower.

Fabio Antonini
  Nov. 21

Benjamin Oppenheimer
    University of Colorado - Boulder
    The Circumgalactic Medium in EAGLE Simulations: Mass and Dynamics

I will discuss EAGLE zoom simulations that I have worked on with Alex Richings that incorporate non-equilibrium ionization and cooling. In this talk, I will focus on mass budgets: cool gas traced by HI and hot gas traced in the X-ray, as well as the dynamical state of the circumgalactic medium and whether halo gas can be described as being in hydrostatic equilibrium.

Alex Richings
  Nov. 30

Ben Ryan
    University of Illinois
    General Relativistic Radiation Magnetohydrodynamic Simulations of Slowly Accreting
    Black Holes

Disk accretion onto black holes at very low mass accretion rates is geometrically thick, optically thin, and radiatively inefficient. At near-Eddington rates, accretion is geometrically thin, optically thick, and radiatively efficient. These inconsistencies must be matched across an intermediate regime in which the thermal state is set by the balance of advection due to MHD turbulence, viscous dissipation, Coulomb coupling of electrons and ions, and radiative cooling. In particular, Compton scattering globally couples the disk thermodynamics through the radiative transfer equation. Such intermediate accretion disks are probably broadly represented in both low-luminosity active galactic nuclei and the low-hard state of X-ray binaries. To simulate these systems, I have developed a numerical method for full transport frequency-dependent general relativistic radiation magnetohydrodynamics, bhlight. I will begin with a review of the successes of previous nonradiative GRMHD simulations applied to accreting black holes. I will then present our recent work on intermediate disks, emphasizing electron thermodynamics and spectral properties. I will discuss our current work on M87, especially in the context of the upcoming Event Horizon Telescope. Finally, I will discuss possible future directions.

Sasha Tchekhovskoy
  Dec. 19

Drummond Fielding
    University of California - Berkeley
    A Multi-scale Portrait of the Circumgalactic Medium

The flow of gas through the circumgalactic medium (CGM) regulates galaxy growth over cosmic time. Observations have recently revealed a complex multi-phase structure in the CGM that has challenged many of the established theories and highlights significant gaps in our understanding of this critical aspect of galaxy formation. The spatial scales relevant to the CGM span a huge range with its structure and evolution determined by small-scale processes—such as the launching of galactic winds by clustered supernovae and thermal instability in the hydrostatic halo—and large-scale processes—such as cosmological accretion. I will describe my efforts to understand the details and interplay of these multi-scale processes in order to develop a coherent picture of the CGM that is consistent with observations.

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Past CIERA Special Seminars