Join Our E-Mail List
Research Events
For the Public
Northwestern University

Weekly Astrophysics Seminars 2016-2017

Seminars are held at 4:00 PM on Tuesdays in Room F160
on the first floor of the Technological Institute (2145 Sheridan Road) unless otherwise noted.


Winter Quarter 2017

  Date      Speaker / Seminar Host
  Jan. 10

Robert De Rosa
   University of California - Berkeley
   The Gemini Planet Imager Exoplanet Survey

Measuring the frequency and distribution of extrasolar planetary systems provides crucial context to understand the formation of our own solar system. While ongoing radial velocity and transit surveys are beginning to have sensitivity to terrestrial planets at separations similar to those within our own solar system, wide orbit Jovian-mass planets remain out of reach. Direct imaging provides a complementary detection technique, being sensitive to the thermal emission from young gas giants in wide (>10 au) orbits around nearby (<100 pc) stars. Using a high-order adaptive optics system, an apodized pupil coronagraph, and an integral field spectrograph, the recently commissioned Gemini Planet Imager (GPI) was designed to directly image such planetary systems. Our team is currently undertaking an 890-hour campaign — the Gemini Planet Imager Exoplanet Survey (GPIES) — to search for planetary-mass companions in wide orbits (4-40 au) around a sample of 600 nearby, young (<300 Myrs) stars. I will discuss the current status of the survey, which began in late 2014, and highlight several of the key science results that I have been involved with, most notably the discovery of the young, cool exoplanet 51 Eridani b.

Mel Ulmer

  Jan. 17

Ashley Zauderer
    Harvard University
    Tidal Disruption Events: Observational Signatures of Quiescent Black Holes

I will present a brief overview of theoretical predictions for observational signatures of the tidal disruption of stars by supermassive black holes. In 2011, the Swift satellite observed a source, J1644+57, which arguably is the most clear-cut case for a tidal disruption event. This event is unique because it deviates from the simple predictions: strong radio emission was observed starting at early times, providing evidence for a collimated, mildly-relativistic jet. I will outline the utility of radio observations to monitor the expansion and energy scale of the relativistic outflow and to probe the parsec-scale environment around a supermassive black hole that may have been previously dormant. I will conclude with a summary of interpretations for this event and open theoretical questions.

Raffaella Margutti

  Jan. 24

Tyler Groff
    Goddard Space Flight Center
    Integral Field Spectroscopy for Exoplanet Imaging

Direct Imaging of exoplanets using a coronagraph has become a major field of research both on the ground and in space. Key to the science of direct imaging is the spectroscopic capabilities of the instrument and our ability to fit spectra and understand the composition of the observed planets. Direct imaging instruments tend to use an integral field spectrograph (IFS), which encodes the spectrum into a two-dimensional image on the detector. This results in more efficient detection and characterization of targets, and the spectral information is critical to achieving detection limits below the speckle floor of the imager. The most mature application of these techniques is at more modest contrast ratios on ground-based telescopes, achieving approximately 5-6 orders of magnitude suppression. In space, where we are attempting to detect Earth-analogs, the contrast requirements are more severe and the application of spectra less understood. I will discuss this evolution in exoplanet spectroscopy in the context of the The Coronagraphic High Angular Resolution Imaging Spectrograph (CHARIS) at the Subaru telescope and highlight how lessons learned are driving the development for the WFIRST integral field spectrograph, which is currently in Phase A of its design. CHARIS has been delivered to the observatory and now sits behind the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) and AO188 adaptive optics systems. CHARIS has a ‘high' and 'low' resolution operating modes. The high resolution mode is used to characterize targets in J, H, and K bands at ~R70. The ‘low-resolution’ prism is meant for discovery and spans J+H+K bands (1.15-2.37 microns) with a spectral resolution of ~R18. This discovery mode has already proven better than 15-sigma detections of HR8799c,d,e when combining ADI+SDI. Using SDI alone, planets c and d have been detected in a single 24 second image. The CHARIS team is optimizing instrument performance and refining ADI+SDI recombination to maximize our contrast detection limit. In addition to the new observing modes, CHARIS has demonstrated a design with high robustness to spectral crosstalk. CHARIS is in the final stages of commissioning, with the instrument open for science observations beginning February 2017. Here we review the science case, design, on-sky performance, and specific lessons learned for extremely high contrast imagers such as crosstalk optimization, wavefront correction using the IFS image, lenslet tolerancing, the required spectral resolution to fit exoplanet atmospheres, and the utility of the spectrum to both rule out false-positives and provide higher contrast detection limits.

Mel Ulmer

  Jan. 31

Smadar Naoz
   University of California - Los Angeles
   On the Dynamics of Planets and Also Stars and Black Holes - New Insights from Triples

Many observed triple systems in our Universe are in a hierarchical configuration: two objects orbit each other in a relatively tight inner binary while the third object is on a much wider orbit. Furthermore, the secular approximation for the evolution of hierarchical triple body systems has been proven to be very useful in many astrophysical contexts, from planetary to triple-star systems and even black holes. In this approximation the orbits may change shape and orientation, on timescales longer than the orbital periods, but the semi-major axes are constant. In early studies of hierarchical secular three-body systems (Kozai 1962; Lidov 1962), the wider orbit was set to be circular and one of the tight binary members was assumed to be a test (massless) particle. In this situation, the component of the tight orbit’s angular momentum along the total angular momentum is conserved, and the lowest order of the approximation (i.e., the quadrupole approximation) is valid. I will discussed recent developments that showed that considering systems beyond the test particle approximation, or circular orbits, requires the next level of approximation for a correct representation of the physics, called the octupole-level. This leads to qualitative different behavior of the system. In this case, the angular momenta component of the tight and wide orbits along the total angular momentum is not conserved. Most interestingly, at this level of approximation, for an eccentric wide orbit, the tight orbit can reach extremely high eccentricities and undergo chaotic flips of its orientation. This behavior has important implications to the evolution of many systems, and I will present some nominal examples, such as retrograde hot Jupiters, blue stragglers and low-mass X-ray binaries.

Fabio Antonini

  Feb. 7

Jessica Werk
   University of Washington
   Multiphase Gas Flows in Gaseous Galaxy Halos

The circumgalactic medium (CGM; non-ISM gas within a galaxy virial radius) regulates the gas flows that shape the assembly and evolution of galaxies. Only in the last several years, primarily because of vastly improved capabilities in space-based UV spectroscopy (HST/COS), observations and simulations of the CGM have emerged as the new frontier of galaxy evolution studies. My recent work suggests a rapid cycling of massive amounts of gas on scales of hundreds of kiloparsecs that in turn has raised pressing questions concerning the physical characteristics of the gas in the halos of galaxies. In this talk, I will discuss new constraints we have placed on the origin and fate of this material by studying the gas kinematics, metallicity and ionization state, and using hydrodynamical simulations in a cosmological context. I will end by describing a large survey, now underway, that will quantify the influence of assembly history on the fate of the galaxy and its gaseous halo for the first time.

Alex Richings

  Feb. 14

Dustin Lang
   University of Toronto
   DESI: the Next-Generation Galaxy Mapping Project

DESI, the Dark Energy Spectroscopic Instrument, is under construction and will measure the expansion history of the University using the Baryon Acoustic Oscillation technique. DESI will measure a total of 35 million galaxies (to redshift z=1.6) and quasars over 14,000 square degrees, with survey operations starting in 2019. There are also plans to survey millions of relatively bright galaxies and Milky Way stars during the bright time. I will present the amazing engineering challenges and science opportunities that DESI presents.

Ben Nelson

  Feb. 21

Nadia Zakamska
   John Hopkins University
   Observations of Quasar Feedback

Quasars are now thought to have made critical impact on galaxy formation. Feedback from accretion onto supermassive black holes is implicated in establishing the black hole mass vs galaxy bulge correlations and in limiting the maximal mass of galaxies. In this talk, I will review the indirect evidence for quasar feedback as required by galaxy formation models. I will then present recent multi-wavelength observations of powerful quasar-driven winds and outflows on galaxy-wide scales. These data may provide direct observational evidence for one of the long-standing paradigms in galaxy formation.

Daniel Angles Alcazar


Past Astrophysics Seminars