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Northwestern University

Theory Group Meetings 2015-2016

Theory Group Meetings are held at 12:30 PM in Tech F160 (2145 Sheridan Rd) unless otherwise noted

Theory Group Meeting is the informal weekly discussion group of theoretical astrophysicists at CIERA. Discussions usually center around blackboard talks given by graduate students and postdocs at Northwestern, but can include prepared talks given external speakers, as well. Whatever slides or material the speaker brings in, Theory Group Meeting often becomes a quite detailed discussion of the fundamental theoretical astrophysics topic being addressed. Also, while topics under discussion often relate to theory, observational astrophysicists are definitely welcome!

Spring Quarter 2016

  Date Speaker / Seminar
  Apr. 8

Daniel Angles-Alcazar
   The Baryon Cycle in Galaxy Evolution


Michael Katz
   Study of Near-Band Edge Sources for the LISA Detector


Brandon Miller
   Using Single-spin Templates for GW Parameter Estimation

  Apr. 15

Cliff Johnson
   University of California - San Diego
   Insights on Star Cluster Formation from M31


Laura Fissel
   The Role of Magnetic Fields in Star Formation

  Apr. 22

Mads Sørenson
   Geneva Observatory of the University of Geneva
   The Evolution of Black Hole X-ray Binaries


Sourav Chatterjee
   Dynamical Effects of Black Holes on Star Clusters

  Apr. 29

Jason Hwang
   Creating Kepler-36 Through a Hit and Run


Michael Zevin
   Machine Learning Applications in LIGO Detector Characterization and Binary Evolution

  May 13

Fabio Antonini
   Slow Mass Segregation and the Distribution of Stellar Remnants at the Galactic Center


Fani Dosopoulou
   Dynamical Friction in Eccentric Massive Black Hole Binaries

  May 20

Ben Nelson
   Push it to the (Roche) Limit: Inferring Multiple Populations of Hot Jupiters


Katie Breivik
   Distinguishing Binary Black Hole Formation Channels with (e)LISA

  May 27

Jeff Andrews
   Crete Astrophysics Group, Greece
   Beyond Population Synthesis: Markov-Chain Monte Carlo Models of High Mass X-ray Binaries


Niharika Sravan
   Massive Star Winds and Implications for Binary Evolution

Winter Quarter 2016

  Date Speaker / Seminar
  Jan. 15

Michael Tremmel
   University of Wisconsin
   A New Hope: Improving on Formation, Dynamics and Accretion Models of SMBHs in Cosmological Simulations

Supermassive black hole physics is a crucial aspect of galaxy evolution theory, but modeling their dynamics and accretion also present a unique challenge for large-scale cosmological simulations. I present a new, physically motivated sub-grid model for SMBH formation, dynamics, and accretion in large-scale cosmological simulations that better accounts for the local (resolved) environment . I will highlight how each aspect of our approach leads to testable outcomes compared to common SMBHs prescriptions. Finally, I will present a first application of this scheme studying the properties and time evolution of dual, active SMBHs.

  Jan. 22

Betsy Mills
   Do star formation laws break in the center of the Galaxy?

I will review our understanding of molecular gas conditions in the central 500 parsecs of the Milky Way, and summarize recent studies that find that the Galactic center deviates from universal star formation relations. It is suggested that the amount of star formation in the Galactic center is less than expected, given the quantity of dense gas in this region. However, in order to conclude that the Galactic center truly breaks these ‘laws' of star formation, two possibilities must be ruled out: that our indicators in this region could underestimate the amount of star formation, and that prior observations could have overestimated the amount of dense gas. I will analyze new evidence for ongoing star formation in the Galactic center and present new measurements of the gas densities in the Galactic center that show it to be less dense than originally thought. However, I will ultimately argue that the average density of the gas is less relevant to explaining the dearth of star formation than the fraction of gas at each density.

  Jan. 29
Alex Richings
    Non-equilibrium Chemistry in Simulations of Galaxy Formation
  Feb. 5
Sam Hadden
    Tutorial on REBOUND, an Nbody program
  Feb. 19
Chris Pankow
    Implications from the discovery of GW150914, a heavy binary black hole binary
  Feb. 26
Laura Sampson
    Testing General Relativity with GW150914
  Mar. 11

Manos Chatzopoulos
   University of Chicago
   A Decade of Super-Luminous Supernova Discoveries: The Physics of Extreme Stellar Catastrophes

Fall Quarter 2015

  Date Speaker / Seminar
  Oct. 9

Laura Sampson
    Astrophysical Inference with Pulsar Timing Arrays

Jason Hwang
    SPH Simulations of Planet—Planet Collisions

  Oct. 16
Chris Pankow
    Bayesian Parameter Estimation for Binary Neutron Stars in the Advanced LIGO Era
  Oct. 23
Ben Nelson
     Statistical and Dynamical Remastering of Classic Exoplanet Systems
  Oct. 30

Blake Sherwin
   UC Berkeley
   Mapping Mass Across the Universe: CMB Lensing Measurements Past and Future

Measurements of gravitational lensing in the CMB directly probe the projected distribution of mass out to high redshifts, and thus encode a wealth of information about cosmology. In my talk, I will explain how CMB lensing measurements will allow us to precisely determine the masses of neutrinos, probe the relation between dark and luminous matter at high redshifts, and constrain the properties of inflation and the early universe. After reviewing first measurements of CMB lensing, I will discuss new studies of the lensing effect with CMB data from the ACTPol experiment. I will show the remarkable precision achievable by CMB lensing surveys over just the next five years and explain the great scientific returns and challenges we can expect from these surveys.

  Nov. 6

John Chisholm
   UW Madison
   Shining a Light on Star Formation Driven Outflows: the Physical Conditions within Galactic Outflows

Stellar feedback drives energy and momentum into the surrounding gas, which drives gas and metals out of galaxies through a galactic outflow. Unfortunately, galactic outflows are difficult to observe and characterize because they are extremely diffuse, and contain gas at many different temperatures. Here we present results from a sample of 37 nearby (z < 0.27) star forming galaxies observed in the ultraviolet with the Cosmic Origins Spectrograph on the Hubble Space Telescope. The sample covers over three decades in stellar mass and star formation rate, probing different morphologies such as dwarf irregulars and high-mass merging systems. Using four different UV absorption lines (O I, Si II, Si III and Si IV) that trace a wide range of temperatures (ionization potentials between 13.6eV and 45eV), we find shallow correlations between the outflow velocity or the equivalent width of absorption lines with stellar mass or star formation rate. Absorption lines probing different temperature phases have similar centroid velocities and line widths, indicating that they are comoving. Using the equivalent width ratios of the four different transitions, we find the ratios to be consistent with photo-ionized outflows, with moderately strong ionization parameters. By constraining the ionization mechanism we model the ionization fractions for each transition, but find the ionization fractions depend crucially on input model parameters.


Daniel Angles-Alcazar
    Tracing Cosmic Inflows, Galactic Outflows, and Gas Recycling in Realistic Environments

  Nov. 13
Greg Sloan
   Carbon-rich Interstellar Dust from Fullerenes to Aliphatics

Josh Fixelle
    Resolving the Complexities of Hot Jupiter Mass Transfer via Roche Lobe Overflow

  Nov. 20

Shuo Zhang
   Hard X-ray Outburst History of Sgr A*

The Galactic center supermassive black hole (SMBH) Sagittarius A* (Sgr A*) is remarkably underluminous with a bolometric luminosity about 10^-9 times its Eddington luminosity. It is the closest SMBH and thus an ideal target for investigation of galactic nuclei and their activity cycles. This goal can be fulfilled by studying its past, current and possible future outbursts. Its current X-ray quiescent state, with a luminosity of Lx ~ 10^33 erg/s, is punctuated by flares up to a few times 10^35 erg/s, whose origin is poorly understood. I collected nine hard X-ray flares detected up to 79 keV, and studied their timing behavior using the Bayesian block analysis. The broadband 3-79 keV spectroscopic studies show that the X-ray flares can have a range of photon indexes, which can be explained by the magnetic reconnection scenario. During the flares, Sgr A* is still orders of magnitudes lower than its Eddington luminosity. Whether it has ever experienced more substantial increases in activity as observed in low-luminosity Active Galactic Nuclei is still under discussion. Indication of such past activity of Sgr A* has come from the Galactic center molecular clouds (GCMCs). I use the hard X-ray emission from GCMCs to constrain the past Sgr A* X-ray outburst, resulting in Lx~5x10^38 erg/s with a photon index of ~2.2. I also discovered different timing variability from different cloud substructures, which can further constrain the past outburst. Future Sgr A* flaring activity could be affected by the G2 infall event. There has been evidence that G2 could cause increased Sgr A* flaring activities. Ongoing multi-wavelength monitoring of Sgr A* will address this remaining puzzle.

Katerina Chatziioannou
   Montana State University, Bozeman
   Studying the Universe using Gravitational Waves from Compact Binaries

The second generation era of ground-based gravitational wave detectors has just begun. Years of instrument upgrades have led to unprecedented sensitivity and the first direct detection of gravitational waves should be close. Compact binaries of neutron stars and/or black holes hold a prominent role among the possible sources of these gravitational waves. Not only are they one of the most common and well understood sources, but they also carry information that could answer a plethora of questions ranging from the validity of general relativity to the equation of state of the supra-nuclear matter encountered in neutron star interiors. In this talk, I will show how gravitational waves can be used to study the spin and mass distribution of stellar remnants and probe the internal composition of neutron stars. I will describe the strong spin interactions compact binaries exhibit, the rich information these interactions carry, and how they can enhance our understanding of the Universe.

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Past Theory Group Meetings