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

Weekly Astrophysics Seminars 2017-2018

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


Spring Quarter 2018

  Date      Speaker / Seminar Host
  Apr. 3

Shami Chatterjee
   Cornell University
   The Deepening Mystery of Fast Radio Bursts

Fast radio bursts, dispersed millisecond flashes of radio waves originating from beyond the Milky Way, are an enigmatic phenomenon, with more competing models to describe them than there are burst detections. With the recent localization of the repeating FRB 121102 and the measurement of a redshift z = 0.193 for its host galaxy, we have confirmed the cosmological origin of FRBs. The precise localization of a repeating FRB source has also enabled observations over a broad spectral range, revealing intriguing new clues about the central engine. The detection of a rotation measure exceeding 100,000 rad/m^2 reveals its extreme magneto-ionic environment, and dramatic structure in the burst dynamic spectra point to the role of propagation effects in the intergalactic and interstellar media. Understanding the central engine of FRBs as a class and unlocking their future use as cosmological probes will require a larger sample of FRBs, the discovery of other repeating sources, and the identification of more host galaxies.

Deanne Coppejans

  Apr. 17

Vikram Dwarkadas
   University of Chicago
   Triggered Star Formation inside the Shell of a Wolf–Rayet Bubble as the Origin of the Solar System

A critical constraint on solar system formation is the high 26Al/27Al abundance ratio of 5e-5 at the time of formation, which was about 17 times higher than the average Galactic ratio, while the 60Fe/56Fe value was about 2e-8, lower than the Galactic value of 3e-7. This challenges the assumption that a nearby supernova was responsible for the injection of these short-lived radionuclides into the early solar system. We show that this conundrum can be resolved if the Solar System was formed by triggered star formation at the edge of a Wolf-Rayet (W-R) bubble. Aluminium-26 is produced during the evolution of the massive star, released in the wind during the W-R phase, and condenses into dust grains that are seen around W-R stars. The dust grains can survive passage through the reverse shock and the low density shocked wind, reach the dense shell swept-up by the bubble, detach from the decelerated wind and are injected into the shell. The grains are destroyed releasing the 26Al into the dense shell. Some portions of this shell subsequently collapses to form the dense cores that give rise to solar-type systems. The subsequent aspherical supernova, if it does indeed occur, would not be expected to inject appreciable amounts of 60Fe into the proto-solar-system, thus accounting for the observed low abundance of 60Fe. We discuss the details of various processes within the model using numerical simulations, nucleosynthesis modelling, and analytic and semi-analytic calculations. We conclude that it is a viable model for solar system formation, that can explain the initial abundances of 26Al and 60Fe, as well as other isotopes such as 41Ca.

Raffaella Margutti

  Apr. 24

Suvi Gezari
   University of Maryland
   Hungry Black Holes

We are entering an era of increasingly powerful wide-field optical imaging surveys that are transforming the study of the variable night sky. I will highlight the capability of time domain observations to probe supermassive black holes (SMBHs) lurking in the centers of galaxies by catching them light up in the act of feeding on stars and gas. Watching how these “hungry” black holes digest their meals can reveal their mass, spin, and binarity; the fundamental parameters of an astrophysical black hole. I will present results from our systematic studies of nuclear transients with the Pan-STARRS1, iPTF, and ZTF surveys, and conclude with a discussion of the exciting potential of the LSST to map the demographics of SMBHs over cosmic time.

Giacomo Terreran

  May 1

Rachel Somerville
   Rutgers University
   Momentum-driven winds from radiatively efficient black hole accretion and their impact on
   galaxy properties

Accretion onto supermassive black holes in galactic nuclei can produce energetic radiation and jets. The impact of these physical processes (collectively referred to as "AGN Feedback") on galaxies and the circum- and intergalactic gas is a central topic of investigation in galaxy formation and cosmology. Many state-of-the-art cosmological simulations of galaxy formation and evolution have focussed on the "jet mode" of AGN feedback, and have largely neglected the momentum that may be imparted to gas via radiation pressure on gas and dust. Observations indicate that most or perhaps even all active galaxies may contain high-velocity outflows, but there is an ongoing debate about the mass-loading factors and overall impact of these outflows on large scales. In this talk I will present results from a recent suite of high-resolution "zoom in" cosmological hydrodynamic simulations in which we model both thermal and momentum feedback from radiatively efficient accretion onto supermassive black holes in massive galaxies. We carry out an analysis of gas and metal inflows and outflows in these simulations, and in a matched suite that is identical except that they do not include black holes and AGN feedback. I will discuss the properties of the winds driven by the combined effects of stars and black holes, and how they differ in the full and no-AGN simulation suites. In addition I will discuss the impact of these winds on observable properties of galaxies and the circumgalactic medium.

Sarah Wellons

  May 8

Justin Crepp
   University of Notre Dame

Ben Nelson

  May 15

Jason Tumlinson
   Johns Hopkins University

Jonathan Stern

  May 22

Charlie Conroy
   Harvard University

Cliff Johnson

  May 29

Ramesh Narayan
   Harvard University

Sasha Tchekhovskoy

For more information, contact:

Past Astrophysics Seminars