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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
   Ultra-precise Spectrographs that Operate at the Diffraction Limit

The Doppler radial velocity (RV) method continues to inform our understanding of extrasolar planets: their formation and evolution, orbital architectures, masses and composition, and demographics. Although much progress has been made in generating precise RV time series measurements, basic physics considerations related to the way that spectrographs are designed and built limit the utility of Doppler observations much below one meter per second. As a result, effects involving stability, image quality and spectral resolution, and consequently the handling of stellar activity, currently preclude the study of Earth-mass analogues orbiting Sun-like stars. In this talk, I will describe a new type of spectrograph that uses “extreme” adaptive optics to inject starlight directly into single mode fibers. By correcting for the image-blurring effects introduced by Earth’s turbulent atmosphere, I will argue that a diffraction-limited instrument can address outstanding questions in exoplanetary science by generating unprecedented RV precision. We are constructing the first-such spectrograph of this kind for the Large Binocular Telescope in Arizona. The instrument, named “iLocater,” will benefit from input images that achieve ~20 times higher spatial resolution than seeing-limited designs, enabling high spectral resolution (R=200,000) observations using an ultra-stable, compact optical design at low cost. This capability shows promise to open new vistas of exploration in the study of extrasolar planets and stellar astronomy.

Ben Nelson

  May 15

Jason Tumlinson
   Johns Hopkins University
   The Circumgalactic Medium: What It Is, and Why It Matters

The “Circumgalactic Medium” is the vast reservoir of diffuse gas surrounding galaxies that acts as their fuel supply, waste dump, and recycling center. Thanks to recent revolutions in observing and theory we now appreciate that the CGM has a vital role in galaxy formation, but there are many open questions. I will talk about how we got to this point in our understanding of the CGM, draw out the major open questions, and point to some possible routes through new instruments and new simulations to answering these questions.

Jonathan Stern

  May 22

Charlie Conroy
   Harvard University
   Unraveling Galaxy Formation Histories with Semi-Resolved Stellar Populations

Description: Unraveling Galaxy Formation Histories with Semi-Resolved Stellar Populations Charlie Conroy (Harvard University) The analysis of stellar populations has traditionally been pursued in two limiting cases: spatially-resolved stellar populations in the color-magnitude diagram, and integrated light observations of distant systems. In between these two extremes lies a rich and relatively unexplored realm of observational phenomena. In this talk I will describe our efforts to develop techniques and analyze datasets exploiting the “semi-resolved” stellar population regime, both spatially and temporally. We have used these techniques to unravel the stellar populations of nearby galaxies, which has offered new insights into their formation histories. These new tools will play an important role in maximizing the science returns from next generation ground and space-based facilities.

Cliff Johnson

  May 29

Ramesh Narayan
   Harvard University
   Numerical Simulations of Black Hole Accretion

Accreting black holes are observed in a large variety of systems in astronomy: active galactic nuclei, X-ray binaries, tidal disruption events, gamma-ray bursts. While analytical one-dimensional models have been enormously useful for understanding several aspects of accretion physics, some aspects such as the formation of jets and winds are beyond the scope of these models. Numerical general relativistic MHD simulations have filled the gap, and are being used to explore the multidimensional gas dynamics and radiative properties of accretion flows. The talk will review some recent progress in this field.

Sasha Tchekhovskoy

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Past Astrophysics Seminars