Research Events
For the Public
Northwestern University

Weekly Astrophysics Seminars 2014-2015

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 2015

  Date      Speaker / Seminar Host
  Jan. 6

Doug Watson
    University of Chicago
    Modeling Galaxy Star Formation Across Cosmic Time

How galaxies form and evolve remains one of the most profound problems in astrophysics today. The backbone of all theories of galaxy formation is the favored Cold Dark Matter (CDM) model for structure growth. Galaxies are believed to be tracers of the CDM cosmic web and are inextricably tied to the evolution and properties of bound regions of dark matter, so-called dark matter halos. We are in a particularly exciting era for understanding galaxy formation thanks to precision dark matter N-body simulations coupled with a rapid influx of exquisite measurements from galaxy redshift surveys out to high redshift. This allows, for the first time, the exploration of truly high-precision models of galaxy formation out to z~3.
In this talk, I will focus on the following topics. (1) The empirical “age matching” model that is predicated on the co-evolution of galaxies and halos. This simple formalism has been shown to be remarkably powerful, yielding accurate predictions galaxy color- and SFR- dependent clustering and galaxy-galaxy lensing, as well as a variety of galaxy group-based statistics. (2) New observational statistics that have the power to discriminate between competing galaxy formation models. (3) Future prospects for self-consistent, precision modeling across most of cosmic time.

  Jan. 20

Mariska Kriek
   UC Berkeley
    Reconstructing the Formation Histories of Massive Galaxies

In past years, large and deep photometric and spectroscopic surveys have significantly advanced our understanding of galaxy growth, from the most active time in the universe (z~2) to the present day. In particular, the evolution in stellar mass, star formation rate, and structure of complete galaxy samples have provided independent and complementary insights into their formation histories. In addition, detailed studies of the properties of distant galaxies have lead to a better apprehension of the physical processes which govern galaxy growth. Nonetheless, many outstanding questions remain. In this talk I will give an overview of our current picture of galaxy growth in the past 11 billion years, discuss current challenges and outstanding questions, and introduce new and ongoing efforts to further unravel the formation histories of massive galaxies.


  Feb. 3

Joshua Peek
   Columbia University
    The Diffuse Universe: Catalogs, Continuum, Combination

Astronomy has always been a science of catalogs: catalogs of stars, galaxies, quasars, and planets. While most of the light in the Universe comes from these dense objects in the darkness, the contents of the universe are largely diffuse. Dark energy, dark matter, plasma, and gas make up 99.8% of the mass-energy budget of the Universe and cannot be easily cataloged. If we want to understand how the objects in the universe came to be, we must appeal to the largely invisible diffuse phase that formed them. I will try to make sense of this conundrum in three ways. The first is to use the cataloged objects to discern things about the intervening gas: through dust reddening we can measure the location and density of metal enriched gas in the Galaxy and beyond. The second is to examine the velocity-resolved emission from the gas. To do this we invent statistically robust ways to measure the shape structure of the gas, where much of the information about the diffuse phases hides. The third is to combine the two, catalogs and continuum, to delve into the 4D structure of our Galaxy. I will discuss a method in development, Kinetic Tomography, which will hopefully help us understand how the Galaxy came to be the way it is today.

Giles Novak

  Feb. 17

Lucianne Walkowicz
   Adler Planetarium and LSST

Francesca Valsecchi

  Feb. 24

Sarah Gallagher
   University of Western Ontario
    Winds, Winds Every Where: Radiatively Driven Outflows from Supermassive Black Holes?

Supermassive black holes reside in the centers of every massive galaxy. In relatively brief spurts, black holes grow as luminous quasars through the infall of material through an accretion disk. Remarkably, the light from the accretion disk can outshine all of the stars in the host galaxy by a factor of a thousand, and this radiation can also drive energetic mass outflows. Mass ejection in the form of winds or jets appears to be as fundamental to quasar activity as accretion, and can be directly observed in many objects with broadened and blue-shifted UV emission and absorption features. A convincing argument for radiation pressure driving this ionized outflow can be made within the dust sublimation radius. Beyond, radiation pressure is still important, but high energy photons from the central engine can now push on dust grains. This physics underlies the dusty wind picture for the putative obscuring torus. I'll describe our model of the dusty wind and evaluate its success in accounting for observed properties of quasars such their mid-infrared spectral energy distributions, fractions of hidden objects, and column densities of important ions.

John Everett

  March 3

Mark Reid
   Harvard CfA
    Measuring the Milky Way: the BeSSeL Survey

Over 2000 years ago, Hipparcus measured the distance to the Moon by triangulation from two locations across the Mediterranean Sea. However, determining distances to stars proved much more difficult. Many of the best scientists of the 16th through 18th centuries attempted to measure stellar parallax, not only to determine the scale of the cosmos but also to test Heliocentric cosmologies. While these efforts failed, along the way they lead to many discoveries, including atmospheric refraction, precession, and aberration of light. It was not until the 19th century that Bessel measured the first stellar parallax.
Distance measurement in astronomy remained a difficult problem even into the early 20th century, when the nature of galaxies ("spiral nebulae") was still debated. While we now know the distances of galaxies at the edge of the Universe, we have only just begun to measure distances accurately throughout the Milky Way. Using the Very Long Baseline Array, we now can achieve positional accuracy approaching 10 micro-arcseconds! I will present new results on parallaxes and motions of star forming regions. These measurements address the nature of the spiral structure, size, rotation speed, and mass of the Milky Way.

Farhad Zadeh


Spring Quarter 2015

  Date      Speaker / Seminar Host
  April 7

Dawn Erb
   University of Wisconsin, Milwaukee


  April 21

Geoffrey Vasil
   The University of Sydney


  May 19

Edwin Bergin
   University of Michigan
    Dissecting the Explosions and Environments of Nearby Supernovae

Evidence has mounted that Type Ia and core-collapse (CC) supernovae (SNe) can have substantial deviations from spherical symmetry; one such piece of evidence is the complex morphologies of supernova remnants (SNRs). In this talk, I will highlight techniques we have developed to ascertain the role of explosion geometry and environment in shaping these SNRs. I will discuss application of these methods to extensive multiwavelength data of Milky Way and Magellanic Cloud SNRs, and I will explore the implications regarding the nature of the progenitors and the dynamical evolution of these sources.

Farhad Zadeh

  May 26

Tom Abel
   Stanford University
    Dark Matter Dynamics

Computational Physics allows us to study extremely non-linear systems with fidelity. In astrophysical hydrodynamics and studies of galaxy formation much of the last two decades we have explored various discretization techniques and found subtle differences in some applications. Interestingly numerical studies of collisionless fluids such as e.g. the collapse of cold dark matter to form the large scale structure of the Universe has only been studied meaningfully with one approach; N-body Monte Carlo techniques. I will introduce a novel simulation approach, and demonstrate its feasibility, that for the first time can study a collisionless system in the continuum limit in multi-dimensions. I will also show this new technique opens a new window in making sense of structure formation as well as plasma physics. In this context we have developed a novel rasterization/voxelization algorithm applicable in computational geometry, computational physics, CAD design and other fields. I show how these approaches allow also for much improved predictions for gravitational lensing, dark matter annihilation, properties of cosmic velocity fields , and many other applications.

Fred Rasio

Fall Quarter 2015

  Date      Speaker / Seminar Host
  Oct. 20

Laura Chomiuk
   Michigan State University

Fred Rasio

For more information, contact: Janet Howe (janet.howe@northwestern.edu)

Past Astrophysics Seminars