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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


Spring Quarter 2017

  Date      Speaker / Seminar Host
  Mar. 28

Amy Bender
   KICP; Argonne National Laboratory
   The South Pole Telescope: Searching for Cosmological Answers with the Cosmic
   Microwave Background

Observations of the Cosmic Microwave Background (CMB) are a cornerstone of our current cosmological model. The CMB contains the imprint of signals from both early and late times in the evolution of the universe, enabling a diverse set of fundamental probes. CMB experiments are currently focused on measuring the B-mode polarization signature, as it has the potential to constrain inflationary gravitational waves as well as the effect of massive neutrinos on structure formation. The South Pole Telescope (SPT) is a large-aperture millimeter-wavelength telescope that has observed the CMB for the past decade. In this talk, I will highlight recent results from the SPT surveys including measurements of the CMB polarization. Over the past three months, the SPT was upgraded with a new receiver. I will discuss some of the technological advances implemented in this receiver and how it will open a new regime in multiband polarized observations of the CMB.

Deanne Coppejans

  Apr. 4

Norman Murray
   On the Origin of Turbulence in Giant Molecular Clouds in the Milky Way

The first detection of CO line emission in space by Wilson, Jefferts, and Penzias revealed a line width of 6.2 km/s, far broader than the thermal line width. A decade later, Larson noticed that the line width varied with the size of the emitting region, interpreting his result in the context of a turbulent cascade. This is still the accepted interpretation, which raises a puzzle. Turbulence is believed to decay on an eddy turnover time, about 5-10 Myrs in Milky Way GMCs, so that a source of energy to power the steady state turbulence would seem to be required. Two types of energy sources have been proposed, gravitational potential energy, or nuclear energy from stars. Gravitational sources can be further broken down to accretion of gas from the Milky Way's halo onto the gas disk, accretion through the gas disk (triggered either by the magneto-rotational stability or by gravitational instability), accretion of atomic gas onto GMCs, contraction of GMCs, or proto-stellar jets and their associated outflows. Nuclear energy can be expressed in the form of O star winds, ionizing radiation, radiation pressure, or supernovae.

Using a new GMC catalog (Mivilles-Deschenes, Murray, and Lee 2016), we show that in the outer disk (beyond the solar circle) the turbulence in CO emitting gas can be powered by accretion through the disk, or by accretion onto GMCs. But the dissipation rate in inner Galaxy GMCs is factor of 100 larger than that in the outer disk. We show that most of the GMCs lack sufficient young stellar mass to provide this much energy, and that the accretion rate through the disk or onto the GMCs cannot supply enough power either. It appears that the turbulence is powered by contraction of the GMCs.

Alex Richings

  Apr. 11

Jo Bovy
   University of Toronto
   Stellar Streams and the Milky-Way’s Dark-Matter Halo

Stellar tidal streams originating from disrupting globular clusters in the Milky Way’s halo hold enormous promise as probes of both the large-scale structure of the Milky Way halo’s density distribution and its small-scale structure. As such, the observed density, spatial, and kinematic structure of stellar streams can provide important new constraints on the interactions and small-scale structure of dark matter. I will discuss the simple gravitational dynamics of tidal-stream formation and evolution and how we can use it to build simple and fast models for tidal streams. I will show some examples of this machinery in fitting observed streams and what it tells us about the shape of the Milky Way’s halo. I will further present a fast perturbation theory for computing the effects of impacts between a stream and many small dark-matter subhalos and its application to existing and future data sets.

Daniel Angles-Alcazar

  Apr. 25

Nicholas Stone
   Columbia Astrophysics Laboratory
   Exotic Astrophysical Channels for Black Hole Binary Formation

The detection of gravitational waves (GWs) by Advanced LIGO opens the door on a new era of physical and astronomical discovery. A key astronomical question that can be answered by GW observations is the following: what process produces the majority of stellar mass black hole (BH) binaries in the universe? The two traditionally considered processes have been evolution of isolated field binaries and dynamical assembly of binary systems in globular clusters. However, the unexpected properties of GW150914 should motivate a careful consideration of alternative scenarios. I will discuss two novel scenarios for binary BH assembly recently proposed by myself and collaborators. First, wide binary BHs embedded in the accretion disks of active galactic nuclei (AGN) can be driven to merger through torques from circumbinary mini-disks. Pre-existing BH binaries can be captured into AGN disks dissipatively, or they may form in situ in Toomre-unstable regions of an AGN disk. While BH mergers in these gas-rich environments raise the tantalizing possibility of electromagnetic counterparts, strong AGN are sufficiently rare that the modest localization capabilities of GW detectors alone may be able to confirm or falsify the importance of this scenario with a sufficiently large BH binary sample. Next, I will present work from an ongoing series of papers investigating the dynamical assembly of triple star systems due to binary-binary scattering in globular clusters. The inner binary of a triple system may be driven to merger by the Kozai-Lidov effect, and in certain circumstances can reach the LIGO band with nontrivial eccentricity. The rate of such mergers depends on the outcomes of the chaotic non-hierarchical four-body problem, and we have developed a statistical mechanics formalism to describe the distributions of these outcomes.

Fabio Antonini

  May 9

Desika Narayanan
   Haverford College
   Making Galaxy Formation Great Again

Our understanding of galaxy formation has gotten weak. Very very weak. We have many more problems than we used to, and we can thank the advent of new telescopes for that. Lots and lots of problems. So many problems. For some of these problems, only an understanding of interstellar medium physics can solve them. Believe me, these are bigly problems. But the physics of the ISM can save us. If you come to this talk, you're going to see a magnificent victory over these problems. A historic victory. You've never seen a victory over galaxy formation problems like this folks. Believe me. Any solutions to the problems that we'll describe -- ranging from dust attenuation in starbursts to carbon emission from the EoR -- that don't involve ISM physics are fake news. The interplay between ISM physics and galaxy formation in this talk will be magnificent. Believe me.

Alex Richings

  May 23

Nanda Rea
   Institute of Space Sciences (CSIC-IEEC), Barcelona
   API Astronomical Institute, University of Amsterdam

Raffaella Margutti

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Past Astrophysics Seminars