Talks & Events
Astronomy Colloquia: 2017
Mark Voit, Michigan State University
Feedback from a central supermassive black hole is an essential component of galaxy evolution models. Without it, those models cannot produce realistic massive galaxies and galaxy clusters. However, the black-hole feedback mechanism remains mysterious. Somehow, accretion of matter onto the central black hole of a massive galaxy becomes precisely tuned so that it regulates radiative cooling and condensation of gas in a volume many orders of magnitude larger than the black-hole's gravitational zone of influence. I will discuss how the required coupling can arise through condensation and precipitation of cold clouds out of a galaxy's circumgalactic medium, and will show how a feedback mechanism that suspends the circumgalactic medium in a marginally unstable state can regulate star formation within galaxies.
The Yin and Yang of Slowly-Pulsating B Stars: Asteroseismology and Angular Momentum Redistribution
Richard Townsend, University of Wisconsin-Madison
During their main-sequence evolution, almost all B-type stars will pass through a phase where they are unstable toward oscillation in one or more global internal gravity waves ('g modes'). The g modes, driven by iron and nickel opacity in the stars' outer envelopes, generate surface temperature and velocity changes with periodicities on the order of days.
In the 'Yin' part of my talk, I'll discuss how time-series spectroscopy and photometry of these `slowly-pulsating B' (SPB) stars can be leveraged into asteroseismology --- probing the stars' interiors by careful analysis of their oscillation spectra. I'll highlight in particular how the Kepler mission, together with the MESA stellar evolution code and my GYRE stellar oscillation code, has allowed novel constraints to be established on the internal rotation and mixing physics of SPB stars.
I'll then pivot to the 'Yang' part of my talk. Although we typically regard stellar oscillations as passive tracers of stellar structure, they can also modify this structure. I'll present recent work by my group exploring angular momentum redistribution by g modes. Modeling this process in SPB stars, we find that significant modification of internal rotation profiles can occur on timescales as short as centuries. This suggests that the g modes can impact the stars' life trajectories, a possibility that's been hitherto ignored in stellar evolution calculations.
Why interstellar grain align and why you should care
B-G Andersson, SOFIA Science Center
More than 70 years after the discovery of interstellar polarization, we now have a quantitative, empirically tested, theory of grain alignment giving ride to the observed effect. This Radiative Alignment Torque (RAT) theory predicts that dust grains are spun up by an anisotropic radiation field, if the wavelength of the light is less than the grain diameter. If the grain is made of a paramagnetic material, it will then align with the magnetic field.
A number of specific, observationally testable, predictions follow from the theory, many of which have already been addressed. With a full testing of the theory and quantification of its parameters, polarimetry has the promise to not only allow efficient and reliable tracing of interstellar and interplanetary magnetic fields, but also to provide new and unique probes of the dust and the interstellar environment.
I will review RAT alignment and its observational testing, and discuss some of the probes of ISM environmental parameters and dust that the verified theory allows.
Space astrometry: the Hipparcos and Gaia missions
Michael Perryman, Priceton
Alone amongst the space agencies, ESA made its entry into space astrometry with the adoption of the Hipparcos mission in 1981. Outside of the astrometric community, it was viewed at the time as fundamental if not particularly exciting, although Freeman Dyson described it as "... the first time since Sputnik in 1957 that a major new development in space science has come from outside the US". In his ASP Millennium Essay in 2001, Cavendish Professor Malcolm Longair stated that "It is invidious to single out surveys which I find particularly impressive, but I make an exception for the Hipparcos astrometric satellite".
Hipparcos delivered its high-accuracy catalogue of 120,000 star distances and space motions in 1997. As a follow-up, ESA accepted the Gaia mission in 2000. Launched in 2013 and expected to operate into the next decade, Gaia will represent a revolution in its dynamical stereoscopic mapping of our Galaxy, promising a catalogue of more than a billion stars to 20 magnitude at microarcsec-level accuracy. The talk will provide a short historical context and describe the scientific motivation for these missions, outline the essential experimental principles which underpin their measurements, and give an overview of the science objectives, including Gaia's expected yield of many thousands of astrometrically-detected exoplanets.
Michael Perryman obtained his PhD in 1980 (Cambridge, UK) and spent most of his subsequent career with the European Space Agency. He was project scientist for Hipparcos from adoption in 1981 to catalogue finalisation in 1997, holding the dual role of overall project manager (1989-1993) after the satellite failed to achieve its nominal geostationary orbit. With Lennart Lindegren (Lund, Sweden) he was the co-originator of Gaia, and responsible for driving many of its principal attributes. He was study scientist from the Gaia's origins in 1995 to mission adoption in 2000, and thereafter ESA project scientist until the Critical Design Review in 2008.
Young Star Fundamentals and Surprises
Lynne Hillenbrand, Caltech
Young stars associated with regions of recent star formation are both predictably, and enigmatically, variable over much of the electromagnetic spectrum -- due to processes occurring on the stellar surface, within the disk-to-star accretion zone, in the inner circumstellar disk regions, and perhaps in the outflow. The talk will begin with an overview of the relevant young star phenomena, then proceed to discuss recent work on fundamental stellar parameters for young stars, including several young planet discoveries, and then to the revelations of circumstellar phenomena based on state-of-the-art time domain data sets.
How Black Holes get their Kicks: Dynamical Evolution and Coalescence
Steinn Sigurdsson, Penn State
Recent observations have increased interest in the possibilities of a significant population of black hole binaries in the local universe. Natal kicks may play a crucial role in the merger rate of stellar mass black holes. Dynamical evolution can lead to an enhanced interaction rate for compact binaries in dense stellar systems and a distinct and richer population of compact binaries. I discuss some of the issues related to black hole binary formation and coalescence, the issue of retention in globular clusters and possible contribution to the LIGO rate.
Probing Chemical Enrichment in the Circumgalactic Medium -- Combining Absorption Spectroscopy and Direct Imaging Observations
Hsiao-Wen Chen, University of Chicago
Tremendous progress has been made over the last decade in our empiricaland theoretical understanding of how galaxies form and evolve across cosmic time. In particular, state-of-the-art cosmological simulations can not only match the large-scale statistical properties of galaxies, but they can also successfully reproduce the observed small-scale features of star-forming disks. However, these models have fallen short in matching the empirical properties of diffuse gas, which constitutes 90% of all baryons in the universe, beyond visible galaxy disks and into circumgalactic space. An accurate characterization of the complex physical processes that govern the interactions between star-forming regions and this diffuse circumgalactic medium (CGM) is a critical next step toward a comprehensive understanding of galaxy formation and evolution. In this talk, I will summarize the progress and challenges in CGM studies from traditional absorption-line observations, and discuss future prospects in direct imaging of the CGM around distant galaxies.
First results from LIGO: past, present and future
Nergis Mavalvala, MIT
The Laser Interferometer Gravitational-wave Observatory (LIGO) detected gravitational waves for the first time in 2015. Since then there have been a couple more detections of binary black hole mergers. I will discuss the instruments that made these discoveries, the science so far, and plans for future improvements and upgrades to LIGO.
Novel detectors for next-generation mm-wavelength instruments
Erik Shirokoff, University of Chicago
The kinetic inductance detector (KID) is a novel superconducting photon detector. It offers simple fabrication, intrinsic multiplexing of thousands of detectors per cable, and much higher dynamic range than competing technologies, and has now demonstrated background limited operation suitable for ground-based instruments at mm and submm wavelengths. I'll discuss two specific applications that make use of these new devices. The first, SuperSpec, is an compact on-chip, mm-wavelength spectrometer. Its small size, wide spectral bandwidth, and highly multiplexed detector readout will enable construction of powerful multi-object spectrometers able to catalog thousands of dusty star forming galaxies at high redshift. I will discuss the design, optimization, and measured performance of our prototype devices, our upcoming engineering run with the SuperSpec demonstration camera, and the unique observational opportunities accessible to future large-scale facility instruments based upon this technology. The second project, the Chicago CMB-KIDs program, is developing a KID-based, polarization sensitive, multi-band focal plane array optimized for CMB observations. I'll discuss our pixel designs and progress toward producing laboratory demonstration of a full-scale array suitable for deployment in a future CMB instrument.
Dust polarization and interstellar turbulence
Marc Kamionkowski, Johns Hopkins
Perhaps the most surprising result from the Planck satellite is the observation that the E-mode power in the dust polarization is twice that in the B mode. In this talk I will show how the E and B modes in the dust polarization are related to fluctuations in the magnetized interstellar medium. I will argue that the observed E/B ratio, as well as the TE (temperature-polarization) cross-correlation are not easily reconciled with expectations from MHD turbulence. I will then discuss some alternative explanations for the dust-emission patterns seen in the Planck temperature-polarization maps and outline some interesting directions for future related research.
AGN-driven outflows at z~2
Alison Coil, University of California, San Diego
AGN-driven outflows are assumed to be a key driver of galaxy evolution, determining the shape of the galaxy stellar mass function at high masses and regulating, perhaps even quenching, star formation as galaxies become quiescent. However, the details of how common this feedback is and how it impacts the host galaxy are generally unclear. I will present new results using Chandra data in the CANDELS and UltraVISTA surveys showing which galaxies host AGN of a given accretion rate and how this correlates with star formation in the host galaxy from z~0 to z~4. I will further present new results from the MOSDEF survey on AGN-driven outflows at z~2, discussing their incidence, kinematics, and physical extent. We find that fast, galaxy-wide AGN-driven outflows are common in typical star-forming galaxies at z~2 and that they likely help regulate star formation at the cosmic peak of galaxy growth.
Imaging All the Sky All the Time in Search of Radio Exoplanets
Gregg Hallinan, Caltech
All the magnetized planets in our solar system, including Earth, produce bright emission at low radio frequencies, predominantly originating in high magnetic latitudes and powered by auroral processes. It has long been speculated that similar radio emission may be detectable from exoplanets orbiting nearby stars, which would provide the first direct confirmation of the presence, strength and extent of exoplanetary magnetospheres, as well as informing on their role in shielding the atmospheres of potentially habitable exoplanets. Despite 4 decades of observations, no detection has been achieved. Surprisingly, however, brown dwarfs have been found to produce both radio and optical emissions that are strikingly similar to the auroral emissions from solar system planets, albeit 10,000 times more luminous, bolstering the continued search for similar emission from exoplanets. I will discuss the auroral radio emission from exoplanets and brown dwarfs and introduce a new radio telescope, consisting of 352 antennas spaced across 2.5 km, that images the entire viewable sky every ten seconds at low radio frequencies, thereby monitoring thousands of stellar systems simultaneously in the search for radio emission from exoplanets.
The WIMP is dead. Long live the WIMP!
Dan Hooper, University of Chicago
Although weakly interacting massive particles (WIMPs) have long been the leading class of candidates for the dark matter of our universe, the lack of a confirmed detection of these particles has left the community increasingly skeptical of their existence. In this talk, I will ask the following questions: How surprised should we be that WIMPs have not yet been detected? What assumptions might we change in order to explain the lack of any clear signals of dark matter? In light of the current experimental situation, what are the prospects for future direct, indirect and collider searches for dark matter? And lastly, may we already be observing evidence of annihilating WIMPs in the gamma-ray sky?
Pulsar Magnetosphere: The Incredible Machine
Anatoly Spitkovsky, Princeton
Pulsars are rotating magnetized neutron stars that emit repeating pulses of radiation spanning all of the electromagnetic spectrum. 50 years after their discovery, more than 2000 pulsars are known, and they have been used as sensitive astronomical probes of diverse phenomena ranging from the properties of interstellar medium to the predictions of general theory of relativity. Despite great observational successes, our theoretical understanding of how pulsar magnetospheres work is woefully incomplete. Pulsars bring together aspects of classical and quantum electrodynamics, coupled with strongly magnetized plasma physics in curved rotating spacetime of a massive compact object. The nonlinear interplay of these effects makes it a very difficult but rewarding problem to study. I will review the status and progress of pulsar magnetospheric modeling in various approximations, including force-free and relativistic magnetohydrodynamics, culminating with recent developments of fully kinetic simulations of pulsar magnetospheres. These simulations allow us to find the shape of the magnetosphere and the location of particle acceleration regions, constraining the origin of high energy emission. The pulsar magnetosphere is a prototype for other strongly magnetized astrophysical objects, and I will discuss how the lessons from pulsar modeling can be useful in understanding the physics of black hole disks and in predicting electromagnetic counterparts to gravitational wave sources.
Core-Collapse Supernova Explosion Physics
Adam Burrows, Princeton
Core-collapse supernovae have challenged theorists and computational science for more than half a century. Such explosions are the source of many of the heavy elements in the Universe and the birthplace of neutron stars and stellar-mass black holes. However, determining the mechanism of explosion remains the key goal of theory. Recently, there have been breakthroughs in understanding and simulating these explosions, and I will describe our recent calculations that lead to robust explosions and the physics behind them. All these events have gravitational-wave and neutrino signatures that could be diagnostic of the internal dynamics of the mechanism and explosion phenomenology in real time. I will discuss such signatures and how their detection might bear on a definitive experimental resolution of the core-collapse puzzle.