MIT Astrophysics Colloquia - Fall 2015

Tuesdays at 4:00 PM in the Marlar Lounge, Room 37-252
MIT Kavli Institute for Astrophysics and Space Research
70 Vassar Street, Cambridge, MA
(unless location otherwise noted)
Refreshments are served at 3:45 PM.

Sponsored by
the Astrophysics Division of the MIT Department of Physics and
the MIT Kavli Institute for Astrophysics and Space Research.

Tuesday Sep 15:
Massive Elliptical Galaxies and Supermassive Black Holes
Chung-Pei Ma
UC Berkeley
Host: Rob Simcoe

Abstract: Massive elliptical galaxies exhibit the most massive black holes, most extreme stellar initial mass functions, and most dramatic size evolution over cosmic time. Yet, their complex formation histories remain obscure. I will describe the ongoing MASSIVE Survey, a volume-limited, multi-wavelength, spectroscopic and photometric survey of the structure and dynamics of the 100 most massive early-type galaxies within 100 Mpc. A combination of integral-field spectroscopy on sub-arcsecond and large scales allows us to perform simultaneous dynamical modeling of the supermassive black holes, stars, and dark matter. I will present first results from the survey and discuss the implications of black hole binaries as sources of gravitational waves for pulsar timing array experiments.

Tuesday Sep 22:
The Energy Balance of Short-Period Planets
Nicolas Cowan
McGill University
Host: Nevin Weinberg

Abstract: What started as a trickle in the mid 1990’s is now a torrent, with over one thousand extrasolar planets currently known, and thousands of candidates awaiting confirmation. The study of exoplanets has already revolutionized our view of planet formation, and will soon do the same to our understanding of planetary atmospheres and interiors. Since we view them from the top-down, one of the first aspects of exoplanet atmospheres to be constrained is their global thermal emission. By combining infrared emission measurements at a variety of orbital phases, we can infer a planet's Bond albedo, the efficiency of its day-night heat transport, and, in the case of planets subject to seasons, its thermal inertia. Multi-wavelength emission measurements can also constrain a planetary atmosphere's composition and vertical temperature structure. Such inferences are particularly sensitive to the uncertainty in emission measurements, however, and the accuracy of eclipse measurements has typically been over-stated. Fortunately, improved analysis techniques and next-generation instruments should allow us to resolve outstanding questions about hot Jupiters, and to extend our methods to temperate terrestrial planets.

Tuesday Sep 29:
What X-rays tell us about exoplanet systems
Katja Poppenhaeger
Queen's University Belfast
Host: Zach Berta-Thompson

Abstract: With thousands of exoplanetary systems known to date, characterizing the atmospheres of exoplanets and how they evolve over time has become an intensely studied question. While optical and near-infrared observations probe lower layers of exoplanetary atmospheres, high-energy observations reveal what happens to the upper atmospheric layers that interact with the stellar wind. Also, high-energy photons directly trace the magnetic activity of the host stars, and can be used to draw inferences about the stellar corona, stellar flares, and the rotational history of stars. I will discuss what this exciting observational window has told us so far about exoplanets and their host stars, and will also highlight some upcoming high-energy missions and how their capabilities can be used for the exoplanetary field.

Tuesday Oct 6:
From the Heliosphere, to Galaxy Clusters, Gamma Ray Bubbles, Pulsars and Black Holes: Unexpected Plasmas, Relevant and Laboratory Based Theories
Bruno Coppi
Host: Brendan Griffen

Abstract: The in situ exploration of the Heliosphere has revealed the existence of unexpected kinds of plasmas and magnetic field configurations around the most distant planets (e.g. Uranus and Neptune) for which reliable theories can be formulated and be of help to envision the environments of a variety of recently discovered exo-planets. On larger scales, radically different kinds of plasmas have been found: in particular plasmas with “temperatures in the tens of keV are observed to be the main visible component of Galaxy Clusters, while g-ray emitting plasma structures (“bubbles”) have been seen to emerge from the disk of Our Galaxy with dimensions of the same order as those characterizing the Galaxy. Although the theory of the plasmas that can surround pulsars has a long history, the fact that the plasmas on the surface of pulsars can have inhomogeneous features (such as hot spots) and dynamics has gained attention recently. Given the very high magnetic fields involved, the role of the electron thermal conductivity anisotropy is shown to be an important factor in these. There are important issues to be resolved in order to envision the plasmas that can surround black holes. These involve the structures that can form, such as rings and tridimensional spirals, the kind of transport of angular momentum that is needed to allow plasma accretion on the black hole, etc. In fact, laboratory experiments on high-energy plasmas have cast new light on basic processes that include the nature of angular momentum transport not described by a diffusion equation, magnetic reconnection events and associated production of high-energy particles, self-organization processes, etc.

Tuesday Oct 13:
Implausible Life: An Unappealing But Plausible Scenario for Life's Origin on Earth
Ed Turner
Host: Joshua Winn

Abstract: There is no evidence which strongly contradicts the hypothesis that life arose on Earth due to such extraordinarily improbable events that it is extremely unlikely it has arisen or will ever arise elsewhere within the observable universe. Moreover, a few bits of evidence and lines of reasoning support this hypothesis, though none in a conclusive or compelling way. The ways in which molecular biology suggests combinatoric improbability, the Fermi(-Hart) paradox, "rare Earth" lines of argument and the failure to date of SETI are among them. Moreover, some of the most common counterarguments to these inferences are flawed in fundamental ways. However unappealing it may be (to most, but not all), we should take the hypothesis quite seriously at our current state of knowledge. It is arguably as good as any other available scenario on a purely empirical basis.

Tuesday Oct 20:
Atmospheric dynamics of brown dwarfs
Adam Showman
University of Arizona
Host: Michael McDonald

Abstract: A variety of observations provide evidence for vigorous motion in the atmospheres of brown dwarfs and directly imaged extrasolar giant planets; these observations include spectral evidence for clouds, disequilibrium chemistry, lightcurve variability, and maps of surface patchiness. These observations raise major questions about the nature of the atmospheric circulation on these exotic worlds, which resemble high-heat-flux, high-gravity, rapidly rotating versions of Jupiter. In this talk, I will survey the dynamical regime of these objects with an eye toward explaining these observations. Brown dwarfs rotate rapidly, and for plausible wind speeds, the flow at large scales will be rotationally dominated. The interaction of convection with the overlying, stably stratified atmosphere will generate a wealth of atmospheric waves, and I argue that, just as in the stratospheres of planets in the solar system, the interaction of these waves with the mean flow will lead to a significant atmospheric circulation at regional to global scales. One-layer and 3D global simulations of the atmospheric circulation suggest that, under relevant conditions, brown dwarfs can develop large-scale atmospheric circulations comprising banded flow patterns, zonal jets, turbulence, and in some cases stable vortices. These large-scale coherent structures will be accompanied by a vertical (overturning) circulation that will modulate the condensate cycle and lead to cloud patchiness analogous to that inferred from the observations. The simulations provide a foundation for understanding lightcurves and surface maps of brown dwarfs, and moreover illuminate the continuum of dynamical processes between brown dwarfs and Jupiter itself.

Tuesday Oct 27:
The Search for Alternate Universes
Ranga-Ram Chary
Host: Jacqueline Hewitt

Abstract: In the last few years, we have made remarkable progress in understanding the properties of our observable Universe which appears to have evolved from a hot Big Bang 13.7 billion years ago. The fine-tuning of initial conditions required to reproduce our present day Universe suggests that our Universe may merely be a region within an eternally inflating super-region. Many other regions could exist beyond our observable Universe with each such region governed by a different set of physical parameters than the ones we have measured for our Universe. Collision between these regions, if they occur, should leave signatures of anisotropy in the cosmic microwave background. I present spatial and spectral analysis of the cosmic microwave background data from Planck. I will focus on constraining properties at the epoch of recombination 270,000 years after the Big Bang and present the observational evidence in favor of alternate Universes.

Tuesday Nov 3:
The Heart of the Beast : The Relativistic Astrophysics of Active Galactic Nuclei
Chris Reynolds
University of Maryland
Host: Mark Bautz

Abstract: At the heart of an AGN is a relativistic accretion disk around a spinning supermassive black hole (SMBH) with an X-ray emitting corona and, sometimes, a relativistic jet. On larger scales, the outer accretion disk and molecular torus act as the reservoirs of gas for the continuing AGN activity. And on all scales from the black hole outwards, powerful winds are seen that probably affect the evolution of the host galaxy as well as regulate the feeding of the AGN itself. In this talk, I will discuss how current and future X-ray spectroscopy can be used to study each of these components. Specifically, (1) I shall discuss recent NuSTAR measurements of the high-energy cutoff in the X-ray continuum which are pushing us to conclude that X-ray coronae have electron temperatures regulated by electron-positron pair production. (2) I shall review the latest state of play regarding the measurement of SMBH spins, and show that the predominance of rapidly-rotating objects in current surveys of SMBH spin is entirely unsurprising once one accounts for the observational selection bias resulting from the spin-dependence of the radiative efficiency. Finally, I shall discuss recent progress in our understanding of fast ($v\sim 0.1-0.3c$), highly-ionized (mainly visible in FeXXV and FeXXVI lines), high-column density winds that may dominate quasar-mode galactic feedback.

Tuesday Nov 10:
Investigating the Composition and Structure of Hot Exoplanets with HST and JWST
Avi Mandell
NASA Goddard Center for Astrobiology
Host: Hans Moritz Guenther

Abstract: Hot transiting exoplanets represent an extreme end of the exoplanet distribution. They orbit very close to their host stars, which subjects them to intense heating from stellar radiation. However, they almost certainly did not form in their current short orbits, and their complex formation histories provide both an opportunity and an impediment for learning about their formation and migration. Investigating their structure and composition through atmospheric characterization can help us break model degeneracies and develop a more holistic picture of the evolution of planetary systems in general. In this talk I will discuss the current state of atmospheric characterization of hot transiting exoplanets, focusing primarily on results from the Hubble Space Telescope and future observations with the James Webb Space Telescope. HST has provided the first opportunity for spectroscopy of a significant number of transiting exoplanets, and results over the last several years give us the first indication of trends in composition and atmospheric structure. However, HST only barely provides the needed wavelength coverage and sensitivity for measuring key atmospheric absorption features across the near- and mid-infrared. With an order of magnitude improvement in both IR wavelength coverage and sensitivity, JWST will revolutionize our ability to constrain the atmospheric composition of a wide array of planets and break degeneracies between different absorbers, leading to new breakthroughs in our understanding of the atmospheric temperature structure, the role of clouds and hazes, and the overall classification of planetary atmospheres and what this tells us about how planets form.

Tuesday Nov 17:
The Milky Way - evidence for Seyfert activity in the recent past
Joss Bland-Hawthorn
University of Sydney
Host: Anna Frebel

Abstract: The Galaxy's supermassive black hole is a hundred times closer than any other massive singularity. It is surrounded by a highly unstable gas disk so why is the black hole so peaceful at the present time? This mystery has led to a flurry of models in order to explain why Sgr A* is radiating far below (1 part in 108) the Eddington accretion limit. But has this always been so? Evidence is gathering that Sgr A* has been far more active in the recent past, on timescales of thousands of years and longer. The bipolar wind discovered by MSX, the gamma-ray bubbles discovered by Fermi-LAT, the WMAP haze, the positronium flash confirmed by INTEGRAL, are suggestive of something truly spectacular in the recent past. We present new evidence that the Galactic Centre was a full blown "active galaxy" just two million years ago. The echo of this incredible event can be seen today imprinted across the Galaxy.

Tuesday Nov 24:
Dark Matter Signal or New Gamma-Ray Point Sources: How Can We Tell?
Tracy Slatyer
Host: Scott Hughes

Abstract: Studies of data from the Fermi Gamma-Ray Space Telescope have revealed bright gamma-ray emission from the central regions of our galaxy, with a spatial and spectral profile consistent with annihilating dark matter. However, I will present a new model-independent analysis that suggests that rather than originating from dark matter, the GeV excess may arise from a surprising new population of as-yet-unresolved gamma-ray point sources in the heart of the Milky Way. I will briefly discuss other possible astrophysical explanations, and whether a population of millisecond pulsars could explain these new results.

Tuesday Dec 1:
Gravitational Wave Astronomy with Advanced LIGO
Chad Hanna
Pennsylvania State University
Host: Nergis Mavalvala

Abstract: Advanced LIGO has completed construction and is now collecting gravitational wave data with unprecedented sensitivity. Advanced LIGO will soon be sensitive enough to detect gravitational waves from several potential sources, the most promising of which are compact binary mergers. Measuring gravitational waves will broaden our understanding of fundamental physics and give birth to a new branch of astronomy whereby we can infer the dynamics of distant objects not only through their electromagnetic signatures, but through their imprint on space-time itself. The LIGO Scientific Collaboration has already established partnerships with observing facilities spanning the electromagnetic spectrum, as well as several astroparticle observatories, in the hopes of detecting transient sources through a variety of channels. In this seminar, I will give an update on the progress of our first ever advanced LIGO observing run, focusing specifically on searches for colliding neutron stars and black holes and their even more elusive electromagnetic counterparts.

Tuesday Dec 8:
Reconstructing the Formation Histories of Massive Galaxies
Mariska Kriek
UC Berkeley
Host: Paul Schechter

Abstract: 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 over 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.

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