MIT Astrophysics Colloquia - Spring 2017

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 Feb 7:
Cosmology with Shadows in the Microwave Sky
Nick Battaglia
Princeton University
Host: M. McDonald

Abstract: Our current 6-parameter LambdaCDM cosmological model is well constrained by observations in the linear regime, such as measurements of the cosmic microwave background (CMB) radiation. A new frontier for cosmology is to search for departures from this model, which pushes future measurements into the non-linear regime (e.g., late-time growth of structure). The thermal Sunyaev Zel'dovich (tSZ) effect offers a unique way to probe the growth of structure using high-resolution CMB experiments, such as the Atacama Cosmology Telescope. I will review the recent tSZ cosmological constraints and discuss the astrophysical uncertainties that currently limit them. I will discuss current efforts to mitigate these astrophysical uncertainties highlighting the role that optical surveys like the Large Synoptic Survey Telescope will play. I will describe the prospects for future CMB experiments like the Simons Observatory and CMB-S4 to constrain departure from our standard cosmological model with tSZ observations.

Tuesday Feb 14:
New Probes of Old Structure: Cosmology with 21cm Intensity Mapping and the Cosmic Microwave Background
Laura Newburgh
Yale University
Host: J. Hewitt

Abstract: Current cosmological measurements have left us with deep questions about our Universe: What caused the expansion of the Universe at the earliest times? How did structure form? What is Dark Energy and does it evolve with time? New experiments like CHIME, HIRAX, and ACTPol are poised to address these questions through 3-dimensional maps of structure and measurements of the polarized Cosmic Microwave Background. In this talk, I will describe how we will use 21cm intensity measurements from CHIME and HIRAX to place sensitive constraints on Dark Energy between redshifts 0.8 -- 2.5, a poorly probed era corresponding to when Dark Energy began to impact the expansion history of the Universe. I will also discuss how we will use data from new instruments on the ACT telescope to constrain cosmological parameters like the total neutrino mass and probe structure at late times.

Tuesday Feb 21:
Tracing the Cosmic Shutdown of Star Formation in Massive Galaxies
Kate Whitaker
University of Connecticut
Host: M. Vogelsberger

Abstract: Over the last few decades, astronomers have progressed from archeological studies of nearby galaxies to direct observations of the early universe. We have uncovered billions of years of cosmic growth that present new challenges to galaxy formation theories. In this talk, I will review the recent innovative techniques developed to probe the distant universe, and the key observations constraining the formation histories of galaxies over the past 11 billion years. We have discovered a population of surprisingly compact and massive “red and dead” (quiescent) galaxies that are no longer actively forming stars. The physical mechanisms responsible for shutting down star formation and the subsequent buildup of this quiescent population at such early times is one of the most outstanding questions in astrophysics today. We don’t yet understand why these enigmatic galaxies are so compact, with sizes a factor of 5 smaller than nearby galaxies of similar mass. I will present promising paths forward towards solving this puzzle that leverage the capabilities of the Hubble Space Telescope, as well as a look toward the future with exciting upcoming public facilities.

Tuesday Feb 28:
The Physics and Astrophysics of Merging Neutron-Star Binaries
Luciano Rezzolla
Goethe University of Frankfurt
Host: S. Hughes

Abstract: I will argue that if black holes represent one the most fascinating implications of Einstein's theory of gravity, neutron stars in binary system are arguably its richest laboratory, where gravity blends with astrophysics and particle physics. I will discuss the rapid recent progress made in modelling these systems and show how the inspiral and merger of a binary system of neutron stars is more than a strong source of gravitational waves. Indeed, while the gravitational signal can provide tight constraints on the equation of state for matter at nuclear densities, the formation of a black-hole--torus system can explain much of the phenomenology of short gamma-ray bursts, while the ejection of matter during the merger can shed light on the chemical enrichment of the universe.

Tuesday Mar 7:
Stellar Streams and the Milky Way's Dark Matter Halo
Jo Bovy
University of Toronto
Host: P. Schechter

Abstract: 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.

Tuesday Mar 14:
Imaging All the Sky All the Time in Search of Radio Exoplanets
Gregg Hallinan
Host: M. McDonald

Abstract: 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. In the case of the latter, the absence of detection thus far may simply be due to the highly variable nature of the radio emission. The auroral radio emission from Earth can increase by three orders of magnitude during a geomagnetic storm and a similar dependence on local space weather conditions can be assumed for exoplanets, requiring dedicated monitoring of stellar systems to enable detection. 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.

Tuesday Mar 21:
Dark Matter Substructure: Cosmological Treasure Trove or a Pandora's Box?
Frank van den Bosch
Yale University
Host: P. Schechter

Abstract: Hierarchical structure formation in a LCDM cosmology gives rise to virialized dark matter halos that contain a wealth of subtructure. Being able to accurately predict the abundance and demographics of dark matter subhaloes is of paramount importance for many fields of astrophysics: gravitational lensing, galaxy evolution, and even constraining the nature of dark matter. Dark matter substructure is subject to tidal stripping and tidal heating, which are highly non-linear processes and therefore best studied using numerical N-body simulations. Unfortunately, as I will demonstrate, state-of-the-art cosmological simulations are unable to adequately resolve the dynamical evolution of dark matter substructure. They suffer from a dramatic amount of artificial subhalo disruption as a consequence of both inadequate force softening and discreteness noise amplification in the presence of a tidal field. I discuss implications for a variety of astrophysical applications, and briefly discuss potential ways forward.

Tuesday Mar 28:

Tuesday Apr 4:
Probing New Physics with the Cosmic Microwave Background
Abigail Vieregg
University of Chicago
Host: TBD

Abstract: Measurements of the Cosmic Microwave Background (CMB) have been extremely important for our understanding of the origin, contents, and evolution of the universe. Increasingly precise maps of the CMB are a unique and powerful tool for understanding new physics, including inflation, the superluminal expansion of the universe during the first moments after the Big Bang. I will discuss the current status of CMB measurements, focusing on constraints on inflation with the BICEP series of experiments at the South Pole (BICEP2, The Keck Array, and BICEP3). I will then discuss the rapidly moving plans for a next-generation ground-based program called CMB-S4. The sensitivity leap of CMB-S4 is needed to pass critical thresholds in our understanding of inflation, neutrino mass, and in searches for additional light relativistic species.

Tuesday Apr 11:
Synergy of molecular clouds and a supermassive black hole in our Galactic Center.
Eugene Churazov
Max Planck Institute für Astrophysik
Host: C. Canizares

Abstract: While the supermassive black hole Sgr A* at the center of the Milky Way is currently very dim, we believe that it experienced a powerful outburst of X-ray radiation hundreds of years ago. The historical records of this outburst are revealed by reflection/reprocessed radiation coming from dense molecular clouds. The imprints left by the outburst in spatial and time variations of the reflected emission suggest that the outburst happened some hundred years ago. It lasted less than several years and Sgr A* was about hundred million times brighter than today. These characteristics are consistent with a relatively modest tidal disruption event. Thus, molecular clouds offer us a convenient tool to study Sgr A*’s past history. At the same time, the outburst serves as an extremely powerful probe of molecular gas. Essentially, this is the only opportunity to reconstruct a full 3D structure of molecular clouds and their mass distribution. Future X-ray observatories, including cryogenic bolometers and polarimeters, will further boost our ability to conduct in-depth studies of molecular gas and outbursts of Sgr A*.

Tuesday Apr 18:
The Demographics of Exoplanets with Microlensing
Scott Gaudi
Ohio State University
Host: S. Seager

Abstract: Measurements of the demographics of exoplanets over a range of planet and host star properties provide fundamental empirical constraints on theories of planet formation and evolution. Because of its unique sensitivity to low-mass, long-period, and free-floating planets, microlensing is an essential complement to our arsenal of planet detection methods. I summarize the results to date from microlensing surveys, and discuss the expectations from near-future ground-based surveys. I then discuss efforts to combine results from microlensing, radial velocity, and direct imaging surveys to constrain the distribution of planets over roughly four orders of magnitude in planet mass and nearly five orders magnitude in period. Finally, I motivate a microlensing survey with WFIRST, which when combined with the results from Kepler, will yield a nearly complete picture of the demographics of planetary systems throughout the Galaxy.

Tuesday Apr 25:
Transmission Spectra of Exoplanetary Atmospheres
Kevin Heng
University of Bern
Host: N. Weinberg

Abstract: Transmission spectra are vital for probing the atmospheres of exoplanets, which motivates a deep theoretical understanding of how to interpret them. In the first part of the talk, I will review the theory of transmission spectra using both order-of-magnitude arguments and heuristic highlights of theoretical formalism. I will use a validated analytical formula for the transit radius to discuss the degeneracies inherent in interpreting transmission spectra. Specifically, transmission spectra lack an absolute normalisation, due to the a priori unknown relationship between a reference transit radius and its corresponding reference pressure. For ~1000 K atmospheres (or hotter), transmission spectra are insensitive to the pressure and temperature variations probed, with the implication that parameter exploration using a fixed grid in pressure (for the model atmosphere) may not completely elucidate the degeneracies involved. In the second part of the talk, I will discuss how the sodium and potassium lines may be used to quantify the degree of cloudiness in an exoplanetary atmosphere. Specifically, the difference in transit radius between the line core and wing can be straightforwardly calculated for a cloud-free atmosphere, and any deviation from this reference indicates the presence of clouds/hazes (of unknown composition, geometry and size distribution). I will discuss the tentative trend that hotter atmospheres appear to be less cloudy at the wavelengths probed by the alkali metal lines. I will also compare my findings to the study of Stevenson (2016), which uses the 1.4-micron spectral feature of water in the HST-WFC3 bandpass to quantify cloudiness. More generally, I will discuss how a combination of the alkali metal lines and CHEOPS and TESS photometry will enable us to efficiently screen a sample of objects for whether they possess atmospheres and if they are cloudy.

Tuesday May 2:
Surveying the sky with the LSST: software as the instrument of the next decade
Andrew Connolly
University of Washington
Host: S. Vitale

Abstract: The development of a new generation of telescopes, wide-field detectors, and computational facilities has led to an era where it is now possible for deep optical surveys to survey a large fraction of the visible sky. One of the largest of these surveys, the Large Synoptic Survey Telescope (LSST), will comprise an 8.4 m primary mirror with a 9.6 square degree field-of-view and a 3.2 Gigapixel camera and begin operations at the end of this decade. Over the ten years of its operation, the LSST will survey half of the sky in six optical colors, discovering 37 billion stars and galaxies and detecting about 10 million variable or transient sources every night. In this talk, I will describe some of the latest developments from the LSST, introduce new algorithmic approaches for detecting variable and moving objects (including using atmospheric refraction to extract spectral information), show how we can increase the yield of variable and transient sources by modifying the cadence and observing strategy of the LSST, and describe some of the computational challenges we face with the LSST when working with petabyte data sets.

Tuesday May 9:
How rare are Earths and how common are Jupiters?
David Hogg
Host: D. Chakrabarty

Abstract: The NASA Kepler and K2 missions have increased vastly the number of planets known around other stars. I will describe how we have found some of the hardest-to-find planets (those with small radii, like Earth, or those with long periods, like Jupiter) in these data sets. It involves noise modeling and linear algebra. I will also discuss how it is possible to make inferences about the full population of planets in the Galaxy, given the noisy measurement and uncertain detection of every important known system. It involves building statistical models with hierarchical structure. The questions in the title are not conclusively answered yet: Earths and Jupiters both appear common, but there certainly are some respects in which our Solar System appears unusual. I discuss the prospects and possible show-stoppers in the next generations of data, and concentrate on methodologies that will be critical in the era of TESS.

Tuesday May 16:
Illuminating the Cosmic Dawn
Andrei Mesinger
Scuola Normale Superiore
Host: J. Hewitt

Abstract: The birth of the first stars, black holes and galaxies heralded the end of the cosmic Dark Ages and the beginning of the Cosmic Dawn. The light from these objects heated and ionized almost every atom in existence, culminating in the Epoch of Reionization: the final major phase change of the Universe. This final frontier of astrophysical cosmology is undergoing a transition from an observationally-starved epoch to a "Big Data" field. This process will culminate with interferometric observations of the redshifted 21-cm line, eventually providing a 3D map of the first billion years of our Universe. I will discuss the recent clues to reionization, obtained from the CMB, high-redshift galaxies, and quasars. I will present the first detection (2 sigma) of ongoing reionization, found in the spectrum of a z=7.1 quasar. Finally, I will showcase the physical bounty of the upcoming 21-cm observations. I will discuss the innovative modeling techniques we are developing to tap into this bounty, allowing us to constrain astrophysical parameters in a fully Bayesian framework.

This page is maintained by Mike McDonald