Astrophysics Seminar and Colloquium Abstracts
Spring 2018

January 18 - Seminar

Blakesley Burkhart (Harvard-Smithsonian CfA)
Turbulent Beginnings: A Predictive Theory of Star Formation in the Interstellar Medium

Our current view of the interstellar medium (ISM) is as a multiphase environment where magnetohydrodynamic (MHD) turbulence affects many key processes. These include star formation, cosmic ray acceleration, and the evolution of structure in the diffuse ISM. In this talk, I shall review the fundamentals of galactic turbulence and then discuss progress in the development of new techniques for comparing observational data with numerical MHD turbulence simulations. I shall highlight a number of exciting problems that our statistical, numerical and observational progress in the field of MHD turbulence has opened up to quantitative analysis. In particular, I will demonstrate how the star formation rate can be analytically calculated from our understanding of how turbulence and gravity induced density fluctuations in the ISM via a probability distribution function analysis. This analytic calculation predicts star formation rates from pc size scales (GMCs) to kpc size scales in galaxies. These studies represent just the beginnings of a bright future for research in galactic and extragalactic turbulence.


January 23 - Seminar

Kristen McQuinn (University of Texas at Austin)
The Low-Mass Galaxy Frontier

Dwarf galaxies are key to unlocking our understanding of structure formation and galaxy evolution. Traditionally defined as galaxies below a mass threshold of ~10^9 Msun, these small systems preserve the history of the Universe in their stars, gas, and chemical elements. Due to their low-masses, they also provide stringent tests for cosmological simulations. In this talk, I will give an overview of how we are now mapping the full baryon cycle of these traditional dwarf galaxies, with an emphasis on providing quantitative benchmarks for the leading cosmological models. In addition, I will show how we are moving to the next frontier in low-mass galaxy research with the discovery and characterization of smaller systems hovering at the brink of what we call a galaxy. The physical properties of these extremely low-mass systems lie below many thresholds from theoretical predictions and allow us to address fundamental questions about structure formation, survival, and evolution.


January 25 - Seminar

Robyn Sanderson (Caltech)
Insights into dark matter from the stellar halos of galaxies

Cosmological simulations can now make specific and detailed predictions for the shapes, masses, and substructure fractions in galactic dark matter halos that depend on the dark matter model assumed. Comparing these predictions to the observed mass distributions of galaxies should in principle lead to constraints on the nature of dark matter, but observable dynamical tracers can be scarce in regions where the dark matter distribution is best able to discriminate between models. One such region is the distant outskirts of galaxies, where the influence of baryonic matter on the dark matter halo is limited and the effect of dark substructures most prominent. New surveys of Milky Way stars like Gaia, alongside next-generation instruments and giant telescopes, are for the first time providing accurate positions, velocities, and abundances for large numbers of stars in faint tidal streams: remnants of tidally-disrupted satellite galaxies that trace out the mass distribution in the distant reaches of galaxy halos. I will show how state-of-the-art simulations play a crucial role in interpreting and analyzing this wealth of new information about stellar halos, and how stellar halo observations over the next decade will characterize the dark matter distribution in galaxies, test theories of the nature of dark matter, and illuminate the role of dark matter in galaxy formation.


February 1 - Seminar

Daniel Scolnic (University of Chicago)
Measuring Dark Energy With Supernovae and Kilonovae

The next decade will be the golden age of cosmology with transients. In this talk, I will present new analyses of Type Ia Supernovae that mark the most precise measurement of dark energy to date. I will go over how this analysis ties together with the analysis of the local value of the Hubble constant, for which tension persists with the inferred value from the CMB - an exciting hint at possible departures from the standard cosmological model. I will then discuss the first measurements of the Hubble constant with kilonovae and gravitational waves. I will review the large amount of overlap between the issues that must be tackled for future progress using supernovae and kilonovae to measure cosmological parameters. Finally, I will discuss the roles that surveys like LSST and WFIRST will play and how we can harness the millions of transients discovered to make generation-defining cosmological measurements.


February 13 - Seminar

Eli Visbal (Flatiron Institute)
The First Billion Years of the Universe

Upcoming data from instruments such as JWST, large ground-based telescopes, and future 21cm experiments will soon open a new window into the early Universe. These observations will allow us to address a number of exciting questions related to the first billion years after the Big Bang: What were the properties of the first stars and galaxies? How did the first supermassive black holes form? How and when did reionization of neutral hydrogen in the intergalactic medium occur? In this talk, I will discuss some of the theoretical work required to interpret future observations and answer these questions. I will begin by describing models of the large-scale distribution and abundance of the first metal-free stars and how these models can be utilized to make predictions for a range of different observables. I will then describe how the existence of billion solar mass supermassive black holes only a billion years after the Big Bang presents an interesting puzzle and discuss one possible solution, the formation of massive (~100,000 solar mass) black hole seeds formed through direct collapse of gas in pristine dark matter halos. I will conclude by discussing galaxy intensity mapping, a promising new observational technique to study cosmology, galaxy evolution, and reionization by measuring large-scale spatial fluctuations in galaxy line emission.


February 15 - Seminar

J. Colin Hill (Institute for Advanced Study)
New Information in Ancient Photons: Novel Approaches to CMB Secondary Anisotropies

Studies of the cosmic microwave background (CMB) radiation have driven the current era of precision cosmology. The tightest cosmological constraints to date have been derived from the primary CMB anisotropies, which predominantly probe the universe in its infancy. However, CMB experiments have recently entered a new regime in which constraints derived from the secondary anisotropies - sourced by effects between our vantage point and the surface of last scattering - substantially improve upon those derived from the primary anisotropies alone. Moreover, the secondary anisotropies contain valuable astrophysical information about the distribution of baryons and dark matter at late times. I will describe new approaches to extract information from these signals, focusing in particular on the kinematic Sunyaev-Zel'dovich effect, which refers to the Compton-scattering of CMB photons off free electrons with non-zero bulk momentum along the line-of-sight. I will show how I have used this effect to probe the abundance of ionized gas in and around modern-day galaxies and thereby resolve the long-standing "missing baryon problem." I will conclude with a look ahead to such measurements with the Advanced Atacama Cosmology Telescope and Simons Observatory, which will measure CMB secondary anisotropies - and search for primordial gravitational waves - with unprecedented sensitivity.


February 20 - Seminar

Chihway Chang (University of Chicago)
Cosmic Surveys in the Next Decade: Mapping the Landscape of the Universe

Cosmology in the next decade will be driven by data. Exploiting the information one can extract from the ongoing and upcoming large surveys will give us the power to stress-test the LCDM model with unprecedented precision and open up windows for new physics. In this talk I will present some of our work in the Dark Energy Survey Collaboration and the Large Synoptics Survey Telescope Dark Energy Science Collaboration, to analyse state-of-the-art galaxy survey data as well as getting ready for the next generation of data. I will focus on topics surrounding weak lensing analyses, including cosmology from 2-point functions, generating weak lensing mass maps, and measuring the mass profiles at the outskirts of galaxy clusters.


February 22 - Seminar

Jacob Simon (University of Colorado)
The Nature of Planet Formation

One of the single most important questions in astrophysics is how planets and planetary systems are born out of their parent protoplanetary disks. With parallel advances in computational astrophysics and observational capabilities, now is the quintessential time for unraveling the mysteries of planet formation. In this talk, I will present my vision for combining observational and theoretical tools to determine how planetary systems form. I will begin by describing how a new generation of simulations of protoplanetary disk dynamics has led to a paradigm shift in disk evolution and how we are using observations with ALMA to test and refine this paradigm. I will then discuss theoretical work on the formation of planetesimals within the gas disk environment implied by observations and how the predicted properties of these planetesimals compare with those observed in the asteroid and Kuiper Belt populations. I will conclude with an outlook for protoplanetary disk and planet formation studies, including future efforts to use dynamical models of planet formation to explain the wide variety of exoplanet systems that we observe.


March 21 - Colloquium

Rebecca Surman (Notre Dame)
Forging the heaviest elements

The origin of the heaviest elements has long been one of the greatest mysteries of nuclear astrophysics. The only known means to synthesize nuclei up to uranium and thorium is rapid neutron capture, or r-process, nucleosynthesis, and exactly where and how the r-process occurs has remained uncertain for decades. Recently disparate lines of evidence---from astronomical observations, modeling of galactic chemical evolution and individual astrophysical events, neutrino and nuclear experiment and theory, and gravitational wave detections---appear to be converging on a preferred site of production: neutron star mergers. Here we will review the available evidence and discuss the role nuclear physics can play in a definitive resolution to this mystery.


March 22 - Seminar

Brett Salmon (STScI)
Improving Our View of the z>6 Universe with Dust Laws, Lensed Galaxies, and a Candidate Spatially Resolved Arc at z~10

The most distant galaxies known are at z~10-11, observed 400-500 Myr after the Big Bang. The few z~10-11 candidates discovered to date have been exceptionally small --- barely resolved, if at all, by the Hubble Space Telescope. In this talk I will present the discovery of SPT0615-JD, a fortuitous z~10 galaxy candidate stretched into an arc over 2.5" by the effects of strong gravitational lensing. Discovered in the Reionization Lensing Cluster Survey (RELICS) Hubble and Spitzer program, this candidate has a lensed H-band magnitude of 25.7 AB mag and lensing magnification of 4-7. The unprecedented lensed size of this z~10 candidate offers the potential for the James Webb Space Telescope to study the geometric and kinematic properties of a galaxy observed 500 Myr after the Big Bang. I will also present the sample of bright, lensed galaxy candidates at z>6 found in RELICS that include other rare high-z arcs. Finally, I will discuss how we can improve our estimates of high-z star formation rates by constraining the shape of the dust-attenuation law in individual galaxies.


April 5 - Seminar

Esra Bulbul (Harvard-Smithsonian CfA)
Clusters of Galaxies: Laboratories for Probing the Properties of Baryons and Dark Matter

As the most massive collapsed structures in the Universe, galaxy clusters are unique laboratories for studying the evolution of baryons in concert with dark matter particles, as well as for exploring the nature of dark matter. In this talk, I will highlight recent efforts to understand the nature of dark matter and evolution of metals utilizing high spectral resolution X-ray observations of clusters of galaxies. In addition, I will summarize the current state of galaxy cluster astrophysics and briefly discuss the potential of next-generation X-ray telescopes.


April 12 - Seminar

Jane Rigby (NASA/GSFC)
Galaxy Evolution in High Definition Via Gravitational Lensing

In hundreds of known cases, "gravitational lenses" have deflected, distorted, and amplified images of galaxies or quasars behind them. As such, gravitational lensing is a way to "cheat" at studying how galaxies evolve: lensing can magnify galaxies by factors of 10--100 times, transforming them from objects we can barely detect to bright objects we can study in detail. For such rare objects, we are studying how galaxies formed stars at redshifts of 1--4, the epoch when most of the Universe's stars were formed. For lensed galaxies, we can obtained spectral diagnostics that are currently unavailable for the distant universe, but will become routine with next-generation telescopes. Two programs using this technique are Megasaura and TEMPLATES.

Megasaura, the Magellan Evolution of Galaxies Spectroscopic and Ultraviolet Reference Atlas, is a set of high signal-to-noise, medium spectral resolution spectra of 15 extremely bright gravitationally lensed galaxies at redshifts of 1.6 < z < 3.6, including many of the brightest lensed galaxies known. The Megasaura spectra reveal a wealth of spectral diagnostics: absorption from the outflowing wind; nebular emission lines that will be key diagnostics for JWST, GMT, and TMT; and photospheric absorption lines and P Cygni profiles from the massive stars that power the outflow.

TEMPLATES is an approved Early Release Science (ERS) program for the James Webb Space Telescope, that will characterize the nature of star formation, including star formation hidden by dust, in four extremely bright lensed galaxies with a range of extinction.


April 19 - Seminar

Vasily Belokurov (Cambridge University)
Gaia and the Galactic halo

In this talk, the Gaia data is used to study the properties of the Milky Way stellar halo. I will present new constraints on the halo spin, the halo accretion history, measurements of the tangential motion of the largest halo substructures such as the Sgr stream and the Monoceros Ring, the detection of multiple new streams in the Solar neighbourhood as well as the discovery of new faint satellites. All of this with the Gaia DR1. Imagine what you will be able to do with the Gaia DR2 in less than one week.


April 25 - Colloquium

David Charbonneau (Harvard University)
The Terrestrial Planets of Other Stars

When exoplanets transit their parent stars, we are granted an unparalleled opportunity to detect their presence and study their bulk properties. I will review recent analyses of data from the NASA Kepler Mission to deduce the rate of occurrence of small planets. I will then present follow up measurements of the acceleration of a subset of the host stars, which allows us to estimate the planetary masses and hence densities. Finally, I will discuss recent findings from the MEarth Project, which seeks to discover the most spectroscopically accessible terrestrial exoplanets. Our recent discovery of a temperate rocky world orbiting the nearby small star LHS1140 provides an unprecedented opportunity to detect the molecules present in the atmosphere of a terrestrial exoplanet.


April 26 - Seminar

Ben Wandelt (Flatiron Institute, Sorbonne University, Princeton University)
How to learn from cosmological data

The mysteries of the cosmic beginning, gravitational clustering, and cosmic acceleration persist. How can we distill cosmological information from the next generation of data sets? Taking examples from the cosmic microwave background, large scale structure, and supernova cosmology, I will discuss inference strategies, artificial intelligence, and computational approaches that promise to extract more information from current and upcoming data sets. The philosophy is to allow maximum freedom to design realistic forward models, to be robust to systematic nuisances, to accurately combine multiple probes, move beyond simplistic likelihood assumptions, naturally allow quantitative model comparison, characterize tensions in the data, and maintain (near-)optimality whenever possible.