Astrophysics Seminar and Colloquium Abstracts
Fall 2017

September 14 - Seminar

Mi Dai (Rutgers)
Supernova cosmology in the era of LSST

Type Ia supernovae (SNe Ia) have been playing an important role in discovering and constraining dark energy -- the unknown cause behind the observed cosmic acceleration. The field of SN cosmology remains exciting, and challenging, as the number and quality of SNe increase dramatically in the era of LSST (and other next-generation surveys). On the one hand, it is important to characterize the systematic effects better as they will become dominant over the statistical uncertainties. On the other hand, it will be impractical to obtain SN types and redshifts via spectroscopic follow-up. I will discuss my thesis work that focuses on these two aspects of SN cosmology. I will describe a method that samples the SN Ia light curve parameter probability distributions in a cosmological analysis, and an approach for SN photometric classification and redshift estimation.


September 28 - Seminar

Chuck Steidel (Caltech)
Reconciling (and using) the stellar and nebular emission of high-redshift galaxies

Massive stars produce the most readily-observed signatures of forming galaxies at high redshift -- the FUV stellar continuum and the nebular/recombination emission excited by the stellar EUV radiation field. Encoded in these spectra, which can be observed from the ground in the observed-frame optical and near-IR, are details of the stellar and gas-phase chemical abundances, the physical conditions in the galaxy ISM, and the nature of the massive stellar populations. Most of the efforts to understand the rapidly-improving observations of high-redshift star-forming galaxies have used low-redshift samples both as statistical baseline and to establish calibrations for extracting physical insight from comparatively crude data such as that available at high redshift. I will discuss why this type of approach is dangerous, and may lead to incorrect conclusions for galaxies observed at high redshift. Using data obtained as part of the Keck Baryonic Structure Survey (KBSS), I will show that the simultaneous analysis of FUV stellar and FUV/optical nebular spectra of the same galaxies can be used successfully to "close the loop" with internally consistent models of high-redshift galaxies -- without reference to low redshift, where the conditions that once prevailed at high redshift have become extremely rare. I will discuss what one can hope to measure reliably, and what will remain challenging, using current and future surveys based on rest-frame UV/optical spectra of high redshift galaxies -- which will continue to be our principal route to understanding galaxy formation in the JWST era.


October 4 - Colloquium

Josh Winn (Princeton)
The architecture of exoplanetary systems

The basic geometry of the Solar System -- the shapes, spacings, and orientations of the planetary orbits -- has long been a subject of fascination as well as inspiration for planet-formation theories. For exoplanetary systems, those same properties have only recently come into focus. I will review our current knowledge of the occurrence of planets around other stars, their orbital distances and eccentricities, the orbital spacings and mutual inclinations in multiplanet systems, the orientation of the host star's rotation axis, and the properties of planets in binary-star systems. I will also discuss opportunities to improve our understanding with data from the upcoming TESS space mission.


October 5 - Seminar

Lou Strolger (STScI)
The rates of supernovae, far and near

For nearly two decades, the Hubble Space Telescope has been heavily used to locate supernovae in high-redshift environments, with the primary goal of improving constraints on the nature of dark energy. Along the way we have made surprising observations on the nature of supernovae themselves, and clues to their elusive progenitor mechanisms, some of which are difficult to reconcile with observations at much lower redshift. From complete volumetric supernova rate histories that now extend to z > 2 we find type Ia supernova delay-time distributions are consistent with a power law index of -1, but with the fraction of prompt (td < 500 Myr) sources less than expected from various ground-based surveys. Core collapse supernova rates trace the cosmic star formation rate history, but require stellar progenitors more massive than have been seen in deep studies of nearby events (M > 20 M). I'll detail some interesting discoveries from our recent campaigns on clusters of galaxies, and also discuss what we expect to find with the James Webb Space Telescope, launching in 2018, and with WFIRST in the 2020s.


October 12 - Seminar

Kathleen Eckert (Penn)
The mass functions of galaxies and galaxy groups in the RESOLVE survey

There are several outstanding problems facing our understanding of galaxy formation and evolution, including the apparent shortfall of observed dwarf galaxies compared to theoretical expectations. The galaxy mass function is often used to characterize the galaxy population for comparison with the halo mass function from simulations of dark matter. Although galaxy mass is typically defined by the stellar mass, for low-mass galaxies, the cold atomic gas can be the dominant observable mass component. Thus, I will present both stellar and cold-baryonic (stars + cold atomic gas) mass functions of galaxies and galaxy groups in the RESOLVE and ECO surveys, two volume-limited galaxy surveys that are complete for galaxies with cold baryonic mass > 109 M. I will focus on several key results from my thesis, including that the galaxy cold-baryonic mass function rises with a steeper low-mass slope than the stellar mass function and that the galaxy mass function is dependent on group halo mass, suggesting that group formation processes potentially cause the shortfall of observed galaxies.


October 19 - Seminar

Adrian Price-Whelan (Princeton)
Very wide binaries and comoving stars from the Gaia mission

Multiplets of conatal, coeval stars are important tracers of kinematics, stellar models, and star and planet formation processes in the Galaxy. The disruption of these systems is sensitive to the clumpiness of the Galactic gravitational field, their spectra can be used to calibrate stellar models at fixed age and chemical abundances, and changes or differences in their surface abundances likely relate to the stability and mass in planetary systems. Open cluster stars are the gold standard for such studies because they are easily identifiable and contain stars that span a range of masses at fixed birth conditions. However, there are few well-understood open clusters, the dynamic range in chemical abundances is small, and they typically have young ages. Widely-separated binary stars are far more numerous, span a larger range in age and chemistry, and are now readily identifiable with precise astrometry from the Gaia mission. I'll talk about our group's efforts to identify large samples of widely-separated binary stars in anticipation of (1) using their kinematics to study the gravitational field of the Milky Way, and (2) using their chemical abundances to study star formation and the outcomes of planet formation. I'll also highlight an interesting wide binary discovered along the way that appears to have had a catastrophic rocky accretion event.


October 26 - Seminar

Meredith Hughes (Wesleyan)
Planet formation through radio eyes

Circumstellar disks provide the raw material and initial conditions for planet formation. Millimeter-wavelength interferometry is a powerful tool for studying gas and dust in planet-forming regions, and it has recently undergone an immense leap in sophistication with the advent of the ALMA interferometer. I will discuss some ways in which millimeter-wavelength interferometry is being used to study the process of planet formation in circumstellar disks, with particular emphasis on the kinematics of turbulence in protoplanetary disks and the degree to which debris disk structure reflects the dynamics of embedded planetary systems.


November 2 - Seminar

Daniel Grin (Haverford)
Ultra-light axions and the cosmic microwave background

The dark matter holding together galaxies and the dark energy driving today's cosmic acceleration are persistent mysteries. In contrast to the prevailing WIMP paradigm, I will examine the possibility that dark matter is composed of a hypothetical ultra-light particle, called an axion, whose quantum mechanical properties are manifest on much larger length scales than WIMPs. I'll begin by summarizing the particle physics that first lead to the axion hypothesis. By changing the expansion history of the universe and the clustering properties of dark matter, axions could have a detectable presence in astronomical data, changing the properties of the microwave afterglow of the Big Bang the distribution of galaxies today. I'll discuss the physics of these imprints and the work of collaborators and me to test for the presence of axions using Planck cosmic microwave background data, as well as data from galaxy surveys. I'll present constraints from completed experiments, survey the promise of upcoming/hoped-for experiments, and conclude by explaining what axions might have to do with the physics of inflation, the exponential growth of the universe during the first 10-32 seconds of cosmic time.


November 9 - Seminar

Rachael Beaton (Princeton)
Engineering the measurement of the Hubble Constant

The local expansion rate of the Universe, the Hubble Constant, is one of the fundamental parameters in our current concordance cosmology and one that anchors the expansion history of the Universe. The resolution of the historical factor-of-two controversy in the Hubble Constant nearly two decades ago (e.g., the Hubble Space Telescope Key Project; Freedman et al. 2001) has evolved into a 3.4σ tension between the traditional Cepheid-distance ladder measurements (Riess et al. 2016; Freedman et al. 2012; Freedman et al. 2001) and that determined from modelling anisotropies in the cosmic microwave background (CMB; Planck Collaboration et al. 2016). At the heart of the tension is not only a difference in method, but also a fundamental difference in the state of the observed Universe: the distance ladder measures the local rate in the nearby universe (e.g., z ~ 0), whereas the CMB anisotropy measurements uses the very young Universe (z ~ 1100). Resolution of the tension requires (i) a full scale evaluation of the systematic effects in either technique or "new physics" added to the standard cosmological model. The trigonometric parallaxes provided by Gaia in the near term permit an unprecedented opportunity to use alternative standard candles and construct a full end-to-end distance ladder without Cepheids. The Carnegie-Chicago Hubble Program is doing just that; we are in the middle of building a new distance ladder that relies on the tip of the red giant branch (TRGB; Beaton et al. 2016). As I will demonstrate, this not only provides a direct cross-check on the Cepheid path, but there are numerous advantages to using a distance indicator that, as a standard candle from old stellar populations, is nearly ubiquitously present in low-crowding and low-extinction components of galaxies. More specifically, by being able to calibrate every "local" SNe Ia and easily probing ever-larger volumes with JWST and WFIRST, the TRGB-based distance ladder paves a clear path to a 1% measurement within the foreseeable future.


November 15 - Colloquium

Brian Metzger (Columbia)
The multi-messenger picture of a neutron star merger

On August 17 the LIGO/Virgo gravitational wave observatories detected the first binary neutron star merger event (GW170817), a discovery followed by the most ambitious electromagnetic (EM) follow-up campaign ever conducted. Within 2 seconds of the merger, a weak burst of gamma-rays was discovered by the Fermi and INTEGRAL satellites.  Within 11 hours, a bright but rapidly-fading thermal optical counterpart was discovered in the galaxy NGC 4993 at a distance of only 130 million light years.  The properties of the optical transient match remarkably well predictions for "kilonova" emission powered by the radioactive decay of heavy nuclei synthesized in the expanding merger ejecta by rapid neutron capture nucleosynthesis (r-process).  The rapid spectral evolution of the kilonova emission to near-infrared wavelengths demonstrates that a portion of the ejecta contains heavy lanthanide nuclei.  Two weeks after the merger, rising non-thermal X-ray and radio emission were detected from the position of the optical transient, consistent with delayed synchrotron afterglow radiation from an initially off-axis relativistic jet with the properties resembling those of (on-axis) cosmological gamma-ray bursts.  I will describe a unified scenario for the range of EM counterparts from GW170817 and their implications for the astrophysical origin of the r-process and the properties of neutron stars (particularly their uncertain radii and maximum mass, which are determined by the equation of state of dense nuclear matter).  Finally, I will preview the upcoming era of multi-messenger astronomy, once Advanced LIGO/Virgo reach design sensitivity and a neutron star merger is detected every few weeks.


November 16 - Seminar

Marc Favata (Montclair State)
Highlights of LIGO's second observing run

LIGO has had an exciting year. Some highlights include more black hole mergers, the first detections with a network of three interferometers, and the first detection of a binary neutron star merger (which was seen by a few other telescopes as well). I will give a partial overview of this science and prospects for the future.


November 30 - Seminar

Mathew Madhavacheril (Princeton)
Neutrino mass from CMB lensing and cross-correlations

High-resolution cosmic microwave background experiments are beginning to provide ever more sensitive maps of the matter distribution in the Universe through its gravitational lensing signal. Understanding and mapping this signal will help us determine the properties of neutrinos as well as reveal possible signatures of primordial gravitational waves. I will survey the present status of CMB lensing measurements, on both cluster and cosmological scales, focusing on recent results from the Atacama Cosmology Telescope. I will also discuss the prospects for measuring neutrino mass with future surveys like the Simons Observatory and CMB-Stage-4, and the challenges that need to be met to make accurate inferences in the presence of astrophysical foregrounds.


December 7 - Seminar

Luke Hovey (Los Alamos)
Constraints on cosmic-ray acceleration efficiency in Balmer shocks of two young Type Ia supernova remnants in the Large Magellanic Cloud

There now exist multiple lines of evidence that shocks in supernova remnants (SNRs) are the dominant accelerators of ~90% of cosmic rays (CRs) up to the "knee" of the CR spectrum. The hunt for direct evidence of efficient CR acceleration in the blast-waves of remnants has thus far proved elusive, despite the detection of gamma-ray emission from a number of such sources. I will present my work on constraining the efficiency of CR acceleration in Balmer-dominated shocks of two of the youngest Ia supernova remnants in the Large Magellanic Cloud and how those values compare to the Tycho SNR, where there have been significant detections of gamma-rays. I will also highlight my future work that will lead to direct measurements of CR acceleration efficiencies with additional spectropolarimetry observations of the remnants.