Astrophysics Seminar and Colloquium Abstracts
Spring 2017

January 26 - Seminar

Carles Badenes (University of Pittsburgh)
Neutrons in Type Ia Supernovae: the most dangerous and terrifying particles in the Universe!

Despite decades of continuing observational and theoretical efforts, the identity of the progenitor systems of Type Ia Supernovae (SN Ia) remains obscure. Recent results have added to the controversy about the nature of the binary companion of the exploding white dwarf, which must be either another white dwarf (double degenerate systems, DD) or a non-degenerate star (single degenerate systems, SD). On the one hand, there are no clear signs of dynamical interaction between SN ejecta and circumstellar material, which seems to favor DD systems. On the other hand, the abundances of neutron-rich isotopes in the ejecta suggest that at least some exploding SN Ia have masses very close to the Chandrasekhar limit, which is more naturally explained by the SD scenario. I will describe recent X-ray observations of Type Ia Supernova Remnants that can shed light on the properties of SN Ia progenitors, the SD vs. DD debate, and its implications for the advent of precision cosmology in the next decade.


February 2 - Seminar

Romeel Davé (University of the Western Cape)
A Multi-Scale Approach to Modeling the Baryon Cycle

Galaxy growth is driven by a complex interplay of gravity, dissipational processes, star formation, and energetic feedback from massive stars and black holes. Understanding this so-called baryon cycle that connects galaxies and their surrounding cosmic ecosystem within a modern structure formation context is a central challenge in galaxy formation theory today. I will describe two large-scale approaches to understanding the physics of the baryon cycle: the Mufasa Project, a new suite of cosmological hydrodynamic simulations that yields among the most realistic descriptions of the galaxy population to date; and the Equilibrium Model, an analytic framework that encapsulates the baryon cycle in a simple yet remarkably effective MCMC-based formalism to provide valuable constraints on inflow and outflow processes. I will highlight some key physical insights and generic properties for how the baryon cycle must operate within and around galaxies in order to yield the galaxy population as observed across cosmic time.


February 9 - Seminar - postponed due to snow

Mathew Madhavacheril (Princeton University)
Cosmic Lever Arms with CMB Lensing

The cosmic microwave background (CMB) provides a snapshot of the universe at a redshift of around 1100. At the same time, it acts as a backlight that is gravitationally lensed by structure that formed at low redshifts. CMB fluctuations are therefore especially sensitive to physics that becomes influential at late-times such as dark energy and neutrino mass. Recent measurements from the Planck satellite indicate a tension between the amplitude of structure measured at low redshifts and that expected from the primary CMB alone. Ongoing high-resolution CMB experiments like the Atacama Cosmology Telescope (ACT) and planned experiments including the Simons Observatory and CMB Stage IV will significantly improve the precision on cosmological parameters and clarify these tensions. I will survey the status and prospects of using CMB lensing in conjunction with low-redshift optical and spectroscopic probes to constrain the properties of neutrinos and dark energy, focusing on recent results from the ACTPol experiment.


February 16 - Seminar

Tjitske Starkenburg (Simons Center for Computational Astrophysics)
Dark influences: the impact of dark satellites on dwarf galaxies

The structure in our Universe is thought to have formed in a hierarchical fashion. This means that galaxies, from small to large, grow through accretion, and through interactions and mergers with other galaxies. Within the concordance LCDM cosmological framework, galaxies are embedded in dark matter halos, but the vast majority of the smallest halos have likely been unable to host a galaxy. Detecting these small, star-less but very numerous halos is important as it may provide novel clues to the nature of the dark matter. Indirect detection may be possible through their gravitational effects on luminous matter.

In this talk, I present a suite of simulations studying the effects of dark satellites on dwarf galaxies. We find that these interactions can lead to the formation of irregular, starbursting, and spheroidal systems. Furthermore, we characterize the effects of the dark satellites quantitatively through morphological and kinematical indicators to aid their observational identification. Our models predict that a significant fraction of the dwarf galaxies at the present day likely has recently experienced such an interaction. Therefore, interactions with smaller dark matter halos may well contribute to the diversity of the dwarf galaxy population.


February 23 - Seminar

Jia Liu (Princeton University)
Weak lensing in the nonlinear regime

Within the next decade, galaxy and CMB lensing datasets of unprecedented precision will come online from large surveys (LSST, Euclid, WFIRST, AdvACT, SPT-3G, CMB-S4, etc.). These surveys will be sensitive to structure evolution in the strongly nonlinear regime, warranting the study of higher-order (non-Gaussian) statistics that contain information beyond the traditional second-order statistics. In this talk, I will present our recent study of one simple non-Gaussian statistic, the lensing peaks –– their origin (1606.01318), application to galaxy lensing data (1412.0757), and forecasts for CMB lensing (1608.03169).


March 2 - Seminar

Keith Hawkins (Columbia University)
Galactic Archaeology with Spectroscopy and Gaia

One of the key objectives of modern astrophysics is to understand the formation and evolution galaxies. Dissecting the assembly history of the Milky Way, which is a fantastic testing ground for our theories of galaxy formation, requires a detailed mapping of the structure, dynamics, chemical composition, and age distribution of its stellar populations. We have entered an era of large spectroscopic and astrometric surveys, which has begun to pave the way for the exciting advancements in this field. Combining data from the many multi-object spectroscopic surveys, which are already underway, and the rich dataset from Gaia will undoubtedly be the way forward in order to disentangle the full chemo-dynamical history of our Galaxy. In this talk, I will discuss my current work in Galactic archaeology and how I have used large spectroscopic surveys to dissect the structure of our Galaxy. I will also explore the future of Galactic archaeology through chemical cartography.


March 9 - Seminar

Ben Shappee (Carnegie Observatories and Princeton University)
The All-Sky Automated Survey for Supernovae

For the first time, the entire visible sky is being surveyed for the violent, variable, and transient events that shape our universe. To accomplish this, my collaborators and I built the All-Sky Automated Survey for Supernovae (ASAS-SN), which is a long-term project to monitor the whole sky, at a high cadence, using a global network of robotic telescopes. The primary goal of ASAS-SN is to find the closest and brightest supernovae (SNe) with an unbiased search: ASAS-SN now discovers about two-thirds of all bright (V<17 mag) supernovae. These nearby supernovae are critical in studying the physical nature of their progenitor systems because we can study them in unprecedented detail across the electromagnetic spectrum which cannot be done for their more distance counterparts. However, this systematic all-sky technique also allows ASAS-SN to discover many other interesting galactic and extragalactic transients. During this talk, I will give an overview of the ASAS-SN survey and highlight some of our more interesting discoveries. These discoveries include ASASSN-15lh, likely the most luminous supernovae ever discovered; ASASSN-14lp, one of the earliest observed Type Ia supernovae; ASAS-SN16ae, the largest (Delta V > 11 mag) and second-ever L-dwarf flare; and ASASSN-14ae, ASASSN-14li, and ASASSN-15oi, the three brightest tidal disruption events discovered in the optical. These discoveries, however, are just the beginning. In 2017 ASAS-SN will more than double in size, allowing us to survey the visible entire sky with better than a daily cadence while being more resistant to weather.


March 23 - Seminar

Susan Clark (Columbia University)
Using neutral hydrogen to measure cosmic magnetism and CMB foregrounds

Sensitive, high-resolution observations of Galactic neutral hydrogen (HI) reveal an intricate network of slender linear features. Across the high Galactic latitude sky, this HI is aligned with the magnetic field as traced by both starlight polarization (Clark et al. 2014) and Planck 353 GHz polarized dust emission (Clark et al. 2015). The structure of the neutral interstellar medium is more tightly coupled to the magnetic field than previously known. At high Galactic latitudes, where the Planck data are noise-dominated, the HI data provide an independent constraint on the Galactic magnetic field orientation, and hence the local dust polarization angle. The HI data thus provide a new tool in the search for inflationary gravitational wave B-mode polarization in the cosmic microwave background, which is currently limited by foreground dust contamination. I will discuss this polarized foreground problem as an opportunity: the search for primordial cosmological signals is now inextricably tied to our understanding of the magnetized interstellar medium.


March 30 - Seminar

Anthony Pullen (New York University)
Enhancing Cosmic Probes of Gravity with Weak Lensing

We explore the potential to use weak lensing to improve probes of gravity on cosmological scales . We first consider using CMB lensing and galaxy surveys to probe E_G, the ratio between curvature and velocity perturbations. This quantity is independent of galaxy clustering bias and is distinct for various gravity models, breaking the degeneracy in current cosmological probes of gravity and dark energy. We present our constraints to E_G using CMB data from Planck and galaxy data from the SDSS BOSS survey, which are in tension with general relativity (GR). We also forecast gravity constraints for upcoming galaxy and CMB surveys, as well as mention potential constraints using intensity mapping (IM). Finally, we discuss our current work to improve forthcoming constraints on gravity.


April 6 - Seminar

Lauren Anderson (Simons Center for Computational Astrophysics)
Improving Gaia Parallaxes with a Data-Driven Model of the Color-Magnitude Diagram

The Gaia TGAS Catalog contains more than 2 million parallax measurements. Converting a noisy parallax measurement into a posterior belief over distance requires inference with a prior. Usually this prior represents some kind of belief about the Milky Way. However, there is also multi-band photometry for the TGAS stars from imaging surveys, and this imaging is incredibly informative about stellar distances. Here we use color information of TGAS stars from 2MASS to build a noise-deconvolved empirical prior distribution of stars in color–magnitude space. This data-driven model contains no knowledge of the physics of stellar interiors or photospheres, nor of the Milky Way, but rather derives its precision from its generative model of noisy parallax measurements and an assumption of stationarity. This prior is used to perform parallax and distance inferences for every star, yielding a more precise stellar parallax estimate and uncertainty (and full posterior) than the TGAS catalogue. We independently validate our distances by looking at members of the Milky Way star cluster M67. This star cluster is not visible in the TGAS parallax estimates, but appears clearly in our posterior parallax estimates. This is a proof of concept with Gaia DR1 which can be applied to future data releases containing larger samples of both precise and noisy parallax measurements.


April 13 - Seminar

Anja von der Linden (Stony Brook University)
Weighing the Giants: Cluster Masses and Cosmology

Surveys of galaxy clusters provide a sensitive probe of cosmology by measuring the evolution of the halo mass function. However, already current cluster surveys are systematically limited by uncertainties in the relation between cluster mass and observables (e.g. X-ray luminosity, cluster richness, SZ decrement). Cluster weak lensing is the most promising observational method to calibrate the mass scaling to the required precision, but requires the control of systematic errors to a few percent each. In the "Weighing the Giants" project, we carefully investigated and quantified all sources of systematic uncertainty, resulting in accurate weak lensing masses for 51 clusters. We use these measurements to improve the precision of cosmological constraints from X-ray selected clusters by a factor of two. Already from a sample of ~200 clusters selected from the ROSAT All-Sky Survey, we place some of the tightest, most robust constraints on a number of cosmological parameters, including the dark energy equation of state, neutrino masses, and modified gravity. Furthermore, we show that when adopting the "Weighing the Giants" mass scale, the results from Planck CMB temperature anisotropies and Planck cluster counts are consistent without invoking the need for new physics. These results bode extremely well for future cluster surveys. In particular, I will show how the "Weighing the Giants" work lays out the path for LSST to become a key cornerstone for cluster experiments in the next decade.


April 19 - Colloquium

Anna Frebel (Massachusetts Institute of Technology)
Observing the signature of a single prolific r-process event in an ultra-faint dwarf galaxy

The heaviest chemical elements in the periodic table are synthesized through the rapid neutron-capture (r-) process but the astrophysical site where r-process element nucleosynthesis occurs is still unknown. The best candidate sites are ordinary core-collapse supernovae (deaths of massive stars) and mergers of two orbiting exotic neutron stars. 13 billion year old small dwarf galaxies preserve a "fossil" record of early chemical enrichment that provides the means to isolate and study clean signatures of individual nucleosynthesis events. Based on new spectroscopic data from the 6.5m Magellan Telescope, we found seven stars in the recently discovered dwarf galaxy Reticulum II that show extreme overabundances of these heavy r-process elements. This "r-process" enhancement implies that the r-process material in Reticulum II was synthesized in a single prolific event. Our results are clearly incompatible with yield predictions from an ordinary core-collapse supernova but instead consistent with that of a neutron star merger. This first signature of a neutron star merger in the early universe holds the key to finally, after 60 years, identifying the cosmic production site of the r-process.


April 26 - Colloquium

Hakeem Oluseyi (Florida Institute of Technology and NASA)
Star Hacking – Finding the Next Earth and Getting There

In modern times, the word hack has come to mean repurposing something in new or creative ways in order to gain benefit or cleverly solve a tricky problem. It is now common in astrophysics to hack stars in order to further human understanding of the universe or invent new technologies. We can use stars to discover new planets, to address the structure and evolution of the Galaxy, or even to develop new technologies. In this talk, I will describe my recent work "star hacking.” Two main projects are being undertaken. The first is a collaboration with the Transiting Exoplanet Survey Satellite (TESS) science team to discover new Earth-like planets around nearby stars. TESS will also observe and discover a large number of variable stars, which may be used to elucidate Galactic structure and evolution. I will also describe a novel magnetic reconnection based, ion drive in-space propulsion technology that my group has invented, which was inspired by plasma acceleration at the Sun's surface.


April 27 - Seminar

Pieter van Dokkum (Yale University)
Exploring the low surface brightness sky with the Dragonfly Telephoto Array

The talk will describe a new telescope concept, optimized for low surface brightness imaging. The Dragonfly Telephoto Array consists of 48 high-end telephoto lenses, creating the equivalent of an f/0.4 refractor with a 1m aperture. Early results from Dragonfly include a measurement of the stellar halos around nearby spiral galaxies down to ~32 mag/arcsec^2 and the identification of new satellite galaxies. Perhaps the most spectacular result so far is the discovery of a population of Milky Way-sized, very low surface brightness galaxies in rich clusters. A follow-up study of one of these "Ultra Diffuse Galaxies" shows that it is massive and extremely dark matter dominated, with a dark matter fraction of 98% inside its effective radius.