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Astrophysics Seminars From 2018 - 2019


Fall 2018

September 26, 2018

 "Imaging Protoplanets with Adaptive Optics and Interferometry"
Steph Sallum

Chancellor's Postdoctoral Fellow & NSF Postdoctoral Fellow
UC Santa Cruz
Host: Konopacky

 Abstract: Understanding the details of planet formation requires direct observations of protoplanets themselves. Transition disks, protoplanetary disks with inner clearings in dust, are the most promising targets for these studies. Their inner clearings and relatively low stellar accretion rates may be caused by forming planets sweeping up material that would have otherwise fallen onto the star. While protoplanets are expected to have low infrared contrasts compared to mature exoplanets, the large distances to transition disks necessitate novel imaging techniques beyond adaptive optics and coronagraphy to make these detections. Non-redundant masking (NRM), which transforms a conventional telescope into an interferometric array, is well suited for imaging protoplanets directly. I will present the results of NRM observations of transition disks, as well as strategies for disentangling accretion signals from light scattered by disk material. I will also discuss the potential for protoplanet characterization using interferometric techniques, and applications of these techniques on next generation facilities such as the Thirty Meter Telescope and James Webb Space Telescope.

October 3, 2018

 "Probing the Large-Scale Environments of Local AGN"
Meredith Powell

Graduate Student
Yale University
Host: Coil

 Abstract: I present the clustering analysis of local AGN in the Swift-BAT Spectroscopic Survey (BASS). With 548 AGN in the redshift range 0.01<z<0.1 over the full sky, BASS provides the largest and least biased sample of local AGN to date due to its hard X-ray selection (14-195 keV) and rich multiwavelength/ancillary data. By measuring the projected cross-correlation function between the AGN and 2MASS galaxies, and interpreting it via HOD and subhalo-based models, we constrain the halo occupation of the full AGN sample as well as in bins of column density an black hole mass. We find that AGN tend to reside in galaxy group environments, and that on average they occupy their dark matter halos similar to inactive galaxies of the same stellar mass distribution. We also find evidence that obscured AGN tend to reside in denser environments than unobscured AGN, even when samples were matched in luminosity, redshift, stellar mass, and Eddington ratio. I show that this can be explained either by significantly different halo occupation distributions or statistically different host halo assembly histories.

October 10, 2018

 "The Miniature X-ray Solar Spectrometer (MinXSS) CubeSats: instrument capabilities and early science analysis on the quiet Sun, active regions, and flares"
Chris Moore

Postdoctoral Fellow
Harvard-Smithsonian CfA
Host: Wright

 Abstract: Detection of soft X-rays (sxr) from the Sun provide direct information on coronal plasma at temperatures in excess of ~1 MK, but there have been relatively few solar spectrally resolved measurements from 0.5 – 10. keV. CubeSats can be a low-cost alternative to rapidly fill astrophysical observations gaps, that large missions are currently missing. The Miniature X-ray Solar Spectrometer (MinXSS) CubeSat is the first solar science oriented CubeSat mission flown for the NASA Science Mission Directorate, and has provided measurements from 0.8 -12 keV, with resolving power ~40 at 5.9 keV, at a nominal ~10 second time cadence. MinXSS design and development has involved over 40 graduate students supervised by professors and professionals at the University of Colorado at Boulder. Instrument radiometric calibration was performed at the National Institute for Standard and Technology (NIST) Synchrotron Ultraviolet Radiation Facility (SURF) and spectral resolution determined from radioactive X-ray sources. The MinXSS spectra allow for determining coronal abundance variations for Fe, Mg, Ni, Ca, Si, S, and Ar in active regions and during flares.

Measurements from the first of the twin CubeSats, MinXSS-1, have proven to be consistent with the Geostationary Operational Environmental Satellite (GOES) 0.1 – 0.8 nm energy flux. Simultaneous MinXSS-1 and Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observations have provided the most complete sxr spectral coverage of flares in recent years. These combined measurements are vital in estimating the heating flare loops by non-thermal accelerated electrons. MinXSS-1 measurements have been combined with the Hinode X-ray Telescope (XRT) and Solar Dynamics Observatory Atmospheric Imaging Assembly (SDO-AIA) to further constrain the coronal temperature distribution during quiescent times. The structure of the temperature distribution (especially for T > 5 MK) is important for deducing heating processes in the solar atmosphere. MinXSS-1 observations yield some of the tightest constraints on the high temperature component of the coronal plasma, in the absence of the intermittent solar observations from the Focusing Optic X-ray Solar Imager (FOXSI) sounding rocket and the Nuclear Spectroscopic Telescope Array (NuSTAR). MinXSS-2 is scheduled to launch in late 2018 for improved solar observations for at least a four year mission.

October 17, 2018

 "Mapping the z~2 Cosmic Web with IGM Tomography with Keck, Subaru, and TMT"
Khee-Gan Lee

Assistant Professor
Kavli IPMU (University of Tokyo)
Host: Kritsuk

 Abstract: In recent years, the CLAMATO survey on the Keck telescope, which observed high area densities of z~2-3 star-forming galaxies spectra. This created a closely-spaced grid of sightlines probing the Lyman-alpha forest in the intergalactic medium at z~2.0-2.5. I will discuss the observations which lead to the highest-redshift detection of cosmic voids and preliminary results for the cross-correlation with co-eval galaxies. Future applications include constrained realizations of the observed volume, constraining galaxy-cosmic web intrinsic alignments, and cosmological parameter measurement. In the final 15 minutes of the talk, I will talk about proposed fiber-based spectrographs for Keck (FOBOS) and TMT (*Please bring your smartphone to this talk for a virtual reality demonstration*)

October 24, 2018

 "The Precision Radial Velocity Landscape"
Arpita Roy

Millikan Postdoctoral Fellow
Host: Konopacky

 Abstract: The field of radial velocity (RV) exoplanet detection is entering a new era with the advent of next-generation instrumentation. Currently in build+commissioning phases are planet hunting optical Doppler spectrographs aiming at 10-30cm/s long-term RV precision in the quest for Earth analogs, and several major NIR facilities aiming for <1m/s that are providing a new level of scrutiny into M dwarf stars and their planets. These massive instruments leverage a range of technological advances, from high-homogeneity illumination delivery setups, to sophisticated wavelength calibration, and ultra stable environmental control. In this talk I will review the status of the field, and our readiness for TESS followup, focusing on the three extreme precision instruments I am helping develop (KPF, NEID, HPF) and including the latest updates shared from instrument teams across the world.

October 31, 2018

 "X-ray Reflection Spectroscopy of Accreting Black Holes"
Javier Garcia

Research Scientist
Host: Laha

 Abstract: In the region close to compact object such as black holes (or neutron stars), the extreme conditions created by the strong gravitational field produces copious amounts of energetic radiation (ultra-violet, X-rays, and Gamma-rays). The interaction of this radiation with the surrounding material results in observables that carry important physical information. X-ray spectral and timing techniques provide direct access to the accretion physics on these systems, such as the black hole spin, the location of the inner-edge of the accretion disk, its ionization stage and composition, among others. In this talk, I will discuss the development of modern relativistic reflection models and how they can be used for the interpretation of the X-ray spectrum from supermassive black holes in AGN and stellar-mass black holes in binary systems. I will show examples of the implementation of our new models to observational data from several X-ray observatories (e.g., RXTE, Swift, XMM-Newton, Suzaku, and NuSTAR), and discuss current outstanding issues, such as the large iron abundances frequently required to fit the reflection spectra, controversies on the disk truncation, the origin of the soft excess in AGN, and the effects of high density in the observed spectra.

November 7, 2018

 "BICEP/Keck: Constraining the primordial gravitational-wave signal with CMB polarization observations from the South Pole"
Lorenzo Moncelsi

Staff Scientist
Host: Galitzki

 Abstract: The inflationary scenario generically predicts the existence of primordial gravitational waves (GW), though over a wide range of amplitudes from slow-roll to multi-field models. Currently the most promising method for constraining, and potentially detecting, an inflationary GW background is to search for the imprint that these tensor perturbations would leave on the cosmic microwave background (CMB) polarization as a parity-odd “B-mode” pattern. The BICEP/Keck experiments (BK) target this primordial signature by observing the polarized microwave sky at degree-scale resolution from the South Pole. Attempting to observe the very faint primordial B-mode signal requires a telescope with exquisite sensitivity and tight control of systematics. The presence of bright Galactic emission, along with the distortion of the CMB polarization field due to gravitational lensing, make this measurement extremely challenging. In order to disentangle the primordial signal from these “foregrounds”, a wide frequency coverage is necessary. I will present the latest BK constraints on the tensor-to-scalar ratio “r” using data taken from 2010 to 2015 at 90, 150, 220 GHz (BK15), in combination with data from the Planck and WMAP satellites. Upcoming observations with the “Stage-3” BICEP Array experiment will extend this frequency range to 30-270 GHz, ultimately improving our sensitivity to r by an order of magnitude with respect to BK15, thus constraining natural inflation and all single-field models.

November 14, 2018

 "Cold Gas Giant Exoplanets: From Detection to Characterization"
Paul Dalba

Postdoctoral Scholar
UC Riverside
Host: Theissen

 Abstract: It is now common knowledge within the astrophysics community that the Galaxy is teeming with exoplanets. With an ever-growing sample of known exoplanets to draw from, the focus of many investigations has begun to shift from detection to characterization. However, this transition is shaped by observational biases. Limited observational baselines and detection efficiencies have so far restricted the majority of characterization efforts to exoplanets with short-period orbits that are strikingly different than the Solar System planets. Yet, the fundamental questions surrounding exoplanetary science—ones of formation, evolution, and even habitability—cannot be addressed without probing the outer reaches of planetary systems as well as the inner regions. In this talk, I will discuss the challenges facing the detection-characterization transition for cold giant exoplanets akin to Jupiter and Saturn. I will present observations dedicated to recovering long-period exoplanets originally discovered in transit and radial velocity surveys. Although somewhat risky, these efforts are necessary to prevent us from "losing track" of known exoplanets. Moving into characterization, I will present research that utilizes Solar System data sets to simulate atmospheric observations of cold giant exoplanets. This work identifies a new method of exoplanet atmospheric characterization and emphasizes the amenability of cold giant exoplanets to characterization.

November 28, 2018

 "Star Forming Clumps in Local Luminous Infrared Galaxies"
Kirsten Larson

Postdoctoral Scholar
Host: Wright

 Abstract: Local luminous infrared galaxies (LIRGS) are a mixture of single disk galaxies, interacting systems, and advanced mergers, exhibiting enhanced star formation rates and AGN activity. This makes them an ideal laboratory of studying resolved star formation in the local Universe. A number of studies have found that high redshift star forming galaxies tend to have turbulent, clumpy disks with extreme star forming clumps that are not seen in normal local galaxies. I will present the results from our HST narrow-band Paα and Paβ imaging study of 48 local LIRGs from the Great Observatories All-Sky LIRG Survey (GOALS). These data allow us to measure the star formation rates, sizes, ages, and masses of 810 spatially resolved star forming regions, and directly compare their properties to those found in both local and high-redshift star forming galaxies. I will show how the star formation rates of the clumps in local LIRGs nicely span the range of star formation rates found in normal local star forming galaxies to the clumps found in high-redshift star forming galaxies at z = 1–3. By comparing star formation in LIRGs to normal low redshift galaxies, high redshift galaxies, and sophisticated hydrodynamical simulations, we can better understand how global galaxy properties and environment influence star formation on smaller scales.

December 5, 2018

 "Update on NASA Exoplanet Exploration Program (ExEP) and Science"
Eric Mamajek

Deputy Program Scientist, Exoplanet Exploration Program
Host: Melis

 Abstract: I'll provide an update on the NASA Exoplanet Exploration Program (ExEP), the Astrophysics Division's program responsible for implementing NASA's plans for discovering and characterizing exoplanets and searching for potentially habitable worlds. One of the key recommendations of the recent NAS Exoplanet Science Strategy (ESS) report, which is providing input to the Astro2020 Decadal Survey, is for NASA to "lead a large strategic direct imaging mission capable of measuring the reflected-light spectra of temperate terrestrial planets orbiting Sun-like stars." I'll summarize some recent NASA ExEP-supported activities (since the 2010 Decadal Survey) which have made critical progress towards informing the design of mission(s) for implementing this recommendation, including 1) exoplanetary occurrence rates (Kepler/K2), 2) observational constraints on levels of exozodiacal dust around nearby stars (LBTI/HOSTS survey), and 3) advances in starlight suppression technology including coronagraphs and starshades. Time permitting - I'll discuss some research results related to the transits of young planets and searches for circumplanetary matter and the ages of exoplanetary systems.

December 12, 2018

 "Star Formation and Galactic Environment"
Eva Schinnerer

Staff Scientist
Max Planck Institute for Astronomy
Host: Sandstrom

 Abstract: High angular resolution observations of nearby galaxies allow us to sample the star formation process in different galactic environments.This provides insights on the importance and role of galactic components such as bulges, stellar bars, spiral arms and active galactic nuclei (AGN) in the conversion of cold (molecular) gas into stars. New instruments can now regularly image with high quality and sensitivity large field-of-views at the scale of individual star-forming units, namely Giant Molecular Clouds (GMCs) and HII region (complexes): ALMA is fundamental for imaging of the molecular gas properties in the star-forming disks, while the optical Integral Field Unit MUSE on the VLT is providing detailed information on the ionised gas and stellar population. I will highlight recent progress in the field and present new results from the Physics at High Angular resolution in Nearby GalaxieS (PHANGS) survey that studies a representative sample of nearby massive, normal star-forming galaxies.

Winter 2019

January 9, 2019

NOTE: Beginning this week, all Astrophysics Seminars will be held from
3:00-4:00pm unless otherwise noted
 "Obstacles and Opportunities in Gravitational-wave Observations"
Rana Adhikari

Professor of Physics
Host: Fuller

 Abstract: The recent pile of black hole mergers has signaled the birth of gravitational astronomy. Just as the Jyotisha and Gan De eventually lead to Galileo, we too can look forward to dramatic improvements in what is measurable with gravitational waves. I will describe how we can utilize coherent quantum feedback to bypass the Poisson limits in interferometry, and modern neural networks to remove terrestrial interference. This imminent increase in our astrophysical reach should allow for high precision probes of black hole horizons and high fidelity maps of the shape of the universe.

January 16, 2019

 "Status of The Thirty Meter Telescope"
Ravinder Bhatia

Associate Project Manager
Thirty Meter Telescope
Host: Keating/Wright

 Abstract: The Thirty Meter Telescope (TMT) is one of the next generation of so-called Extremely Large Telescopes, and currently the only one that will be sited in the northern hemisphere.

In this talk, I will describe the broad range of scientific goals of TMT, which take advantage of its high sensitivity, high spectral resolution and high spatial resolution. Such goals span studying the early Universe through to galactic evolution, star formation and exoplanet characterization. These science goals are being established and refined by our International Science Development Teams.

The construction and operation of the observatory is funded through an international partnership, with each partner contributing to the development and production of the observatory sub-systems. I will describe the status of those different sub-systems, highlighting the technical challenges that we need to overcome. I will also give an update on the choice of the site for TMT, given recent developments in Hawaii and Spain.

In order to strengthen the scientific motivation even further, TMT is looking to collaborate with the Giant Magellan Telescope (GMT). GMT will be located in the southern hemisphere, and this collaboration would therefore allow for access to the full sky. This initiative also aims to secure further funding for each separate telescope project: TMT, GMT the National Optical Astronomy Observatory aim to submit a joint proposal to the National Science Foundation which, if successful, would provide observing access to US astronomers that are currently not affiliated with the existing partners of TMT or GMT. I will provide an update on this initiative, which is being pursued against the backdrop of the next Decadal Survey of Astronomy and Astrophysics.

I will end the talk with a brief commentary on three dimensions of working within a complex international partnership. First is Culture. Second is Governance. And third is the broader landscape of international relations within which the TMT project is embedded.

January 23, 2019

 "Dark Matter, First Light"
Katie Mack

Assistant Professor
North Carolina State University
Host: Keating

 Abstract: Dark matter forms the foundation for all cosmic structure, and its fundamental nature is one of science's most pressing enigmas. As we search for the most distant galaxies in the universe with radio and infrared observations, we are in a position to explore the particle physics of dark matter — the possibility of annihilation, decay, or other particle interactions — through its effects on early stars and galaxies. I will give an update on the quest to identify dark matter both in the lab and in the sky, major unsolved problems in dark matter theory, and how upcoming observations of the epoch of the first cosmic structures can be used to open a new window on the dark universe.

January 30, 2019

 "The Relationship Between Galaxies and the Large-Scale Structure of the
Alison Coil

Professor of Physics
Host: N/A

 Abstract: I will describe our current understanding of the relationship between galaxies and the large-scale structure of the Universe, often called the galaxy-halo connection. Galaxies are thought to form and evolve in the centers of dark matter halos, which grow along with the galaxies they host. Large galaxy redshift surveys have revealed clear observational signatures of connections between galaxy properties and their clustering properties on large scales. For example, older, quiescent galaxies are known to cluster more strongly than younger, star-forming galaxies, which are more likely to be found in galactic voids and filaments rather than the centers of galaxy clusters. I will show how cosmological numerical simulations have aided our understanding of this galaxy-halo connection and what is known from a statistical point of view about how galaxies populate dark matter halos. This knowledge both helps us learn about galaxy evolution and is fundamental to our ability to use galaxy surveys to reveal cosmological information. I will talk briefly about some of the current open questions in the field, including galactic conformity and assembly bias. (Note that this is a plenary talk I gave at the June 2018 AAS meeting, and it is designed to be understood by graduate students.)

February 6, 2019

 "Fission and lanthanide production in r-process nucleosynthesis"
Nicole Vassh

Postdoctoral Research Associate, Department of Physics
Notre Dame University
Host: Fuller

 Abstract: The observations of the GW170817 electromagnetic counterpart last year suggested lanthanides were produced in this neutron star merger event. Lanthanide production in heavy element nucleosynthesis is subject to large uncertainties from nuclear physics and astrophysics unknowns. Specifically, the rare-earth abundance peak, a feature of enhanced lanthanide production at A~164 seen in the solar r-process residuals, is not robustly produced in r-process calculations. The proposed dynamical mechanism of peak formation requires the presence of a nuclear physics feature in the rare-earth region which may be within reach of experiments performed at, for example, the CPT at CARIBU and the upcoming FRIB. To take full advantage of such measurements, we employ Markov Chain Monte Carlo to "reverse engineer" the nuclear masses capable of producing a peak compatible with the observed solar r-process abundances and compare directly with experimental mass data. Here I will present our latest results and demonstrate how the method may be used to the learn which astrophysical conditions are consistent with both observational and experimental data. Uncertainties in the astrophysical conditions also make it difficult to know if merger events are responsible for populating the heaviest observed nuclei, the actinides. Here I will discuss a potential direct signature of actinide production in merger environments. However, an r-process which reaches the actinides is also likely to host fission, which is largely experimentally uncharted for neutron-rich nuclei. The influence of fission on lanthanide abundances, and the potential for future experimental and theoretical efforts to refine our knowledge of fission in the r-process, will be discussed. The question of where nature primarily produces the heavy elements can only be answered through such collaborative efforts between experiment, theory, and observation.

February 13, 2019

 "The Circumgalactic Medium of Star-Forming Galaxies at 2<z<3"
Gwen Rudie

Staff Astronomer
Carnegie Observatories
Host: Rubin

 Abstract: The exchange of baryons between galaxies and their surrounding intergalactic medium (IGM) is a crucial but poorly-constrained aspect of galaxy formation and evolution. I will present results from the Keck Baryonic Structure Survey (KBSS), a unique spectroscopic survey designed to explore both the physical properties of high-redshift galaxies and the connection between these galaxies and their surrounding intergalactic baryons. The KBSS is optimized to trace the cosmic peak of star formation (z~2-3), combining high-resolution spectra of hyperluminous QSOs with densely-sampled galaxy redshift surveys surrounding each QSO sightline. I will present new detailed studies of metal-enriched absorbing gas in the high-z CGM, highlighting the gas kinematics and the diversity of physical conditions found close to galaxies. I will also present new measurements of the evolution of hydrogen and carbon-bearing gas within the CGM from z~2.3 to z~0.2 which exhibit surprising trends. Collectively, these data constrain the nature and sphere of influence of galaxy-scale outflows, intergalactic accretion, and their evolution as a function of time.

February 20, 2019

 "The Wind of Change: create, move, and observe cold gas around galaxies"
Max Gronke

Postdoctoral Scholar
UC Santa Barbara
Host: Rubin

 Abstract: Galactic winds are large-scale, multiphase outflows from galaxies and crucial for the galactic ecosystem. They are, thus, a potent probe for the underlying feedback mechanisms. The usual picture is that the cold gas has been accelerated by ram pressure forces due to the hot gas. However, reproducing this ubiquitous observation in hydrodynamical simulations has proven to be challenging - simply because the destruction time is shorter than the acceleration time. During my talk, I will show some analytical estimates and results from recent (magneto-)hydrodynamics simulations which suggest a solution to this classical "entrainment problem". I will conclude by discussing potential implications for larger scale galactic simulations, and observables of cold gas in the surroundings of galaxies. In particular, I want to show that the Lyman-alpha line is a powerful probe of the (small-scale) structure of neutral hydrogen.

February 27, 2019

NOTE: Hans E. Suess Memorial Lecture 4:00-5:00pm in NSB Auditorium
(Reception in NSB Atrium from 3:00-4:00pm)
 "The Search for Life beyond the Solar System"
Victoria Meadows

University of Washington
Host: Thiemens/Wright

 Abstract: One of the most exciting and interdisciplinary frontiers in exoplanet science is the search for life beyond the solar system. Recently discovered planets, especially Earth-sized planets orbiting nearby red dwarf stars, will provide intriguing near-term targets for large ground-based telescopes and the James Webb Space Telescope, while even larger telescopes are planned to directly image and explore the environments of worlds around stars like our Sun. These telescopes may detect planetary features that suggest a biological origin, but our ability to accurately interpret whether these features are due to life will depend on our understanding of planetary evolution and processes, and environmental context. This talk will provide an overview of the current state of exoplanet biosignature science, and describe interdisciplinary research by NASA’s Virtual Planetary Laboratory team to understand how to identify which planets are most likely to harbor life. I will also describe the VPL’s work to develop a comprehensive framework for exoplanet biosignature assessment by synthesizing research from Earth science, planetary science, stellar astrophysics and biology—and describe the prospects for terrestrial exoplanet characterization and life detection with future telescopes.

March 6, 2019

 "How Long do Quasars Shine?"
Joseph Hennawi

Associate Professor
UC Santa Barbara
Host: Keres

 Abstract: Abstract: Luminous quasars are believed to be the progenitors of the supermassive black holes observed ubiquitously at the centers of all massive galaxies, but we are still in the dark about how these black holes actually formed. Our ignorance largely results from the fact that the expected timescale for supermassive black hole growth of 45 million years is much longer than the mere fifty years that humans have been observing quasars. A holy grail would thus be a direct measurement of quasar lifetimes, shedding light on the physical mechanisms responsible for fueling black hole growth, and how the back-reaction of this growth might influence how galaxies form. I will show how observations of diffuse intergalactic gas in the environs of luminous quasars can be used to chronicle the history of quasar emission on timescales from kiloyears to gigayears. I will also discuss how these same observations can be used to constrain the reionization history of the Universe.

March 13, 2019

 "The Many Faces of Matter inside of Neutron Stars"
Fridolin Weber

Distinguished Professor of Physics / CASS Research Scientist
San Diego State University / UCSD
Host: Weber/Kritsuk

 Abstract: Gravity compresses the matter in the core regions of neutron stars to densities that are several times greater than the density of ordinary atomic nuclei. This provides a high-pressure environment in the core regions of neutron stars where several different subatomic particle processes are expected to compete with each other and even novel states of matter exist. The most spectacular possibilities involve the generation of hyperons and baryon resonances, boson condensates, and/or the formation of color superconducting quark matter. Combined with the unprecedented progress in observational astrophysics, this makes neutron stars superb astrophysical laboratories for a range of physical studies which shed light on the structure and equation of state of dense baryonic matter. In this talk I will begin with providing an overview of our current understanding of the core-composition of neutron stars. Models for the equation of state of dense neutron star matter will then be presented, which are constrained by the latest nuclear and astrophysical data. Particular emphasis will be put on the quark- hadron phase transition in the core regions of neutron stars, which could be driven by changes in the rotation periods of neutron stars. Finally, the phase diagram of hot and dense proto neutron star matter will be discussed and possible instabilities in such matter will be pointed out. The information gained from these investigations provides information about the nuclear equation of state that is complementary to what is expected from the study of gravitational waves from neutron star-black holes and binary neutron star mergers, high baryon density QCD from lattice calculations, and high baryon density physics from low energy heavy-ion collisions at RHIC and GSI.

March 20, 2019

NOTE: No seminar today

Spring 2019

March 27, 2019

NOTE: Spring Break - no seminar today

April 3, 2019

 "Engineering the Next Generation of Exoplanet Imagers"
Dmitry Savransky

Assistant Professor of Mechanical and Aerospace Engineering
Cornell University
Host: Konopacky

 Abstract: As exoplanet direct imaging has rapidly matured and is now providing invaluable data and greatly expanding our understanding of how planetary systems form and evolve, we are now well positioned to consider the ultimate goal of discovering and characterizing Earth-like planets with future, space-based exoplanet imaging instrumentation. In this talk, I will discuss a variety of projects currently being worked on at the Space Imaging and Optical Systems Lab at Cornell University in support of this goal. These projects span numerous disciplines and touch upon various aspects of a space mission, including optimization of the overall mission, linking predicted science yields to engineering decisions, as well as investigating data post-processing techniques and the control of optical systems with mechanical degrees of freedom.

I will present our software framework for the simulation of exoplanet imaging missions (EXOSIMS) and the particular challenges of metric-driven systems engineering in the context of missions of discovery. I will also discuss our recent work on applying common spatial pattern filtering to point source extraction in the weak signal-to-noise regime, as well as our applications of optimal control techniques to wavefront sensing. Finally, I will present a possible new path towards future, giant space-based observatories.

April 10, 2019

 "A Cosmic Test of Quantum Entanglement and Bell's Inequality: Choosing
Measurements with Light from High Redshift Quasars"
Andrew Friedman

Assistant Research Scientist
Host: Friedman

 Abstract: In our recent experimental tests of Bell’s inequality, for the first time, we used observations of astronomical sources to randomly choose measurement settings for polarization-entangled photons sent through free space to two distant detectors. In our 2017 pilot test in Vienna, Austria, we used the color of light from Milky Way stars, and in 2018, we performed the first Cosmic Bell test in the Canary Islands using light from high redshift quasars. In both sets of tests, we observed statistically significant violation of Bell’s inequality, implying that John Bell’s very reasonable assumptions about the world cannot all be true in nature. These assumptions include realism/determinism, locality, and experimental freedom of choice. Our tests aim to put tension on the latter assumption, arguably the most subtle of Bell’s axioms, which holds that each detector’s measurement choices are completely free of any non-quantum degrees of freedom in the causal past of the experiment that could also affect the measurement outcomes. Since the nearest star in the pilot test was ~600 light years away, given our assumptions, the observed Bell violation implies that any non-quantum explanations for entanglement must have been in place prior to ~600 years ago, an improvement of ~16 orders of magnitude compared to previous tests. Similarly, since the nearest quasar in our best experimental run was 7.8 billion light years away, any such mechanism is relegated an additional ~6 orders of magnitude into the cosmically distant past, corresponding to excluding non-quantum explanations from 96% of the space-time volume in the causal past of the experiment. In addition to exploring how free our experimental choices are while investigating the fundamental nature of reality in the subatomic world, such foundational tests are relevant to whether practical quantum encryption schemes will ultimately be as secure as many researchers believe. Our other new theoretical work shows that relaxing freedom of choice by a relatively small amount can still reproduce the quantum predictions, making the associated loophole easier to exploit than other well known entanglement test loopholes.

April 17, 2019

 "The Hidden Baryons of the Milky Way"
Yong Zheng

Miller Postdoctoral Fellow
UC Berkeley
Host: Rubin

 Abstract: Galaxies are not lonely islands floating in the Universe. They host large gaseous envelopes of baryons, a.k.a., the circum-galactic medium (CGM), that exist far beyond a galaxy’s visible extent. Baryonic inflows from CGM replenish star-forming fuel in galaxies, whereas outflows from galaxies enrich the CGM. In this talk, I will describe the theoretical and observed distribution and flows of baryons in the MW’s CGM, including how the MW’s disk hides up to half of its CGM from direct observation. I will then describe new techniques to generate synthetic observations of the CGM using the Enzo & FOGGIE cosmological simulations, and show how these can be used to reveal the hidden baryons in the MW’s CGM. Finally, I will briefly highlight the connections between low- and high-redshift CGM studies, including new applications that rely on fast radio bursts (FRBs).

April 24, 2019

 "Gravitational wave signal from primordial hydro-magnetic turbulence"
Tina Kahniashvili

Associate Research Professor
Carnegie Mellon University
Host: Kritsuk

 Abstract: The first direct detections of gravitational waves from the mergers of binary black holes and binary neutron stars by the LIGO and VIRGO experiments have electrified the physics and astronomy communities. A clear next experimental step is an interferometer in space, which can detect lower frequency signals than a ground-based detector, including supermassive black hole binary coalescences from early galaxy mergers and a known stochastic background from confusion-limited white-dwarf binaries. An even more intriguing signal is the stochastic background from early-universe physics. In this talk I will present our recent work (in collaboration with Axel Brandenburg, Arthur Kosowsky, Sayan Mandal, and Alberto Roper Pol). Using direct numerical simulations of early universe hydromagnetic turbulence with energy densities of up to 10% of the radiation energy density, we show that gravitational waves with energy densities of about 10-10 times the critical energy density of the Friedmann universe today were produced. Their characteristic strain today is found to be about 10-20 and should be observable with the Laser Interferometer Space Antenna (LISA) in the mHz range. The gravitational waves have positive (negative) circular polarization if the magnetic field has positive (negative) magnetic helicity. The gravitational wave energy reaches a constant value after the turbulent energy (kinetic or magnetic) has reached its maximum. Compressive modes are found to produce about 10 times stronger gravitational waves than solenoidal ones. Finally, I will discuss the range of phase transition energy scales and properties that may be detectable with the envisioned space-based interferometer configurations such as LISA.

May 1, 2019

 "Advancing CMB Cosmology: ACTPol and Simons Observatory"
Jeff McMahon

Associate Professor
University of Michigan
Host: Keating

 Abstract: Measurements of the cosmic microwave background (CMB) are a powerful probe of the origin, contents, and evolution of our Universe. CMB measurements continue to improve according to a Moore’s law under which the mapping speed of experiments improves by an order of magnitude roughly every five years. This rapid progression in our ability to measure the CMB has translated into a series of scientific advances including showing our universe to be spatially flat, constraining inflationary and alternative theories of the primordial universe, and providing a cornerstone for our precision knowledge of the Lambda-CDM model. Observations with the current generation of experiments, including Advanced ACTPol, will soon produce improved cosmological constraints. Building on this work, in the coming decade Simons Observatory and ultimately CMB-S4 will: pass critical thresholds in constraints on inflation and light relativistic species; provide improved measurements of dark energy, dark matter, neutrino masses, and a variety of astrophysical phenomena; and enable searches for new surprises.

In this talk I present the design and status of measurements with Advanced ACTPol and how we are building on this work to realize the next generations of experiments including Simons Observatory and CMB-S4. I will highlight the technological advances that underlie the rapid progress in measurements including: polarization sensitive detectors which simultaneously observe in multiple colors; metamaterial antireflection coated lenses and polarization modulators; and overall advances in experimental design. I will present preliminary new results from ACTPol and conclude with science forecasts for the coming decade.

May 8, 2019

 "Multi-object spectrographs: simulation tools for design and trade studies"
Arun Surya

Postdoctoral Scholar
Host: Wright

 Abstract: Several wide-field multi-object spectrographs are proposed for the upcoming extremely large telescopes to provide optical and near-infrared spectroscopy of large samples of astronomical objects. The size of these instruments scale with the telescope aperture size creating unique problems for optics and mechanical design. Mitigating some of these design challenges involves the development of new simulation tools that can interface with different standard optical and mechanical design software to simulate realistic observations and instrument behavior. I will discuss the development of two such tools, the flexure compensation simulation tool and the target allocation tool, developed during the conceptual design phase of TMT-WFOS spectrograph. WFOS is an optical wide field multi-object spectrograph planned for the first light of Thirty Meter Telescope. Flexure Compensation Simulation (FCS) tool provides an interface to accurately simulate the effects of instrument flexure at the WFOS detector plane using perturbation of key optical elements and also derive corrective motions to compensate for such effects. The target allocation tool was developed for the fiber-based design of WFOS to simulate the allocation of fiber units on objects at the focal plane for different target densities and science cases. I will discuss how these tools help in addressing specific challenges in the design of these instruments.

May 15, 2019

 "The Origin of Cosmic Rays: How their Composition Defines their Sources, & Sites, Mixing, Injection & Acceleration"
Richard Lingenfelter

Research Physicist Emeritus
Host: Rothschild/Fuller

 Abstract: Galactic cosmic-ray source abundance ratios (Z/H)GCRS, measured from H to Pb and ~108 to 1014 eV, differ greatly from solar system (Z/H)SS by factors of ~20-200. Yet these two compositions are drawn from essentially the same core collapse (CCSN) and thermonuclear (SN Ia) supernova ejecta: (Z/H)SS from unbiased accumulation over ~Gyr, and (Z/H)GCRS from highly biased sampling during the brief period <30 kyr of homologous early Sedov-Taylor supernova expansion that diffusive shock acceleration (DSA) is most effective. These differences reveal how (Z/H)GCRS can result from just two self-consistent processes: ubiquitous mass mixing (Z/H)SS/(Z/H)CCSN ~4 of shocked, swept-up interstellar medium with high metallicity, core collapse supernova ejecta forming a base; and selective grain injection of elements first condensed FGC as fast grains in freely expanding ejecta, or later implanted in them from ambient gas, then finally Coulomb-sputtered FCS by H and He as suprathermal ions into supernova shocks, where DSA carries them to cosmic-ray energies. This bulk mixing selectively increases source mix abundances (Z/H)SM /(Z/H)SS by ~2-10; and injection by grain condensation and implantation fractions FGC, from meteoritic chondrules, further enhances by ~6, while elemental-charge Z2/3-Coulomb grain sputtering yields, FCS give an added enrichment of ~4-20. Applying these basic processes of mixing and injection to solar system (chondrule) abundances (Z/H)SS produces grain-injected, source-mix (Z/H)SMGI that match major cosmic-ray abundances (Z/H)GCRS to 1±35% with no free parameters. Independently confirming grain injection, (Z/H)GCRS also showsno detectable contribution of Fe from SN Ia, which although they produce ~1/2 Fe in ISM, is quite consistent with there being no dust in SN Ia remnants, while CCSN are a major source.

Lingenfelter, R. E. 2013, Discovery of Cosmic Rays, AIP Conf Proc 1516, 162'
Lingenfelter, R. E. 2018, AdSpR, 62, 2750. arXiv:1807.09726
Lingenfelter, R. E. 2019, arXiv:1903.06330

May 22, 2019

 "The Gobbling Monsters within the Hot DOGs"
Chao-Wei Tsai

NAOC, Beijing
Host: Wright

 Abstract: Hot, Dust-Obscured Galaxies, or Hot DOGs, are a class of distant dust-enshrouded galaxies with extremely high luminosity, including several "Extremely Luminous Infrared Galaxies" (ELIRGs) that reach 10^14 L_Sun. Selected by their utmost red colors in WISE bands, their SEDs incorporating WISE, Spitzer, and Herschel photometry indicate that hot dust dominates the bolometric luminosity. The SED analysis suggests a close-to-spherical dust distribution with a range of temperatures. These hyperluminous sources are likely powered by highly obscured active galactic nuclei (AGN), and are unlikely to be lensed. The measured masses of the monstrous black holes within these Hot DOGs using Keck/MOSFIRE reflects that they are accreting at a rate close to the Eddington limit. This hyperluminous, highly obscured population may represent a special evolutionary stage prior to the red quasar and optical quasar phases.

In the case of W2246-0526, the most luminous Hot DOG yet identified, its total luminosity of 3.6 x 10^14 L_Sun at z = 4.601 make it well into the Extremely Luminous Infrared Galaxy (ELIRG, > 10^14 L_Sun) range, and among the few most luminous galaxies known thus far. Using the broad MgII-2799A emission line from Keck/OSIRIS and the blueshift-corrected broad CIV line from Keck/LRIS, we estimate the black hole mass of the obscured AGN to be ~ 4x10^9 M_Sun, and the corresponding Eddington ratio is L_AGN/L_Edd = 2.8. The high Eddington ratio of W2246-0526 may reach the level where the luminosity is saturating due to photon trapping in the accretion flow, and be insensitive to the mass accretion rate. As a result, the black hole mass growth rate could exceed the apparent accretion rate derived from the observed luminosity.

May 29, 2019

 "Solar Eclipses, Solar Transits by Venus and Mercury, and Pluto/KBO Occultations"
Jay Pasachoff

Professor of Astronomy
Williams College
Host: Keating

 Abstract: On August 21, 2017, a total solar eclipse's band of totality swept across the Continental United States from coast to coast for the first time in 99 years. I will show and discuss some of the the images and spectra my team has obtained at the most recent eclipses, including total eclipses in Easter Island (2010), Australia (2012), Gabon (2013), Svalbard (2015), Indonesia (2016), and the United States (2017) as well as comment on annular or partial eclipses observed elsewhere. I will discuss our observational tests underway for the comparison of models of coronal heating. I will also discuss plans for the 2019 and 2020 total eclipses that cross Chile and Argentina.

I will also report on our observations of transits not only of the Sun by the Moon (that is, a solar eclipse), but also across the Sun by Venus and by Mercury. I will discuss ground-based imaging and Total Solar Irradiance space measurements as well as observations of the 2012 transit of Venus with Hubble by reflection off Jupiter and directly with Cassini from Saturn, providing solar-system close-up analogues to exoplanet transits.

I will close with some discussion of our stellar-occultation observations by Pluto and other objects in the outer solar system, and their relation to the recent New New Horizons’ flyby of Ultima Thule, a billion miles beyond Pluto.

My work at solar eclipses has recently been mainly supported by the US National Science Foundation’s Atmospheric and Geospace Sciences Division, and the Committee for Research and Exploration of the National Geographic Society. The solar-system occultation work has been supported by NASA.

June 5, 2019

 "Reaching Beyond our Spectral Grasp of Star and Planet Formation"
Gregory Mace

Research Associate
University of Texas, Austin
Host: Konopacky

 Abstract: New discoveries generally come from low-resolution observations with weak signals. We are then tasked with significantly increasing the signal and resolution in order to characterize these new objects. Some of the most challenging objects to characterize are young stellar objects in the dusty environments of formation and faint exoplanets close to their host stars. High-resolution infrared spectroscopy allows us to look through the obscuring dust and separate exoplanet and host star spectra. The silicon immersion gratings developed at UT Austin maintain high throughput, while providing a broad spectral grasp, with a fraction of the instrument volume of traditional reflective gratings. I will discuss the application of immersion grating spectrographs (IGRINS, iSHELL, MagNIFIES, GMTNIRS) and the order of magnitude improvement in simultaneous wavelength coverage, throughput, and/or resolution that they enable. Specifically, I will discuss their role in confirming and then characterizing exoplanets, while also tracing the star formation process down to planetary masses. Since these immersion grating instruments are new, or still in development, there is considerable return on efforts to make early use of them to answer a variety of science questions. Additionally, future instrument collaborations for ground, airborne, and space facilities in the 1-10 micron region have the potential to reveal new characteristics at a variety of spectral resolutions.

June 12, 2019

 "Long shot technologies the could enable ground breaking observations
Melville (Mel) Ulmer

Professor of Physics and Astronomy and CIERA
Northwestern University
Host: Konopacky

 Abstract: I will describe the concept of a unique near IR (NIR) camera and give a progress report. This NIR camera, that will operate at faster than 1 kHz frame rates, is especially useful for extreme adaptive optics observations of exoplanets. “Extreme” comes from the concept of being able to image exoplanets that are extremely faint (fainter by more than 1E6) in comparison to their host stars. Assuming success, and the existence of the TMT, we will be able to provide images (albeit not maps of the continents) of habitable planets. However, we can do even better in space, if we can launch a large enough primary mirror. For example, there is a standing request for a 16m diameter version of the Large Ultra-Violet Optical InfraRed (LUVOIR) telescope whose observations will be able detection of (if they exist) molecule spectral signatures in exoplanet atmospheres associated with life (as we know it). Thus, I will then segue into a description of a novel technology that will enable necessary corrections to a deployed space membrane mirror. For, if simply deployed without post-launch corrections, the mirror figure will not be good enough. The technology is so versatile, it can also be applied to any thin space optic that could benefit from post-launch control, e.g., the deformable mirrors in space coronagraphs. As with the NIR camera, I will give a progress report of my team’s efforts.