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Astrophysics Seminars From 2020 - 2021

Contents


Fall 2020


September 30, 2020

NOTE: No seminar today



October 7, 2020

 CASS-SDSU Seminar welcome event led by George Fuller/Eric Sandquist

Zoom link


 Zoom password: cass-sdsu



October 14, 2020

 "WIMP Dark Matter In An Unusual Cosmological History"

Seyda Ipek
UC President's Postdoctoral Fellow
UC Irvine
https://ucsd.zoom.us/j/98208404473


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US: +1 669 900 6833 or +1 213 338 8477 or +1 669 219 2599 or 888 475 4499 (Toll Free) or 833 548 0276 (Toll Free) or 833 548 0282 (Toll Free) or 877 853 5257 (Toll Free)
Webinar ID: 982 0840 4473
International numbers available: https://ucsd.zoom.us/u/adMT0Gnx6h

ABSTRACT: WIMP miracle has been an important paradigm in the search for dark matter. However the WIMP parameter space is under great tension from direct detection experiments. I will give you a WIMP scenario with an alternative cosmological history of our universe where the quarks confined into mesons at an earlier stage compared to the standard cosmology with the Standard Model. This alternate scenario will open up WIMP parameter space that is not excluded by dark matter experiments.



October 21, 2020

 "A Glimpse Below the Lyman Limit: New observational advances on the escape of ionizing photons from local galaxies"

John Chisholm
Professor
UT Austin
https://ucsd.zoom.us/j/98688518404


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Webinar ID: 986 8851 8404
International numbers available: https://ucsd.zoom.us/u/abScIFDBlg

ABSTRACT: The neutral gas between galaxies was rapidly reionized at redshifts between 6-10. The origin of these ionizing photons remains elusive, but fundamentally shapes the subsequent evolution of the galaxies and the formation of large-scale structure in the universe. The key to unraveling the epoch of reionization is an understanding of both the sources emit ionizing photons (stars vs. active galactic nuclei) and how these ionizing photons escape galaxies. I will present an overview of the recent observational advances that have led to the discovery of local star-forming galaxies that emit ionizing photons, including a recent large Hubble Space Telescope survey that has more than doubled the known number of emitters of ionizing photons. A statistical sample allows for the first detailed look into what types of star-forming galaxies emit ionizing photons and how those ionizing photons can escape star-forming galaxies. I will focus on new probes from the rest-frame ultraviolet that promise to diagnose the porosity of the neutral medium within galaxies at high-redshift. These local surveys set the stage for upcoming telescopes to determine the impact of star-forming galaxies on Cosmic Reionization.



October 28, 2020

 "Inference can constrain a model of neutrino flavor transformation in core-collapse supernovae”

Eve Armstrong
Assistant Professor, Department of Physics
New York Institute of Technology
https://ucsd.zoom.us/j/95800291521


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US: +1 213 338 8477 or +1 669 219 2599 or +1 669 900 6833 or 888 475 4499 (Toll Free) or 833 548 0276 (Toll Free) or 833 548 0282 (Toll Free) or 877 853 5257 (Toll Free)

Webinar ID: 958 0029 1521

International numbers available: https://ucsd.zoom.us/u/abrCIcEfzE

ABSTRACT: The multi-messenger astrophysics of compact objects presents a vast range of environments where neutrino flavor transformation may occur and may be important for nucleosynthesis, dynamics, and a detected neutrino signal. Development of efficient techniques for surveying flavor evolution solution spaces in these environments, which augment existing computational tools, could leverage progress in this field. To this end, we continue our exploration of statistical data assimilation (SDA) to identify solutions to a small-scale model of neutrino flavor transformation. SDA is a machine learning (ML) formula wherein a dynamical model is assumed to generate any measured quantities. Specifically, we use an optimization formulation of SDA wherein a cost function is extremized via the variational method. Regions of state space in which the extremization identifies the global minimum of the cost function will correspond to parameter regimes in which a model solution can exist. Our study seeks to infer the flavor transformation histories of two mono-energetic neutrino beams coherently interacting with each other and with a matter background. We require that the solution be consistent with measured neutrino flavor fluxes at the point of detection, and with constraints placed upon the flavor content at various locations along their trajectories, including the point of emission and the locations of the Mikheyev-Smirnov-Wolfenstein (MSW) resonances. We show how the procedure efficiently identifies solution regimes and rules out regimes where solutions are infeasible. Overall, results intimate the promise of this “variational annealing” methodology to efficiently probe an array of fundamental questions that traditional numerical simulation codes render difficult to access.



November 4, 2020

NOTE: No seminar today



November 11, 2020

NOTE: No seminar today: Veteran's Day Holiday



November 18, 2020

 "Solar System Archaelogy: Multidisciplinary Approaches to Understanding Planetary System Formation"

Gerardo Dominguez
Associate Professor, Physics
CSU San Marcos
https://ucsd.zoom.us/j/91549171608


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Webinar ID: 915 4917 1608
International numbers available: https://ucsd.zoom.us/u/aevFEDTGxE

ABSTRACT: Astronomical observations by the Kepler space telescope have confirmed, to date, the existence of over 3,000 planetary systems in our Galactic neighborhood. The distributions of planet size, compositions, and orbital parameters derived from these observations pose a fundamental challenge for models of planetary system formation which currently do not provide an explanation for why some molecular cloud regions collapse to form planetary systems that are rich in terrestrial planets, water, and possibly life while others do not. Historically, our understanding of planetary system formation has relied on astronomical observations of our Solar System. Much less appreciated, however, is the role that that the analysis of planetary materials (i.e. meteorites, cometary dust, and solar wind particles) has played in this understanding. In this talk, I will review how observations of our solar system at the chemical and isotopic level provided us with tight constraints on planetary system formation and the origins of water on terrestrial planets like our own. Incorporating these observations into comprehensive models, I argue, will require combining theoretical and laboratory studies of astrochemical processes that occur in molecular clouds and protoplanetary disks. To illustrate, I will highlight the recent accomplishments made by my research group and collaborators in understanding astrochemical processes with high precision chemical and isotopic resolutions. I conclude by highlighting recent advances in laboratory astrophysical measurement technology including the measurement of radiation induced isotope exchange between water ice and silicate surfaces at 10K, the direct measurement of the optical properties of individual dust grains from the Stardust mission in the laboratory using NanoIR, and the development of planetary exploration instrumentation for the in-situ determinations of the isotopic compositions of lunar ice. I will conclude by discussing how these studies may help us understand the origins of the planets and water in the solar system.



November 25, 2020

NOTE: No seminar today



December 2, 2020

 "Improving CMB foreground dust modeling by 3D mapping the magnetized ISM"

Gina Panopoulou
NASA Hubble Fellow
Caltech
https://ucsd.zoom.us/j/93051810865


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US: +1 669 900 6833 or +1 213 338 8477 or +1 669 219 2599

Webinar ID: 930 5181 0865

International numbers available: https://ucsd.zoom.us/u/abqRDM2tMy

ABSTRACT: Searches for primordial gravitational waves in the CMB are hindered by a major obstacle: foreground dust emission from the Galaxy. In order to surpass this obstacle, the foreground signal must be modeled and removed with unprecedented accuracy. To reach the required levels of precision we must reconstruct the -yet uncharted- 3D structure of the Galactic magnetic field. I will present first steps towards obtaining a tomographic mapping of the dusty, magnetized ISM, by using stellar polarization, astrometric distances and HI data. I will discuss how the use of these state-of-the art datasets can inform future foreground modeling efforts.



December 9, 2020

 "Simulations and Observations of the Circumgalactic Medium"

Ramona Augustin
Postdoctoral Fellow
Space Telescope Science Institute
https://ucsd.zoom.us/j/98794103722


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Webinar ID: 987 9410 3722

International numbers available: https://ucsd.zoom.us/u/ahKOgICXQ

ABSTRACT: Understanding the processes of gas flows in and out of galaxies is crucial in galaxy evolution studies. Yet, observations of the faint and diffuse Circum-Galactic Medium (CGM), where these processes take place, remain challenging.

The most efficient approach to detect this faint and diffuse gas is in absorption towards bright background quasars. However, to investigate the CGM we need to also identify the galaxy counterpart and connect it to the absorption feature. In this context we characterized counterparts to Damped Lyman-alpha Absorbers (DLAs) at z~1 using HST broad-band images. We measured their stellar masses and find them to follow common galaxy scaling relations. We discovered their complex morphology and environments, challenging the interpretation of CGM studies in absorption.

Quasar absorbers are typically limited to a single sightline and hold no information on the spatial structure of the absorber. To overcome this limitation and gain information on the spatial structure and the nature of CGM absorbers, we combine IFU data with high-resolution cosmological FOGGIE simulations. We detect and analyze two MgII absorption systems at z~1 that show significant differences in their equivalent width variations towards the different sightlines. We find that these variations are consistent with expectations from inflow/outflow regions simulations.

Beyond absorption, prospects to map the CGM in emission will offer new information on its extent and clumpiness. To optimize observing strategies of the CGM in emission, we have made predictions from dedicated cosmological zoom-in RAMSES simulations. We post-processed galaxy halos with an emission model to create mock integral field observations. Using the instrument model, our results indicate that ELT/HARMONI will enter a regime of low surface brightness typical of the CGM which is not attainable with current facilities.



December 16, 2020

NOTE: No seminar today


Winter 2021


January 6, 2021

 "The Duration of Star Formation in Galactic Giant Molecular Clouds"

Matthew Povich
Associate Professor, Department of Physics and Astronomy
Cal Poly Pomona
https://ucsd.zoom.us/j/93928292508


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US: +1 213 338 8477 or +1 669 219 2599 or +1 669 900 6833
Webinar ID: 939 2829 2508
International numbers available: https://ucsd.zoom.us/u/aeFR2ELYj

ABSTRACT: Stars and planets are born in vast interstellar clouds of cold gas and dust called giant molecular clouds (GMCs). I have led a collaboration of researchers and undergraduate students in the development a novel infrared (1 - 8 µm) spectral energy distribution modeling methodology to place X-ray-identified, intermediate-mass (2 - 5 Msun), pre-main sequence stars (IMPS) on the Hertzsprung-Russell diagram. Compared to the more numerous and widely-studied low-mass stars, the temperature and luminosity of IMPS changes dramatically over the first few million years of evolution, hence IMPS serve as sensitive chronometers for measuring the ages of the youngest massive stellar populations in the Galaxy. We apply our methodology to constrain the duration of star formation in a sample of ~20 massive star-forming regions in our Milky Way Galaxy that suffer significant differential reddening from obscuring foreground dust. Star formation commenced at different times among our sample GMCs, ranging from <1 Myr to ~9 Myr ago. We find that the nebular IR luminosity surface density decays sharply with time after the onset of star formation. Dust has been evacuated from giant H II regions produced by massive stellar clusters older than ~3 Myr, rendering them IR-faint. This short timescale indicates that radiation pressure and winds from massive, OB stars generally disperse GMCs before the onset of supernovae. Spatially-resolved IR indicators of obscured star formation rates, commonly used for nearby external galaxies, may need to be recalibrated to account for the brief lifetimes of IR-bright, dusty H II regions.



January 13, 2021

NOTE: No seminar today



January 20, 2021

 "The James Webb Space Telescope: Mission Status and the Path to Science"

Michael McElwain
JWST Project Scientist
NASA Goddard Space Flight Center
https://ucsd.zoom.us/j/92297890358


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Webinar ID: 922 9789 0358
International numbers available: https://ucsd.zoom.us/u/azhZfa85d

ABSTRACT: The James Webb Space Telescope (Webb) is a large-aperture space telescope optimized for near- and mid-infrared observations covering 0.6-28.5 microns. This mission was originally conceived in the mid-1990s as an infrared complement and flagship successor to the Hubble Space Telescope. After earning the top recommendation of the 2000 Astrophysics Decadal Survey, this Observatory has had a remarkable development phase that will culminate in a launch later this year! I will provide a brief history of the Webb mission and highlight the technical milestones that were recently achieved. I will then describe the final ground testing activities, the launch segment, commissioning plans, and the Cycle 1 science program.



January 27, 2021

 "The Formation and Growth of Supermassive Black Holes"

Anna-Christina Eilers
NASA Hubble Fellow
MIT
https://ucsd.zoom.us/j/94767960244


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Webinar ID: 947 6796 0244
International numbers available: https://ucsd.zoom.us/u/abjxadfS97

ABSTRACT: Observations of high-redshift quasars show that they host supermassive black holes (SMBHs) already less than ~1 Gyr after the Big Bang. It has been argued that in order to rapidly grow these SMBHs in such short amounts of cosmic time, they need to accrete matter over timescales comparable to the age of the universe, and thus the lifetime of quasars - the integrated time that galaxies shine as active quasars - is expected to be of order ~10^9 yr at z~6, even if they accrete continuously at the Eddington limit.

I will present a new method to obtain model-independent constraints on the lifetime of high-redshift quasars with unprecedented precision, based on measurements of the sizes of ionized regions around quasars, known as proximity zones. The sizes of these proximity zones are sensitive to the lifetime of the quasars, because the intergalactic gas has a finite response time to the quasars’ radiation. Applying this method to quasar spectra at z>6, we discover an unexpected population of very young quasars, indicating lifetimes of only ~10,000 years, which poses significant challenges on current black hole formation models. I will show results from our on-going multi-wavelength survey to detect and characterize young quasars and their environments. Furthermore, I will discuss several modifications to the current SMBH formation paradigm that might explain the results, e.g. super-critical mass accretion rates, massive initial black hole seeds in excess of stellar remnants, or highly obscured quasar growth phases. In the end I will show how we aim to disentangle the various scenarios by means of on-going and future IFU observations with MUSE and the James Webb Space Telescope, in order to shed new light onto the formation and growth of the first SMBHs in the universe.



February 3, 2021

 "Using Supernova Observations to Study their Massive Star Progenitors"

Azalee Bostroem
UC President's Pre-Professoriate Fellow
UC Davis
https://ucsd.zoom.us/j/94526290041


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Webinar ID: 945 2629 0041
International numbers available: https://ucsd.zoom.us/u/abds9DBz4A

ABSTRACT: Although massive stars have a profound influence on the Universe at every scale, from the evolution of galaxies to the formation of the elements necessary for life, the answers to key questions about their mass loss and explodability are unknown. This is because massive stars are rare and the final phases of their evolution and explosions, as supernovae, occur on very short time scales. Over the last 5 years, wide-field surveys have greatly increased the number of supernovae discovered, opening a new window onto massive star evolution. I will discuss the current gold-standard of understanding the properties of massive stars from images taken prior to their explosions. I will then present new insights we are gaining into which stars explode and how these stars lose mass from recently developed post-explosion techniques including light curve modeling, nebular spectra modeling, and multi-wavelength observations.



February 10, 2021

 "Betelgeuse Is Pretty Cool: Cosmic Questions for our Naked-Eye Neighbor"

Emily Levesque
Associate Professor
University of Washington
https://ucsd.zoom.us/j/99713965365


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Webinar ID: 997 1396 5365
International numbers available: https://ucsd.zoom.us/u/acQQi9hDrJ

ABSTRACT: Betelgeuse's "Great Dimming" in the winter of 2020 quickly became a source of fascination for both professional and amateur astronomers. Drastic enough to be easily detected with the naked eye, the star's drop in brightness was unprecedented in observational memory and revealed the extent to which the workings of red supergiants (RSGs) are still a mystery. RSGs are the coldest and physically-largest members of the massive star population, making them ideal sources for studying the extremes of stellar physics and a key turning point in the evolution of post-main-sequence massive stars. They are also the progenitors of Type II-P supernovae, an intermediate evolutionary phase in the lifetimes of some stripped-envelope supernova progenitors, and a crucial step in the formation of massive binary systems (including those that will ultimately produce compact object binaries and gravitational waves). However, our recent studies of Betelgeuse have forced us to confront the many unanswered questions that still surround RSGs, including their binary fraction and evolution, their place in the larger picture of massive star evolution, and the numerous physical phenomena that drive their variability. Using Betelgeuse and its recent behavior as an archetype, this talk will provide an overview of our current knowledge of RSGs, identify some of the most pressing open questions about these stars, and consider the importance of studying RSGs in the coming decade as the next generation of observatories comes online.



February 17, 2021

 "Utilizing Kepler and K2 to Advance Exoplanet Demographics"

Jon Zink
Graduate Student
UCLA
https://ucsd.zoom.us/j/92327014730


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Webinar ID: 923 2701 4730
International numbers available: https://ucsd.zoom.us/u/adTZ9OFABo

ABSTRACT: Over the course of several years the Kepler mission, which continuously collected photometric data from a single patch of the sky, provided a uniform set of transiting exoplanet detections. This catalog remains the gold standard for transiting exoplanet occurrence rate studies. However, 18 additional fields of data, sampling a variety of Galactic latitudes, were collected following the malfunction that led to the end of the Kepler prime mission. Better known as the K2 mission, these fields provide a unique opportunity to understand how exoplanet occurrence is affected by Galactic latitude, stellar metallicity, and stellar age. With a fully automated pipeline now able to detect and vet transit signals in K2 data, we can measure the sample completeness and reliability. Correspondingly, I will present the first uniform analysis of small transiting exoplanet occurrence outside of the Kepler field. Additionally, with the full K2 sample now processed, I will discuss how we can incorporate this new catalog of planets into our current demographics analysis to expand our understanding of system architecture and planet formation mechanisms.



February 24, 2021

 "Bridging Galaxy Evolution Across Cosmic Time With the CLASSY Survey"

Danielle Berg
Assistant Professor
UT Austin
https://ucsd.zoom.us/j/95833100723


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Webinar ID: 958 3310 0723
International numbers available: https://ucsd.zoom.us/u/acArKvGcaG

ABSTRACT: Rest-frame far-ultraviolet (FUV) spectra are fundamental to our understanding of star-forming galaxies, providing a unique window on massive stellar populations, chemical evolution, feedback processes, and reionization. The launch of JWST and construction of the ELTs will soon usher in a new era, pushing the FUV spectroscopic frontier to z~15-20. The success of these future endeavors hinges on a comprehensive understanding of the massive star populations and interstellar medium (ISM) gas conditions that power the observed FUV spectral features. I will present the COS Legacy Archive Spectroscopic SurveY (CLASSY) Treasury as a powerful and promising solution. CLASSY is a large HST program creating the first high-quality, high-resolution FUV spectral catalog of star-forming galaxies at z~0, but which span similar properties as seen at high-z. The spectra contain a suite of emission and absorption lines that characterize the massive stellar populations that populate metal poor galaxies, the physical properties of large-scale outflows that regulate star formation, and the chemical abundance patterns of the gas and stars. CLASSY will improve the diagnostic power of the FUV lines for future JWST/ELT surveys, providing a long-lasting legacy to the astronomical community for decades to come.



March 3, 2021

 "Galactic Archaeology: Piecing Together the History of Our Galaxy"

Keith Hawkins
Assistant Professor
UT Austin
https://ucsd.zoom.us/j/94002866957


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Webinar ID: 940 0286 6957
International numbers available: https://ucsd.zoom.us/u/ad5Ju0Ioxp

ABSTRACT: One of the key objectives of modern astrophysics is to understand the formation and evolution of galaxies. In this regard, the Milky Way is a critical testing ground for our theories of galaxy formation. However, dissecting the assembly history of the galaxy requires a detailed mapping of the structural, dynamical chemical, and age distributions of its stellar populations. Recently, 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 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 large spectroscopic surveys can and have been used to dissect the structure of our Galaxy. I will also explore the future of Galactic archaeology through chemical cartography.



March 10, 2021

 "Turbulence vs. Magnetic Field: which one is more important in the modern picture of star formation?"

Che-Yu Chen
Postdoctoral Staff Scientist
Lawrence Livermore National Lab
https://ucsd.zoom.us/j/92335013501


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Webinar ID: 923 3501 3501
International numbers available: https://ucsd.zoom.us/u/aoRDHjnZN

ABSTRACT: Magnetic field, turbulence, and gas gravity are considered the key agents regulating the star forming process throughout different evolutionary stages. Despite rich observational results, it is still unclear how dynamically significant the magnetic fields are with respect to gravity and turbulence at varying physical scales, because neither the strength nor 3D structure of the magnetic field can be directly measured through observation. Motivated by the advanced polarimetric capabilities of Planck, BLASTPol, and SOFIA, our recent theoretical efforts and statistical examinations utilizing 3D MHD simulations have provided new ways to characterize the 3D field structure and the relative importance of gas gravity and the magnetic field. We are also working toward a comprehensive understanding of the kinematics within star-forming cores through both observations and simulations. With our ongoing survey with GBT-Argus, we aim to establish a broad statistical sample of the internal velocity structure within dense cores, which will provide important insight on the evolution and transport of angular momentum during core collapse under the influence of the local magnetic field.



March 17, 2021

 "Stellar Companions: the inseparable nature of stars and planets"

Kaitlin Kratter
Associate Professor, Department of Astronomy
University of Arizona
https://ucsd.zoom.us/j/92665751568


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Webinar ID: 926 6575 1568
International numbers available: https://ucsd.zoom.us/u/aHxonUnWj

ABSTRACT: Stars rarely reside alone. They are orbited by planets, other stars, and sometimes both. In this talk I will begin with a census of the diversity and importance of stellar companionship in astronomy. These pairs impact nearly everything we study, from cosmology to planetary habitability. I will then illustrate how recent large surveys have helped us improve theoretical models of star and planet formation by revealing subtle trends and correlations between stellar properties and the planets that they host. I will summarize our understanding of binary formation in general, and our recent progress on the origin of close binaries. I will also emphasize the unique lessons we can learn from the companions that occupy the “in between:” not quite stars, and not quite planets.


Spring 2021


March 24, 2021

NOTE: No seminar today



March 31, 2021

NOTE: No seminar today



April 7, 2021

 "The Nature of Nurture: An Optical/NIR Exploration of High-Density Environments in the Intermediate- and High-Redshift Universe"

Brian Lemaux
Postdoctoral Scholar
UC Davis
https://ucsd.zoom.us/j/95857966113


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Webinar ID: 958 5796 6113
International numbers available: https://ucsd.zoom.us/u/adfW7gozRW

ABSTRACT: The past decade has been witness to immense progress in the understanding of the early stages of cluster formation both from a theoretical and observational perspective. During this time, samples of clusters at intermediate redshift, and forming clusters at high-redshift termed “proto-clusters”, once comprised of heterogeneous mix of serendipitous detections or detections arising from dedicated searches around rare galaxy populations, have begun to compete with lower-redshift samples both in terms of numbers and in the homogeneity of the detection methods. Much of this progress has come from optical/near-infrared (NIR) imaging and spectroscopic campaigns designed to target large numbers of typical galaxies to exquisite depth.

In this talk I will discuss work in the intermediate-redshift universe from the Observations of Redshift Evolution in Large Scale Environments (ORELSE) survey, an ongoing wide-field spectroscopic and multiwavelength imaging campaign targeting an ensemble of galaxy groups, clusters, and superclusters at 0.6 < z < 1.3. This survey, which is focused on measuring the effect of environment on galaxy evolution, has recently been used to characterize the color- and star formation rate-density relation at these redshifts. I will review these results and then discuss current surveys and techniques used to find and characterize proto-clusters at higher redshift, including current work from the VIMOS Ultra-Deep Survey (VUDS), a massive spectroscopic campaign targeting 10,000 star-forming galaxies at 2 < z < 6. This survey, in conjunction with other surveys, has uncovered a large number of proto-structures that appear to resemble clusters and groups forming in the early universe. The development of the methods for finding, confirming, and characterizing proto-clusters and proto-groups in the context of VUDS will be discussed, including new techniques and tools developed specifically for these purposes. Several case studies of spectroscopically confirmed massive proto-clusters will be presented, focused both on the diversity of their global properties and that of their member populations as seen through VUDS observations and dedicated Keck/MOSFIRE and Keck/DEIMOS followup observations. I will conclude with a discussion of work incorporating the full ensemble of VUDS proto-structures to explore the relationship of galaxy properties with environment in the early universe.



April 14, 2021

 "The Smallest and Faintest Galaxies: Clues to the Nature of Dark Matter and Galaxy Formation"

Burcin Mutlu-Pakdil
NSF & KICP Postdoctoral Fellow
University of Chicago
https://ucsd.zoom.us/j/92582996652


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Webinar ID: 925 8299 6652
International numbers available: https://ucsd.zoom.us/u/apUhARVKB

ABSTRACT: The smallest and faintest galaxies around the Milky Way are the most ancient, most metal-poor, and most dark-matter-dominated systems known. These extreme objects offer unique access to small scales where the stellar and dark matter content can be studied simultaneously and hold the promise of major breakthroughs in understanding the nature of dark matter, and a more complete picture of galaxy formation. Thus, their discovery and characterization are among the most important goals in the field. I will share our ongoing observational efforts to detect these faint systems around the Milky Way and beyond, and upcoming advances in the era of deep and wide imaging instrumentation.



April 21, 2021

 "Unlocking Multiphase Galactic Gas Flows"

Drummond Fielding
Flatiron Research Fellow
Flatiron Institute
https://ucsd.zoom.us/j/99024162310


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Webinar ID: 990 2416 2310
International numbers available: https://ucsd.zoom.us/u/adyVEzjmpk

ABSTRACT: Multiphase gas flows are ubiquitous in and around galaxies, and are responsible for regulating star formation and black hole growth. During this presentation, I will review recent breakthroughs on the micro scale interaction of turbulent mixing and radiative cooling that have introduced a paradigm shift in understanding multiphase flows. I will present a new model for galactic gas flows that, for the first time, accounts for the interplay of micro-scale phenomena with the macro-scale dynamics. This model illuminates the underlying physical processes governing galaxy formation and is the basis for a novel predictive cosmological simulation sub-grid model.



April 28, 2021

 "From the Weibel instability to flux-tube coalescence – a pathway to seeding
plasma dynamos"

Muni Zhou
Graduate Student
MIT
https://ucsd.zoom.us/j/93420261475


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Webinar ID: 934 2026 1475
International numbers available: https://ucsd.zoom.us/u/abGCHJC3Cb

ABSTRACT: The amplification of seed magnetic fields by the turbulent dynamo is believed to be essential in forming large-scale cosmic magnetic fields with dynamical strengths. The Weibel instability is a crucial mechanism that can produce near-equipartition seed fields from unmagnetized plasmas, but only at the plasma kinetic scale. It remains unclear whether such microscopic seed fields, under the joint action of their own nonlinear evolution and the background turbulence, can contribute to the formation of macroscopic magnetic fields. In the first part of this talk, I will demonstrate how thermal pressure anisotropy can be generated by motions as simple as a shear flow and trigger the Weibel instability. The ensuing generation and nonlinear evolution of seed magnetic fields are studied within a fully kinetic framework. For the second part, I will focus on our study of a system composed of an ensemble of magnetic flux tubes, the dynamics of which can represent the long-term evolution of filamentary Weibel seed fields. The formation of large-scale magnetic fields from initial small-scale fields and the associated inverse energy transfer have been identified as a result of the coalescence of magnetic structures through magnetic reconnection. An analytical model for the time evolution of quantities such as the magnetic energy and the energy-containing scale is constructed within the magnetohydrodynamic description, and is confirmed by our direct numerical simulations. In the end, we apply our study to estimate the scale and strength of the initial seed field for the galactic dynamo problem by considering the interaction between inverse magnetic transfer and ambient turbulence.



May 5, 2021

 "LIMFAST: a Semi-numerical Tool for Line Intensity Mapping"

Lluis Mas Ribas
Postdoctoral Researcher in Astrophysics & Cosmology
NASA/JPL
https://ucsd.zoom.us/j/99395915900


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Webinar ID: 993 9591 5900
International numbers available: https://ucsd.zoom.us/u/ac8X7pmkV1

ABSTRACT: Intensity Mapping is emerging as a new technique to study galaxy formation and evolution over cosmic time, and this creates a pressing need for multi-scale and multi-frequency simulation tools. LIMFAST is a simulation code designed for such a purpose, with emphasis on the epoch of cosmic reionization (z>5). LIMFAST builds on top of the 21cmFAST cosmological code, and it takes advantage of a semi-numerical scheme to enable multiple realizations of different galaxy models and reionization histories, over large cosmic volumes, and in a short time. We have implemented a flexible/customizable star-formation formalism into LIMFAST, accounting for Pop II and Pop III stellar components, as well as the metal enrichment of the galactic environment. LIMFAST then takes into account the properties of these simulated media and derives the emissivity for many emission lines via tabulated results from external photoionization computations. This approach will allow a variety of self-consistent multi-line studies of the high-redshift universe. I will discuss the details of the code and a set of initial results, as well as the variations and extensions that can be easily implemented into LIMFAST when released publicly to the community.



May 12, 2021

 "Simulating Structure and Galaxy Formation"

Mark Vogelsberger
Associate Professor of Physics
MIT
https://ucsd.zoom.us/j/93049625557


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Webinar ID: 930 4962 5557
International numbers available: https://ucsd.zoom.us/u/ai2iluwmV

ABSTRACT: Cosmological simulations of galaxy formation have evolved significantly over the last years.

In my talk I will describe recent efforts to model the large-scale distribution of galaxies with cosmological hydrodynamics simulations. I will focus on large-scale simulations like IllustrisTNG. After demonstrating the success of these simulations in terms of reproducing an enormous amount of observational data, I will also talk about their limitations and directions for further improvements over the next couple of years. Furthermore, I will also talk briefly about our new simulation campaign, the THESAN project, to study the epoch of re-ionization and the early Universe.



May 19, 2021

 "Characterizing the Atmospheres of Low-Metallicity Brown Dwarfs"

Eileen Gonzales
51 Pegasi b Postdoctoral Fellow
Cornell University
https://ucsd.zoom.us/j/97523976404


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Webinar ID: 975 2397 6404
International numbers available: https://ucsd.zoom.us/u/aBnzkjGQ4

ABSTRACT: Ultracool Subdwarfs, objects that have metallicities significantly lower than that of the Sun and ages > 5Gyr, provide insight into understanding how metallicity affects observable features of low-mass stars and brown dwarfs. Currently, substellar subdwarfs are thought to have cloudless atmospheres due to reduced condensate opacities from their low metallicities. In this talk, I aim to explore the nature of clouds in subdwarfs using both observational and theoretical approaches I ask: (1) Are subdwarfs cloudless? and (2) How does their thermal profile compare to objects of similar effective temperature (Teff) or spectral type? By creating distance-calibrated spectral energy distributions (SEDs), I will compare one of the bluest known subdwarfs, SDSS J1256, to various aged sources of similar Teff and/or bolometric luminosity to examine the overall SED shape and features in the NIR bands, as well as compare fundamental parameters and place SDSS J1256 in context with the larger subdwarf population. To explore the nature of clouds in subdwarfs, I use the Brewster retrieval framework to examine a sample of subdwarfs and comparative sources to explore what may be causing the differences seen in the SEDs of these sources. In this talk, I will discuss the results for the widely separated co-moving low-mass d/sdL7+T7.5p pair SDSS J1416+1348AB to determine if the pair formed and evolved together and their cloud properties. Additionally, I will discuss preliminary results from a comparative sample of field sources of similar temperature or spectral type to SDSS J1416A to determine how the PT profile of these objects compares and what may drive the differences we see in their spectra.



May 26, 2021

 "The Multi-Scale Dynamics of Space Debris and Asteroids"

Aaron J. Rosengren
Assistant Professor, Mechanical & Aerospace Engineering
UCSD
https://ucsd.zoom.us/j/92183365838


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Webinar ID: 921 8336 5838
International numbers available: https://ucsd.zoom.us/u/aegZ7YxNDy

ABSTRACT: Like the exotic orbital configurations of many exoplanetary systems, the three-dimensional complex of man-made orbiting debris, brought on by unfettered space activities, has stimulated a renewed interest in applied and fundamental research in celestial dynamics. Despite their reputed normality, Earth satellite orbits can possess an extraordinarily rich spectrum of dynamical phenomena, from stable resonant configurations to significant chaotic drifts in circumterrestrial space throughout their orbital lifetimes. This talk will review these intriguing dynamical phenomena in the Earth orbiter problem and highlight their deeper connections with current aspects of dynamical astronomy, such as the multi-scale dynamics of small Solar-System bodies. One particularly compelling ideology is based on the judicious use of the resulting instabilities to prescribe natural Earth re-entry itineraries to remedy the space debris problem or to navigate space.



June 2, 2021

NOTE: No seminar today



June 9, 2021

 CASS All-Hands Meeting

[Zoom Link TBA ]


 Agenda for a *brief* meeting:

(1) Report on discussions in several Executive Committee meetings held over the last quarter (budgets, seminars, External Advisory Committee, etc.)

(2) Plan for Summer and Fall seminars and seminar committee

(3) Discussion on re-opening protocols, remote/on-site issues; business office challenges;
Report on info from the ORU Director’s meeting (Tuesday, June 8).

—————————————————————————————————————————————————
Time permitting . . .

(4) Exoplanet and Astrobio Initiative — Director Quinn Konopacky has formed a Steering Committee;
Discussion of possible additional future initiatives: Plasma/High energy density; “Big Data”; etc.?

(5) Update progress on Astro-Section and degree program architecture: Shelley Wright, Physics Vice Chair