Colloquia Archives

Jan Eldridge, University of Auckland, New Zealand (remote)
Thursday, December 2, 2021
3:45 p.m.
Zoom
Title:
TBD
No Colloquium- Thanksgiving Recess
Thursday, November 25, 2021
3:45 p.m.
LGRT 1033
Title:
TBD
(Tuesday Schedule) Keith Hawkins, University of Texas Austin
Tuesday, November 23, 2021
3:45 p.m.
LGRT 1033
Title:
TBD
Eileen Meyer, University of Maryland Baltimore County
Thursday, November 18, 2021
3:45 p.m.
LGRT 1033
Title:
The Mysterious Nature of Jets from Black Holes
Abstract:
Astronomers have known for decades that both stellar-mass and super-massive black holes sometimes produce bipolar jets of relativistic plasma. These are extremely energetic events, and in the case of super-massive black holes (also known as radio-loud active galaxies or AGN), can result in extremely Doppler-boosted emission from radio to gamma-rays which outshines the host galaxy. Despite decades of study, we are still in ignorance of both basic properties of jetted AGN, and the mechanism of their production. We still don’t have clear answers to why a jet may be produced in only one of two otherwise near-identical system. Is it related to black hole spin, accretion rate, mass? I will touch on these larger questions as I describe some recent results from my group at UMBC. In particular, I will present our recent study of jet properties and their connection to the central accreting black hole, using the largest-ever catalog of over 2000 spectral energy distributions, and some recent work on the puzzling spectral properties of jets on kiloparsec scales, where we still do not understand the origin of the optical, UV, and X-ray emission. I will end with some thoughts on the future of jet studies as new observatories are planned and launched.
No Colloquium
Thursday, November 11, 2021
3:45 p.m.
LGRT 1033
Elisabeth Newton, Dartmouth College
Thursday, November 4, 2021
3:45 p.m.
LGRT 1033
Title:
Observing the evolution of small stars and planets
Abstract:
Our Galaxy teems with stars and their planets. The evolution of the two are intimately linked: stellar high energy radiation is thought to drive atmospheric evolution in close-orbiting exoplanets. However, studying their evolution is challenging as most changes occur on million-to-billion year timescales. I will discuss two complementary avenues in the study of stellar and exoplanetary evolution. I will examine the evolution of spin and magnetism in M dwarf stars, the smallest and most common type of star in the Galaxy and the most promising stars for the study of temperate planets. I will then turn to the discovery and characterization of young exoplanets, which may still be in the throes of dynamical and atmospheric evolution.
Bob Benjamin, University of Wisconsin Madison
Thursday, October 28, 2021
3:45 p.m.
LGRT 1033
Title:
Progress in Determining the Star Forming Structure of the Milky Way Galaxy
Abstract:
Mapping the Milky Way has been a seventy year long painful slog, and the picture that many researchers currently use for Milky Way structure is more of a “model" than it is a “map". But the combination of (a) high precision parallaxes of radio masers from the BeSSeL and VERA programs and (b) stellar parallaxes for a billion sources from Gaia are drastically reshaping our picture of the structure of the Milky Way Galaxy. In this talk, I will trace the development of the current model of Milky Way spiral structure from its origins in 1951 through to the current day, with a focus on the three nearest spiral arm segments: Perseus, Orion/Local, and Sagittarius. In the modern era, we now have five principal tracers of the star forming structure of the Milky Way: (a) individual “upper main sequence” stars (or just spectroscopically selected OB) stars with Gaia parallaxes, (b) young stellar clusters with Gaia parallaxes, (c) masers in high mass star forming regions with radio parallaxes, (d) three dimension dust extinction maps using main sequence stars with Gaia parallaxes, and (e) YSOs in high mass star forming regions with Gaia parallaxes. I will show a comparison of these maps with each other and discuss how well they compare with the historical model. It is clear that current models of MW star forming structure are in need of some revision. I will provide some speculations on where I see this whole process going in the next few years.
Yvette Cedes, CfA/Harvard
Thursday, October 21, 2021
3:45 p.m.
LGRT 1033
Title:
Using A Violent Demise to Study Extreme Environments
Abstract:
A Tidal Disruption Event (TDE) occurs when a star wanders too close to a supermassive black hole (SMBH) and is unbound by tidal forces. Studying TDEs can allow us to learn not just about the event itself, but also about the outflows and shockwaves they create and the environment surrounding a previously quiescent black hole. In this talk, I will give an overview of TDE observations, primarily focusing on the radio, and a summary of where the field stands today. I will begin with the case of Swift J1644+57, the best-studied example of a TDE which launched a relativistic jet beamed directly at Earth, and has now evolved into a non-relativistic outflow. I will then highlight the example of a non-relativistic TDE, AT2019dsg, and new radio observations that shed additional light on a claimed neutrino association with this source. Finally, I will summarize the state of TDE radio observations today, which point to two populations of TDE outflows, and a potential new class of TDEs that rapidly brighten several years-post disruption and are little understood
Marica Valentini, AIP, Germany (remote)
Thursday, October 14, 2021
3:45 p.m.
Zoom
Title:
Unfolding the assembly history of the Milky Way halo using asteroseismology
Abstract:
Metal poor stars are key to understanding the history of our Galaxy. In their element abundances pattern is encoded the chemical composition of the first stars and therefore, when the stellar age is available, hints on the chemical enrichment and evolution of the Milky Way. However, to obtain precise ages for field metal poor stars is a challenging task: at present only an handful very metal-poor stars have ages, derived by using nucleo-cosmo-chronology (e.g. via Thorium and Uranium abundances). Asteroseismology in recent years has demonstrated to be a powerful tool to derive masses, and hence ages, of red giant stars. When this technique is applied to metal poor stars,it is possible to increase the number of metal poor stars with a precise age measurement. For this purpose I am leading a project combining asteroseismology and high resolution spectroscopy of metal-poor halo giants ([Fe/H]<-1.5 dex). Targets have been identified in RAVE and APOGEE surveys. We obtained seismic information from the light curves collected by the Kepler,K2, and TESS space missions, and detailed chemical abundances, from ESO-UVES and HARPS-N high resolution spectra. We derived iteratively the atmospheric parameters, by taking into account the seismic surface gravity. We then derived precise abundances taking into account NLTE effects. The final atmospheric parameters and abundances, together with the seismic information and, when available, Gaia parallaxes, were used for interfere stellar masses, radii and ages, via Bayesian fitting on a set of isochrones. We obtained a unique set of metal poor stars, for which we determined precise ages. For the first time a consistent and complete approach have been adopted, in order to quantify the impact of temperature shifts, different mass-loss approaches, alpha-enrichment and corrections on seismic scaling relations. Our project shows how it is possible to obtain precise ages for field metal poor giants, and therefore to reconstruct the history of the Galactic halo.
Peter Melchior, Princeton (remote)
Sunday, October 10, 2021
3:45 p.m.
Zoom
Title:
The Data Revolution in Astrophysics
Abstract:
Current astrophysics is characterized by the confluence of three separate developments: large surveys providing more observational data than ever; massive simulations probing ever more complex phenomena; and rapid advances in machine learning establishing entirely new ways of dealing with and interpreting data. None of these three pillars can stand on their own to make meaningful progress in astronomy and the physical sciences. I will present three areas of research that combine accurate statistical modeling, deep neural networks, and numerical simulations: joint-survey processing of LSST, Euclid, and Roman data; simulation-based inference in cosmology and galaxy evolution; and science-driven design of new surveys and observing programs.
Mia Bovill, Texas Catholic University
Thursday, September 30, 2021
3:45 p.m.
LGRT 1033
Title:
Ending the Cosmic Dark Ages: Exploring the First Stars with JWST
Abstract:
The first, Population III stars ignited several hundred million years after the Big Bang. While we know they formed from primordial metallicity gas in 10^5 - 10^7 solar mass dark matter halos, and are thought to be more massive than later generations of stars, their detailed properties remain unconstrained. Current unknowns include; when the first Pop III stars ignited, how massive they were, and when and how the era of the first stars ended. Investigating these questions requires a exploration of a multi-dimensional parameter space including, the slope of the Pop III stellar initial mass function (IMF), and the strength of the non-ionizing UV background. We have developed a novel model which treats these relative unknowns as true free parameters. Our simple model reproduces the results from hydrodynamic simulations, but with a computational efficiency which allows us to investigate the observable differences between a wide range of potential Pop III IMFs. The upcoming launch of the James Webb Space Telescope (JWST) provides us with a unprecedented opportunity to marry theoretical predictions to observations and determine the astrophysics which governs the formation of the first stars. Here we suggest constraining the masses of the first stars with JWST will require a multi-pronged approach. In addition to direct detection via gravitation lensing, this also includes understanding how the Pop III IMF affects observed rates of highly luminous pair instability supernova.
James Lowenthal, Smith College
Thursday, September 23, 2021
3:45 p.m.
Zoom
Title:
Special Talk: Satellite Swarms vs. Astronomy
Abstract:
New technology is allowing massive swarms, or "constellations", of low-Earth communications satellites such as SpaceX's Starlinks to be launched at relatively low cost, leading to a dramatic rise in the number of satellites at altitudes 300-1200 km already in orbit: about 2000 in the last 2 years, with more than 100,000 planned by 2030. The large number and the apparent brightness (by reflected sunlight) of these satellites poses serious and possibly catastrophic challenges to ground-based and even space-based astronomy and the appearance of the starry night sky. There are national and international efforts underway to understand and try to control and respond to these challenges; meanwhile, rockets are launching every 2 weeks, each with 60+ more satellites, observatories and skywatchers are reporting increasing interference from satellite streaks, and the view of the night sky has already been changed.
Science Jamboree II
Thursday, September 16, 2021
3:45 p.m.
Zoom
Science Jamboree I
Thursday, September 9, 2021
3:45 p.m.
Zoom
Kazunori Akiyama (MIT/Haystack)
Thursday, April 29, 2021
3:45 p.m.
Remote - Zoom
Title:
Horizon-scale Imaging of Black Holes and MHorizon-scale Imaging of Black Holes and Magnetic Fields with the Event Horizon Telescopeagnetic Fields with the Event Horizon Telescope
Abstract:
Two years ago, in April 2019, the Event Horizon Telescope (EHT) released the first images of a black hole, resolving the shadow of the supermassive black hole M87* at the center of the nearby active galaxy M87. Furthermore, the EHT recently captured the polarization of the photon ring in M87*, resolving the magnetic field near the event horizon. The EHT uses very long baseline interferometry at 1.3 mm wavelength, allowing to image supermassive black holes at horizon-scale resolutions. In this talk, I will present an overview of the past, present, and future of black hole imaging with the EHT. I will first discuss the major breakthroughs enabling to provide these results and also provided by these horizon-scale image. Finally, I will introduce our next decadal forecast of the forthcoming exciting era to study black holes through direct imaging.
Ivo Labbé (Swinburne)
Thursday, April 22, 2021
3:45 p.m.
Remote - Zoom
Title:
Rise and Fall of the First Galaxies in the era of JWST
Abstract:
The formation of the first galaxies is a most exciting and elusive frontier. A lasting legacy of the Hubble and Spitzer space telescopes is the discovery and characterization of galaxies to redshift z∼11, looking back 97% of the time to the Big Bang. I will review our current sketch of galaxy build-up at cosmic dawn, highlighting fundamental questions that remain unanswered: when did the first galaxies form, what is their role in reionizing cosmic Hydrogen, and how do they evolve into the galaxies we observe at later times? The launch of the James Webb Space Telescope later this year will be an absolute game changer. JWST will deliver ultra-sensitive imaging and spectroscopy at wavelengths previously inaccessible. I will look ahead and discuss several initiatives in Webb's first year that address key outstanding issues, including our program UNCOVER, which aims to explore the so-called "Dark Ages" (10 < z < 20), the period after Recombination when the first stars and galaxies formed.
Dusan Keres (UCSD)
Thursday, April 15, 2021
3:45 p.m.
Remote - Zoom
Title:
Galaxy Evolution With StellarFeedback and Cosmic Rays
Abstract:
Stellar feedback plays an important role in regulation of star formation and stellar mass growth on cosmological timescales. Direct inclusion of this feedback in cosmological simulations has proven difficult but a new generation of cosmological zoom-in simulations shows significant promise. For example, simulations from the FIRE project model stellar feedback on spatial scales of molecular clouds and achieve local self-regulation of star formation and self-consistent driving of galactic outflows via careful treatment of the supernova momentum and energy injected to the surrounding ISM. A prominent feature of these simulations is bursty star formation and spatial clustering of supernovae that drive significant outflows from galaxies, especially in low mass objects. Star formation-driven galactic outflows become inefficient at late times for galaxy masses similar to the Milky Way, which leads to significant buildup of galactic stellar mass. Interestingly, if only a moderate fraction of supernova energy is imparted to the surrounding gas in the form of cosmic rays, this can significantly modify the structure and properties of the circum-galactic medium (CGM), drive large scale winds, and slow down gas infall and growth of L* galaxies. The magnitude of these effects is sensitive to the transport of cosmic rays that can be constrained with diffuse gamma ray emission from galaxies. Potential significant role of cosmic rays in galaxy evolution motivates increased interest in understanding of their transport.
Connor Robinson (Amherst College)
Thursday, April 8, 2021
3:45 p.m.
Remote - Zoom
Title:
Observations and Simulations of Accretion Variability in Young, Low-mass Stellar Systems
Abstract:
Accretion onto young, low-mass stars, known as Classical T Tauri stars (CTTS), is the primary source of ultraviolet (UV) photons in the inner regions of the surrounding protoplanetary disk. Large changes in accretion rate have been observed on timescales of minutes to years, but the driving forces behind this variability and its effect on planet formation are not well understood. To help characterize these changing UV fields, I present the current largest HST UV variability study of CTTS to-date, consisting of 31 UV spectra of 5 systems. I will discuss the typical changes that were observed within this dataset and highlight three interesting epochs: an accretion burst in the transitional disk system GM Aur, the chance alignment of an accretion column along our line of sight in the full disk system VW Cha, and multi-wavelength observations that suggest azimuthal density gradients across the accretion shock. I will also present preliminary results from a short-cadence TESS survey of CTTS in the Taurus star-forming region with simultaneous multi-wavelength observations. Finally, I will present simulations that make predictions about CTTS light curve morphology for a variety of magnetic field configurations and stellar parameters. The lessons learned from these analyses will be applied to data from the ongoing HST Ultraviolet Legacy Library of Young Stars as Essential Standards (ULLYSES), which has devoted 500 orbits to studying accretion onto CTTS.
Lisa Kewley (ANU)
Thursday, April 1, 2021
3:45 p.m.
Remote - Zoom
Sean Linden (UMass)
Thursday, March 25, 2021
3:45 p.m.
LGRT 1033
Title:
A Census of Star Clusters and HII Regions in Extreme Extragalactic Environments
Abstract:
Local luminous infrared galaxies (LIRGs) are the ideal laboratories for studying star formation in the most extreme merger-driven environments; conditions which may be analogous to the star-forming environments of high-redshift galaxies. With HST and the VLA we are able to make fundamental conclusions about the nature of both obscured and unobscured star formation in these systems. In this talk, I will discuss the ongoing effort to identify and characterize the UV-bright population of super star clusters (SSCs) for LIRGs in the Great Observatories All-Sky LIRG Survey. A major result of these studies thus far is the discovery that, relative to the galaxies studied in the PHANGS and LEGUS surveys, the survival rate and maximum mass of SSCs is affected by the active merging-environment of LIRGs. Additionally, with high-resolution radio continuum imaging I will demonstrate that 33 GHz emission from extranuclear HII regions in both normal and starburst galaxies is heavily-dominated by thermal free-free emission, making it one of the most direct and extinction-free probes of the ionizing photon production rate from massive star-forming regions. Finally, I will present new results within the Clusters, Clumps, Dust, and Gas (CCDG) survey to establish the relationship between the stellar and nebular extinction for the most extreme SSCs in LIRGs, and to search for SSCs in the NIR which are completely obscured in the UV. Future JWST observations of LIRGs in the NIR-MIR will allow us to identify the very youngest (1-3 Myr), and most highly-embedded (Av > 50) SSCs, which likely power the bulk of the current SFR in these extreme systems.
Alice Shapley (UCLA)
Thursday, March 18, 2021
3:45 p.m.
Remote - Zoom
Title:
The MOSFIRE Deep Evolution Field (MOSDEF) Survey: A Detailed Census of the Physics of Galaxy Formation in the Early Universe
Abstract:
Understanding the formation and evolution of galaxies remains one of the great challenges of modern cosmology. Key outstanding questions include: What are the physical processes driving the formation of stars in individual galaxies? How do galaxies exchange material with their intergalactic environments? How do the impressive variety of galactic structures that we observe today assemble? How do supermassive black holes affect the evolution of their host galaxies? We present new results from the MOSFIRE Deep Evolution Field (MOSDEF) survey, a comprehensive census of the galaxy population during the peak epoch of activity in the universe ~10 billion years ago. In addition to providing an overview of the MOSDEF survey and its science, we focus on new results regarding the evolving metal enrichment and physical conditions in the interstellar medium of star-forming galaxies towards higher redshift. Our new results suggest many exciting future observational directions for untangling the detailed nature of star formation in the early universe.
Marta Bryan (UC Berkeley)
Thursday, March 11, 2021
3:45
Remote - Zoom
Title:
Exploring the Formation and Evolution of Planetary Systems
Abstract:
Over the past two decades thousands of planets with an extraordinary diversity of properties have been discovered orbiting nearby stars. Many of these exoplanetary systems challenge our narrative for how planets form and evolve, motivating the search for observational clues to the underlying mechanisms that led to this diversity. In this talk I will describe my work using a wide range of observational techniques to uncover these underlying mechanisms. I will constrain the physics of gas giant formation and evolution by discerning population statistics, system architectures, rotation rates, and atmospheric compositions of gas giant planets. I will then discuss the impact that outer gas giants have on the inner architectures of planetary systems by exploring differences in inner planet masses, separations, multiplicities, and orbital properties. Finally, I will highlight the key role that next generation instruments and telescopes such as the GMT/TMT will play by extending these novel observations to entirely new classes of planets.
Enrico Ramirez‑Ruiz (UC Santa Cruz)
Thursday, March 4, 2021
3:45 p.m.
Remote - Zoom
Title:
Cosmic Alchemy In the Era of Gravitational Wave Astronomy
Abstract:
The source of about half of the heaviest elements in the Universe has been a mystery for a long time. Although the general picture of element formation is well understood, many questions about the astrophysical details remain to be answered. Here I focus on recent advances in our understanding of the origin of the heaviest and rarest elements in the Universe. Enrico Ramirez-Ruiz is eager to understand our origins and, in some cases, is simply wild about things that go bang in the night sky. He works with computer models to understand the cataclysmic death of stars and recently led efforts to uncover the origin of the heaviest, most neutron-rich elements in the universe, like gold and uranium.
Chentao Yang (ESO Chile)
Thursday, February 25, 2021
3:45 p.m.
Remote - Zoom
Title:
Properties of the ISM in (strongly-lensed) high-redshift dusty galaxies
Abstract:
The discovery of a high-redshift submillimeter-bright (dusty) galaxies population has revolutionized our understanding of galaxy evolution and star formation in extreme conditions, yet their nature remains hotly debated. Recent wide-area extragalactic surveys at submm/mm bands have discovered hundreds of such galaxies that are strongly lensed at high redshifts. The boosted angular resolution and brightness open new exciting opportunities for studying the interstellar medium within. We have thus carefully selected a sample of the bright lensed dusty galaxies based on the Herschel-ATLAS sample. Through observations of the multiple transitions of the CO lines, we analyzed the physical conditions of the molecular gas. Additionally, we have also conducted the first multi-transition H2O line survey within these high-redshift galaxies. We have studied the properties of the far-infrared radiation fields with the H2O lines. The study provides us new constraints of the warm dense, extreme dust-obscured regions. ALMA 0".2-0".4 follow-up of one of the brightest sources (G09v1.97) in our sample shows very well agreement of the spatial distribution and kinematics between the CO, H2O, and H2O+ lines. Interestingly, this merger source shows a mismatch between the cold-dust continuum peak and the peak of the line emissions, suggesting a significant amount of cold gas is in the interacting region, similar to the local merger prototype, the Antennae galaxies. In parallel, we have conducted a line survey in Band 3 and 4 using ALMA, which resulted in detections of a rich series of molecules including HCN, HNC, HCO+, CCH, 13CO, C18O, CS, N2H+, and isotopologues of CO. The rich detections of molecules in multiple transitions enable us, for the first time, to have a detailed view of the astrochemical process and reveal rich information about the high-redshift physical properties of the molecular gas, the radiation field, and their interactions.
Brenda Frye (U. of Arizona) View(active tab)
Thursday, February 18, 2021
3:45 p.m.
Remote - Zoom
Title:
Planck-Herschel Detection (PHD) of Galaxy Overdensities from z = 3 to the Present
Abstract:
Galaxy clusters as gravitational lenses offer two advantages: to boost the brightnesses of objects in the background, and to study the dark matter in the lens. We introduce here a third use: to unveil properties near the caustic, whose lensing magnification factors can reach 10,000 or more. We begin by recounting a novel approach to detect galaxy overdense regions by their rest-frame far-infrared colors (and not by the Sunyaev-Z'eldovich effect). We will see why this selection picks up a combination of galaxy clusters at z > 1.5 and at z ~ 0.5. To investigate the lensing properties, we analyze Hubble Space Telescope WFC3-IR imaging in the fields of the lower-redshift classical giant arcs. We construct lensing models from the many examples of image multiplicities, and offer an explanation for some of their more nonstandard physical properties. We conclude with a discussion on the emerging field of caustic crossings and other local lensing events which yield insights into our understanding of dark matter substructure.
Grant Tremblay Harvard/CfA
Wednesday, February 17, 2021
3:45 p.m.
LGRT 1033
Title:
TBD
Lynne Hillenbrand (Caltech)
Thursday, February 11, 2021
3:45 p.m.
Remote - Zoom
Dr. Jorge Zavala
Thursday, February 4, 2021
3:45
Remote - Zoom
2nd Year Graduate Project Presentations Part 2: Ben Gregg
Thursday, November 19, 2020
4 p.m.
Remote - Zoom
Title:
Investigating Mid and Far-Infrared Color-Color Relations within Resolved Star Forming Regions in Local Galaxies
Abstract:
We present an extensive archival analysis of a diverse sample of local galaxies, combining multi-wavelength data from GALEX, SST and HSO in order to investigate "blue side" mid-IR and "red side" far-IR color-color correlations within the observed IR SEDs of resolved star forming regions. With archival data from the FUV through 500 micron, we measure photometry of individual star forming regions 36 arcseconds in size. We estimate SFRs and stellar masses and derive metallicity distributions throughout our galaxies. Focusing on the f(70)/f(500) "far-IR" and f(8)/f(24) "mid-IR" colors, we find that a sub-sample of galaxies demonstrate a strong far-IR versus mid-IR color-color correlation within their individual star forming regions, while others demonstrate uncorrelated colors. We identify that this division within our sample of galaxies appears to be driven by two main effects: 1) the local strength of SF and 2) the metal content of the ISM. We find that galaxies uniformly dominated by very high surface densities of SF (e.g. M83) demonstrate strong IR color-color correlations, while galaxies exhibiting relatively lower levels of SF (e.g. NGC5457) tend to demonstrate weaker correlations---likely explained by the increasing effect of varying ISM heating and metal content on the IR colors, specifically in the mid-IR. Our results are found to be consistent with published IR correlations for NGC4449, but present a more complicated picture. We find large dispersion in the SFR versus 8 micron luminosity relation that is traced by the metallicity distributions, consistent with many recent studies, highlighting its problematic use as a SFR indicator.
2nd Year Graduate Project by Luan Luan
Thursday, November 12, 2020
4 p.m.
Zoom
Title:
THE INFLUENCE OF MASS ACCRETION ON THE BAR FORMATION AND EVOLUTION OF GALAXIES
Abstract:
We present several N-body simulations of disk galaxy with mass evolution model to study the influence of mass accretion on the bar formation and evolution. Using techniques of orbit family recognition and harmonic decomposition, we find that the formation of the bar is always accompanied by the formation of a stellar cusp, which may correspond to the bulge in the disk. As the mass growth of the cusp, it destroys the bar, which indicates that bar is not always long-lived and stable structure. We also find that the strength of bar and the time to form the bar depend on the mass accretion rate and the accretion radius.

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