Colloquia Archives

Pratika Dayal, Groeningen
Thursday, October 6, 2022
3:45 p.m.
LGRT 1033
Title:
High-Redshift Galaxy Formation and Reionization
Rohan Naidu, Harvard CfA
Thursday, September 29, 2022
3:45 p.m.
LGRT 1033
Title:
Studying higH-Redshift Galaxies with JWST
Science Jamboree
Thursday, September 22, 2022
3:45 p.m.
LGRT 1033
Title:
Science Jamboree
Science Jamboree
Thursday, September 15, 2022
3:45 p.m.
LGRT 1033
Title:
Science Jamboree
No Colloquium
Thursday, September 8, 2022
3:45 p.m.
LGRT 1033
Title:
No Colloquium
Emily Martin, University of California Santa Cruz
Thursday, April 28, 2022
3:45 p.m.
LGRT 1033
Title:
TBD
Sinclaire Manning, UMass Amherst
Thursday, April 21, 2022
3:45 p.m.
LGRT 1033
Title:
TBD
Sirio Belli, Harvard/CfA
Thursday, April 14, 2022
3:45 p.m.
LGRT 1033
Title:
TBD
Claire Murray, STScI/JHU
Thursday, April 7, 2022
3:45 p.m.
LGRT 1033
Title:
Finding Galaxy Fuel in Multi-wavelength Surveys of the Diffuse Universe
Abstract:
The diffuse interstellar medium (ISM) dominates the mass reservoirs of galaxies and fuels the formation of star-forming clouds. Measuring its physical properties is vital for understanding galaxies as evolving ecosystems, and also for correcting observations of extragalactic light. In this talk, I will discuss how to leverage multi-wavelength surveys with machine learning algorithms to unveil the structure, properties and kinematics of the diffuse ISM in the Milky Way and nearby galaxies. From deep surveys of Galactic 21 cm absorption, we use machine vision to establish the temperature and density of pervasive HI, with important implications for foreground dust reddening constraints. Outside the Milky Way, we combine new, high-resolution surveys of the Magellanic System from the Australia Square Kilometer Array Pathfinder (ASKAP) telescope, with sensitive UV-IR photometry of embedded stars from Scylla, a 500-orbit Hubble Space Telescope imaging survey, to probe the 3D+ structure of the ISM at low metallicity. These efforts combined will prepare us to maximize the potential of large spectroscopic and photometric surveys in the thrilling, incoming era of the Nancy Grace Roman Space Telescope and the Square Kilometer Array.
Blakesley Burkhart, Rutgers
Thursday, March 31, 2022
3:45 p.m.
LGRT 1033
Title:
Turbulent Beginnings: A Predictive Theory of Star Formation in the Interstellar Medium
Abstract:
Our current view of the interstellar medium (ISM) is as a multiphase environment where magnetohydrodynamic (MHD) turbulence affects many key processes: star formation, cosmic ray acceleration, and the evolution of structure in the diffuse ISM. In part 1 of this talk, I shall review the fundamentals of galactic turbulence and discuss progress in the development of new techniques for comparing observational data with numerical MHD turbulence simulations. In part 2, I will focus on how turbulence affects the long-standing problem of star formation. From scales of giant molecular clouds (GMCs), I will demonstrate how the star formation efficiency can be analytically calculated from our understanding of how turbulence, gravity, and stellar feedback induce density fluctuations in the ISM via a probability distribution function analysis. This analytic calculation predicts star formation rates from pc size scales (GMCs) to kpc size scales in galaxies.
Itziar Aretxaga, INAOE
Thursday, March 24, 2022
3:45 p.m.
LGRT 1033
Title:
Dusty star-forming galaxies in deep extragalactic fields: tracing the stellar build-up
Abstract:
We analyze a representative sample of dusty star-forming galaxies in the Extended Groth Strip (EGS) in the L>~1012Lsun regime. Deep infrared HST imaging from CANDELS allows us to assess the recent claim of morphological transformation with redshift. We find a significant fraction of dusty galaxies to be starbursts, and they otherwise tend to populate the upper region of the Main Sequence of star formation. We support the claim that disks are the dominant morphology, with bulges being developed along the way, in a similar way to other optically-selected massive star-forming galaxies in the field. In preparation for the commissioning of TolTEC, we also show a cosmological motivated dusty star-forming galaxy simulation that allows us to explore the expected global properties of the 1011 Lsun<L<1012 Lsun regime.
No Colloquium: Spring Recess
Thursday, March 17, 2022
Joel Leja, Penn State University
Thursday, March 10, 2022
3:45 p.m.
LGRT 1033
Title:
"Brick by Brick: The Road to a Cohesive & Complete Story of Galaxy Formation"
Abstract:
What is the story of galaxy formation -- when, where, and how did these vast cosmic ecosystems assemble? Armed with new cosmic photometric and spectroscopic surveys, and with James Webb data just around the corner, we are in a better position to answer this question than ever before. Yet these new data also breathe new life into a long-standing challenge in observational galaxy evolution: how do we self-consistently model all of these data? In this talk I present recent progress made using the high-dimensional galaxy SED-fitting code, Prospector. By including flexible, model-agnostic prescriptions for the complex physical processes in galaxy formation (e.g., nonparametric star formation histories), Prospector produces qualitatively new and quantitatively distinct solutions for key galaxy observables such as stellar masses and star formation rates. I demonstrate the impact this has on our large-scale view of galaxy formation with new analyses of the stellar mass function and star-forming sequence, produced via Bayesian population modeling combined with the flexible "normalizing flow" ML technique. I argue that this Bayesian hierarchical modeling approach is the key to coherently disentangle the long-standing challenge of the strong dependence of galaxy SED-modeling on the assumed prior. I demonstrate some first steps in this direction with population models of dust attenuation and the rest-frame optical color--(mass-to-light ratio) relationship. I end by discussing how this "learn from the data" approach will be turbo-charged in the near future by lightning-fast ML-powered inference techniques combined with next-generation data (e.g., the PFS survey, JWST, and spatially resolved analysis).
Elena Murchikova, Institute for Advanced Studies
Thursday, March 3, 2022
3:45 p.m.
LGRT 1033
Title:
Milky Way’s Galactic Center black hole in its natural habitat
Abstract:
The Milky Way’s Galactic Center black hole Sagittarius A* is the closest to us supermassive black hole. It is an ideal candidate to explore near horizon effects, to test alternative theories of gravity, and to learn the mechanics of black hole feeding, accretion, and feedback -- forces shaping galaxies and the Universe as a whole. Despite its proximity, the accretion flow onto it is not well understood. At large scales (10^5 R_sch and beyond), the primary source of information about accretion flow comes from observations of hot X-ray emitting gas. At near horizon scales, the density of the flow is constrained by polarization measurements. At intermediate scales, there are too few model-independent probes to reliably determine physical properties of the gas. In 2019, using ALMA observations I discovered a disk of cool gas at intermediate distances (10^4 R_sch) from the black hole, which provides new clues to the physics of the inner accretion flow of the Sagittarius A*. In this talk, I will review what is known about the structure of the accretion flow around the black hole. I will discuss the properties of the cool disk and what we can learn from it about the structure of the accretion flow. I will show our new realistic simulations of the inner two parsecs of the Galactic Center which, for the first time, captures Sagittarius A*’s multiphase accretion physics.
Laura Cadonati, Georgia Institute of Technology
Thursday, February 24, 2022
3:45 p.m.
LGRT 1033
Title:
Exploring the stellar graveyard with gravitational waves
Abstract:
A new era in astrophysics has begun with the 2015 discovery of gravitational waves from the collision of two black holes in data from the Laser Interferometer Gravitational-wave Observatory (LIGO). The additional 2017 LIGO-Virgo detection of gravitational waves from the collision of two neutron stars in coincidence with a gamma ray burst and a kilonova, elevated multi-messenger astrophysics from concept to tool for discovery and exploration. Many more gravitational wave signals have been observed since then from collisions of compact binary coalescence, and gravitational waves are a new, important probe for understanding the universe, with a rich science potential ranging from astronomy to cosmology to nuclear physics. This talk will present a selection of the latest results from LIGO and Virgo, with their GWTC-3 gravitational wave transient catalog, and an outlook for the next decade.
Grant Tremblay- Harvard/CFA
Thursday, February 17, 2022
3:45 p.m.
LGRT 1033
Title:
The Once & Future Great Observatories
Abstract:
The term “Great Observatory” was once a piece of programmatic branding for four space telescopes launched between 1990 and 2003. No longer. While Hubble, Chandra, Compton, and Spitzer will reign as everlasting triumphs of science, the term “Great Observatory” has transcended these missions alone. It now reflects a vision for transformationally powerful, flexible, and long-lived facilities enabling panchromatic synergy, near-simultaneity, and acting as force multipliers for ground-based facilities around the world. With extended missions that can span decades, Great Observatories become something other than a pursuit of important but narrowly-defined science goals. They become discovery platforms for the questions we have not yet thought to ask. In this talk, I’ll review the past, present, and future of the Great Observatories program, particularly in the wake of Astro2020.
Nicolas Gravito-Camargo, Center for Computational Astrophysics
Thursday, February 10, 2022
3:45 p.m.
LGRT 1033
Title:
The Galaxy out of equilibrium and the nature of Dark Matter
Abstract:
The ongoing interaction with the Large Magellanic Cloud (LMC) is revolutionizing our view of the dynamical state of the Milky. The perturbations caused by the LMC open new avenues to test the nature of dark matter. Using results from high-resolution N-body simulations, I will describe the two main perturbations caused by the LMC: the DM wake and the reflex motion. The recent discovery of the stellar wake counterpart of the DM wake opens the opportunity to test the different DM models. Finally, I will discuss how some current challenges to cold dark matter theory, such as the plane of satellites of the MW, are likely natural consequences of the out of equilibrium state due to the MW-LMC interaction
Kaitlin Rasmussen, University of Michigan
Thursday, February 3, 2022
3:45 p.m.
LGRT 1033
Title:
Constraining the Drake Equation: The Present and Future of the Search for Life
Abstract:
Are we alone in the Universe? 60 years ago, Frank Drake posed a unique answer to that question, positing that, given a handful of statistics about the local galaxy, one could calculate the number of intelligent civilizations currently residing in the Milky Way. Today, that equation poses a serious question to astrophysics: how can we better constrain those statistics? First, I will introduce the concept of exoplanet spectroscopy and discuss the latest methods and challenges of the field. Then, I will talk about the "n_e" factor: the number of planets which could support life. My present work studying the atmospheres of hot- and ultra-hot Jupiters at multiple orbital phases serves as a perfect testing ground for the statistical methods which will one day constrain the atmospheres of Earth-sized planets, answering the question: Is it Earth-like? Venus-like? Mars-like? I will also discuss the "f_l" factor, the fraction of habitable planets which could support life. One of the biggest challenges faced by the next generation of telescopes will be the massive degeneracy of possible atmospheres on Earth- and super-Earth-sized worlds. I will discuss new and intriguing techniques for characterizing both biosignatures and anti-biosignatures in the very-low signal-to-noise regime and address the question: Could life develop on a given planet?
Jason Wang, Caltech
Thursday, January 27, 2022
3:45 p.m.
LGRT 1033
Title:
Exoplanets at High Spatial and Spectral Resolution
Abstract:
Although thousands of exoplanets have been discovered, only a small fraction have been studied in detail. By spatially resolving planets from their host stars, we can directly characterize them as individual worlds. I will discuss two novel techniques to study directly imaged exoplanets in unprecedented detail: high-dispersion coronagraphy and long-baseline interferometry. With high-dispersion coronagraphy, we can study exoplanet atmospheres in extreme spectral detail, allowing us to measure molecular abundances, planetary radial velocities, and planetary spins. Long-baseline interferometry with VLTI gives us the spatial resolution of a 140 m telescope, enabling the position of exoplanets to be measured with 10-100x better precision, sub-au resolution of circumplanetary disks, and the first direct detection of a radial-velocity discovered exoplanet. I will present recent science results with both techniques and discuss their future prospects.
Jan Eldridge, University of Auckland, New Zealand (remote)
Thursday, December 2, 2021
3:45 p.m.
Zoom
Title:
"Insights into black hole ecology"
Abstract:
Since the detection of the first gravitational wave transient in 2015 there has been a small revolution in our understanding of the mass distribution of black holes. But there are still many unknowns and theory is still struggling to understand how the observed distribution is determined. There are also other windows onto black holes that are showing that the GW transient masses are only have the story. I'll present the main samples of black holes that are available to us today and in the not-too-distant future. Then describe how theoretical work will allow us to understand what physics is in play in determining the birth and evolution of black holes, their ecology within galaxies and the Universe.
No Colloquium: Thanksgiving Recess
Thursday, November 25, 2021
(Tuesday Schedule) Keith Hawkins, University of Texas Austin
Tuesday, November 23, 2021
3:45 p.m.
LGRT 1033
Title:
The Industrial Revolution in Galactic and Stellar Archaeology
Abstract:
What are the processes that govern the formation evolution and assembly of galaxies across cosmic time? This question is among the most fundamental in modern astronomy yet the answer still eludes us to this day. The Milky Way is an optimal laboratory for answering the questions of galaxy formation and assembly because it is one of the only systems to date where we can obtain detailed and precise data on the positions motions and chemical composition for billions of individual stars. Using our Galaxy as a sandbox for exploring galaxy formation and assembly is the essence of Galactic archaeology. In this context I will present my ongoing work in Galactic and stellar archaeology using large spectroscopic photometric and astrometric surveys together with high resolution spectroscopy. I aim to cover the process of chemo-dynamic tagging metal-poor stars in the Galactic bulge as tracers of the first stars what we can learn from industrial-scale spectroscopic surveys and the future of Galactic archaeology.
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
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.

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