Astrophysics Colloquium

Virtual Colloquium Meetings are held via Zoom.  Meeting information will be sent in email.  
 

Fall 2020

 
10/7/2020  - Tim Brandt (UCSB) 
Title:  Discovering, Weighing, and Characterizing Exoplanets and Brown Dwarfs
 
Abstract: I will present a combination of three observational techniques--astrometry, radial velocity, and imaging--to discover, weigh, and characterize massive exoplanets and brown dwarfs. While thousands of planets are known, only a few have both measured masses (from radial velocity and astrometry) and atmospheric properties (inferred from spectra). Advances in adaptive optics and infrared instrumentation now enable us to see young exoplanets millions of times fainter than their host stars. Despite huge gains in sensitivity, however, high-contrast imaging surveys remain plagued by a lack of discoveries. I have calibrated a huge data set of stellar reflex motions; it can identify unseen planets and brown dwarfs by the gravitational tugs they exert on their host stars, and enable us to measure their masses and orbits. With masses, orbits, and spectra of a growing sample of planets and brown dwarfs, we can finally test models of substellar formation and evolution.
 
 


10/14/2020 - Rachel Bezanson (University of Pittsburgh)
Title:  The Formation of Massive Galaxies: deep, high-redshift spectroscopy from the LEGA-C and SQUIGGLE Surveys and Beyond
 
Abstract:  Massive galaxies reside in the densest and most evolved regions of the Universe, yet we are only beginning to understand their formation history. Today massive galaxies are red and dead ellipticals with little ongoing star formation or organized rotation; naturally they were expected to be relics of a much earlier formation epoch. In this talk I will describe the complex evolutionary history of massive galaxies that has emerged over the last decade, discussing the structural and kinematic evolution of massive galaxies during and after they stopped forming stars (“quenched”) and eventually transformed from rotationally supported disks into kinematically hot ellipticals. I will describe ongoing efforts to better understand this metamorphosis at intermediate redshifts, highlighting results from the ultra-deep LEGA-C spectroscopic survey of ~3000 massive galaxies at z~0.8 and the focused multi-wavelength SQUIGGLE survey of post-starburst galaxies at z~0.6 caught immediately following their cosmic shutdown. I will show that early quiescent galaxies, retained significant rotational support (~twice as much as local ellipticals), implying that the mechanisms responsible for shutting down star formation do not also have to destroy ordered motion. Furthermore, I will describe first results from the SQUIGGLE survey, including the ALMA discovery of vast reservoirs  (~1010 Msun) of cold molecular hydrogen remaining in young post-starburst galaxies caught a few 100 Myrs after quenching their primary episodes of star formation. This enigmatic result suggests that the quenching process does not have to fully heat or deplete the gas to halt star formation. Finally, I will discuss prospects for extending spectroscopic studies of galaxies at cosmic noon - the peak of massive galaxy formation and shutdown - with JWST and eventually 30-m class telescopes. 
 


10/21/2020  - Mark Morris (UCLA)
Title: New Floodlights on the Galactic Wind
 

Abstract:  While there has long been a limited amount of morphological and dynamical evidence

for an outflow from the central regions of our Galaxy, the last decade has seen an

explosive growth in the evidence for a powerful Galactic wind on all scales, starting 

with the discovery of the gamma-ray Fermi Bubbles.  While the lower boundaries of 

the Fermi Bubbles are nested within a hot plasma seen in soft X-rays and in the form

of a “microwave haze,” the connection of these large-scale features to activity in the 

Galactic center was not evident until recent X-ray and radio continuum observations 

revealed the "Galactic center chimneys” extending a few hundred parsecs out from the 

center and connecting to the bases of the Fermi Bubbles.  In this talk I will describe 

what we have learned about these Galactic exhaust channels using a multi-wavelength 

approach.  I will also describe the recent discovery of a population of relatively dense 

atomic and molecular clouds that have apparently been accelerated out of the Galaxy’s 

central molecular zone up to hundreds of km/s by the much faster hot plasma 

constituting the bulk of the Galactic wind.

 


10/28/2020 - Ana Bonaca (CfA/ITC)
Title:  Uncovering the nature of dark matter with stellar streams in the Milky Way
 
Abstract:  Stars orbiting in the halo of our galaxy, the Milky Way, are a window into the distribution of dark matter. Tidally disrupting star clusters are especially valuable tracers, because in pristine conditions they produce thin stellar streams of nearly uniform density. I will present maps of stellar streams based on the latest photometric and astrometric data that reveal variations in the width and density of streams -- typical signatures of dynamical perturbation. Dynamical modeling of a perturbed stream GD-1 suggests it recently had a close encounter with a massive and dense perturber, while precise radial velocities constrain the perturber's orbit and present-day location. Known baryonic objects are unlikely perturbers based on their orbital properties, but observations permit a low-mass dark-matter subhalo as a plausible candidate. Data being delivered by big ground-based surveys of this decade (e.g., DESI, LSST) will enable such studies in hundreds of stellar streams. I will discuss how the upcoming observations can be used to measure the mass spectrum of dark-matter substructures and even identify individual substructures and their orbits in the Milky Way halo.

 


11/4/2020 - Arpita Roy (STScI)
Title:  SECRETS OF THE INSTRUMENT BUILDERS
 
Abstract: The study of exoplanets, arguably more than any other field of astrophysics, has grown in direct consonance with new instrumentation. Currently in build+commissioning phases are planet hunting Doppler spectrographs aiming at 10-30cm/s RV precision in the optical in quest for Earth analogs, and <1m/s in the near-infrared in pursuit of M dwarf planets. These massive instruments leverage a range of technological advances, from high-homogeneity illumination delivery setups, to sophisticated wavelength calibration, ultra stable environmental control, and precision data analysis. Building these instruments is a storied journey of failure and success, innovation and disappointment. In this talk we will delve into the experience and lessons that come from technologically pushing the field forward, as well as the complexity and sophistication of these systems that will enable ground-breaking new science.
 


11/11/2020 - Veterans Day holiday - no Colloquium
 


11/18/2020 - Chris Fassnacht (UCD)
Title:  Investigating the Nature of Dark Matter with Gravitational Lenses
 
Abstract: The nature of dark matter is one of the major questions facing not only astrophysics, but also high-energy physics.  Our "standard model" of cosmology, Lambda CDM, provides an excellent match to observations of the Universe on large scales, but there are some indications that the model may not adequately reproduce observations on galaxy scales and smaller.  As a result, alternative dark matter models have been proposed to explain the discrepancies.  A powerful way to test these non-CDM models is to determine the statistics of low-mass halos, since some of the models (e.g., warm dark matter) predict fewer halos at low masses than CDM.  In the interesting mass range where the predictions from WDM diverge strongly from those of CDM, halos are expected to host very few to no stars, making their detection difficult.  Strong gravitational lensing provides an excellent complement to Local Group observations, especially because it can detect purely dark halos at even cosmological distances.  In this talk I will discuss two methods of using gravitational lenses to detect low-mass halos and present some recent results, as well as giving some thoughts about future efforts to use these techniques.
 
 



11/25/2020 - Thanksgiving holiday - no Colloquium
 

 
12/2/2020 -Jorge G. Moreno (Pomona College)
Title:  TBA
 
Abstract: TBA
 


 
12/9/2020 - Sara Seager (MIT)
Title:  TBA
 
Abstract: TBA
 


 

 

Astrophysics Colloquium Archive