Astrophysics Colloquium

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

Winter 2021

 
1/6/2021 -  Leslie Rogers (U. Chicago) 
Title:  Journey to the Center of the Super-Earth

Abstract: Sub-Neptune, super-Earth size exoplanets are a new planet class. Though absent from the Solar System, they are found by microlensing, radial velocity, and transit surveys to be common around distant stars. In this talk, I'll review both recent developments and outstanding puzzles in our understanding of the nature and origin of these enigmatic planets.
 

1/20/2021 -  Ilya Mandel (Monash University)
Title:  The promise of gravitational-wave astrophysics

Abstract: The first detections of gravitational waves from compact-object mergers have opened up new opportunities and challenges in astrophysics. I will survey the plausible formation scenarios for merging compact-object binaries.  I will then describe ongoing efforts to extract the astrophysical evolution of massive stellar binaries from observations of gravitational waves emitted during mergers of the stellar remnants.

 


1/27/2021 -  Charlotte Mason (Harvard/CfA)
Title:  Constraining Reionization with Lyman Alpha Emission

Abstract: The reionization of hydrogen in the Universe's first billion years was likely started by photons from the first galaxies. We will probably never observe these galaxies directly, but their properties can be inferred by measuring the timeline and morphology of reionization. I will describe how we can use galaxies at our current observational frontiers to measure the reionization process. In particular, both the strength and line shape of Lyman alpha (Lyα) emission can probe the intergalactic medium (IGM), but modelling physics from pc to Gpc scales is required. I will describe how we can constrain the timeline of reionization and properties of ionizing bubbles by comparing observations of both Lyman-break and Lyman-alpha selected galaxies to models and simulations using statistical inference methods. I will present measurements which favour a late and relatively rapid reionization, and place these in the context of high redshift galaxy formation.

 


2/3/2021 - Subo Dong (Peking University) 
Title:  Direct Collision of White Dwarfs as a Possible Major Channel for Type Ia Supernovae

Abstract: The explosion mechanism of Type Ia supernovae (SN Ia) is unknown. The continuity in the properties of SNe Ia across the luminosity function suggests a single dominant channel to explain the population. We argue that direct collisions of white dwarfs may be promising as such a major channel. I will present results of our efforts to test the collision model.
 
 

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.
 

Recorded talk.

 


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:  Spatially Resolved Galaxy Interactions
 
Abstract: For decades, late-stage galaxy mergers have been recognized as naturally occurring events within the hierarchical LCDM paradigm, capable of triggering starburst and quasars. Early-stage mergers (interactions), albeit not as dramatic as their late-stage counterparts, are believed to shape galaxies in gentler and more long-lasting ways: by enhancing star formation, suppressing gas metallicity, igniting AGNs, augmenting H2 fuel, etc. But more importantly, their cumulative effect may ultimately stimulate the transformation of spirals into lenticulars in dense environments. The focus of this talk is to address the spatial structure and evolution of star formation and the interstellar medium (ISM) in interacting galaxies. We use an extensive suite of parsec-scale galaxy merger simulations (stellar mass ratio = 2.5:1), which employs the "Feedback In Realistic Environments-" model (FIRE-2). This framework resolves star formation, feedback processes, and the multi-phase structure of the ISM. We focus on the galaxy-pair stages of interaction. We find that close encounters substantially augment cool (HI) and cold-dense (H2) gas budgets, elevating the formation of new stars as a result. This enhancement is centrally-concentrated for the secondary galaxy, and more radially extended for the primary. This behaviour is weakly dependent on orbital geometry. We also find that galaxies with elevated global star formation rate (SFR) experience intense nuclear SFR enhancement, driven by high levels of either star formation efficiency (SFE) or available cold-dense gas fuel. Galaxies with suppressed global SFR also contain a nuclear cold-dense gas reservoir, but low SFE levels diminish SFR in the central region. Concretely, in the majority of cases, SFR-enhancement in the central kiloparsec is fuel-driven (55% for the secondary, 71% for the primary) -- whilst central SFR-suppression is efficiency-driven (91% for the secondary, 97% for the primary). Our numerical predictions underscore the need of substantially larger, and/or merger-dedicated, spatially-resolved (integral-field spectroscopic) galaxy surveys -- capable of examining vast and diverse samples of interacting systems -- coupled with multi-wavelength campaigns aimed to capture their internal ISM structure.
 
 

12/9/2020 - Sara Seager (MIT)
Title:  The Search for Signs of Life Beyond Earth by Way of Atmospheric Biosignature Gases
 
Abstract: For thousands of years, inspired by the star-filled dark night sky, people have wondered what lies beyond Earth. Today, the search for signs of life is a key factor in modern-day planetary exploration, both for in situ exploration of our own Solar System’s planets and moons and for telescope remote sensing of exoplanets orbiting nearby stars. We aim to detect a gas in a planetary atmosphere that might be attributed to life. A suitable “biosignature gas” must: be able to accumulate in an atmosphere against atmospheric radicals and other sinks; have strong atmospheric spectral features; and have limited abiological false positives. The study of biosignature gases is thus intertwined with chemistry. Life on Earth produces thousands of gases. Which gases might be potential biosignatures in an as yet unknown range of possibly exotic exoplanetary environments? New computer simulations and next generation telescopes soon coming on line make us the first generation with the capability to search for signs of life on what should be a growing number of suitable exoplanets.  We might be surprised by finding biosignature gases on planets closer to home in our own Solar System, such as phosphine on Venus, motivating new space missions for in situ exploration.
 
 


 

Astrophysics Colloquium Archive