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Astronomy Colloquium

Colloquium meetings are held in the Bryant Space Science Center Building (BRT) in Room 217 from 12:45–1:45 pm on Thursdays.

Refreshments will be served after the talk in Room 311

Coordinators:  Zack Slepian (chair), Paul Sell, & Sarah Ballard

Spring/Summer 2020 Schedule


  • Thursday, July 30 [zoom]
    • Eric Koch
    • Unraveling Atomic ISM Physics Across the Local Group

      Encoded in the atomic ISM is physics crucial to the baryonic cycle, including cold gas accretion onto galaxies, the formation of molecular clouds, and the origin of interstellar turbulence from stellar feedback. The 21-cm HI line traces each of these processes but disentangling the effect of each process in observations remains challenging. I will present initial results from a new 21-cm HI VLA survey of the Local Group on 80 pc scales. These new observations combine high sensitivity with fine spectral resolution to reveal remarkable HI spectral complexity across the Local Group. In this talk, I will present two findings using these new observations. First, I will show that the data do not support the existence of monolithic 100 pc cold atomic gas clouds, suggested in previous work from models of opaque HI emission. Instead, we find that HI spectra are best modeled as multiple Gaussian components. Building on this result, I will then show the strong correlation between atomic and molecular ISM kinematics when considering only the HI component associated with CO emission. This correlation persists across molecular clouds in different star-forming states, supporting models of molecular clouds as short-lived dynamic objects rather than long-lived entities decoupled from the galactic environment. These results from our on-going VLA survey exhibit how detailed modelling of 21-cm HI spectra can distinguish between atomic ISM processes.


  • Thursday, May 21 [zoom]
    • Terese Hansen
    • Abundances of Two small Systems

      An increasing number of ultra-faint dwarf (UFD) galaxies and stellar streams
      are discovered in large surveys. The chemical abundances of the stars in these
      systems provide a new venue to explore the chemical and hierarchical build-up
      of the Milky Way. In this talk, I will present the result of abundance
      analysis of stars in an UFD and a stream. (1) Grus II is an UFD galaxy
      discovered in the Dark Energy Survey, detailed abundance analysis for the
      three brightest members Grus II revealed them all to exhibit high [Mg/Ca]
      ratios. This abundances signature is likely the result of nucleosynthesis in
      high mass (30 Msun) core-collapse supernovae pointing at a top-heavy initial
      mass function for Grus II. (2) Gjöll is a stellar stream discovered in the
      northern hemisphere. Inspection of its orbit suggests it is accreted from the
      globular cluster NGC 3201. Abundances for the stream stars confirms this
      connection for some stars while rejecting others, demonstrating the need for a
      combined chemical and dynamical analysis.


  • Thursday, May 14 [zoom]
    • Ian Roederer
    • The astrophysical r-process: what we are learning from gravitational waves, dwarf galaxies, and stellar archaeology

      Understanding the origin of the elements is one of the major challenges of modern astrophysics. The rapid neutron-capture process, or r-process, is one of the fundamental ways that stars produce the heaviest elements, but key aspects of the r-process are still poorly understood. I will describe four major advances in the last few years that have succeeded in confirming neutron star mergers as an important but perhaps not exclusive site of the r-process. These include the detection of freshly produced r-process material powering the kilonova associated with the merger of neutron stars detected via gravitational waves (GW170817), the identification of a dwarf galaxy where most of the stars are highly enhanced in r-process elements (Reticulum II), new connections between the r-process and its Galactic environment (thanks to data from the Gaia satellite), and advances in deriving abundances of previously-undetected r-process elements (such as Se, Te, Pt) in ultraviolet spectra of metal-poor stars. I will highlight opportunities to connect these research directions with future facilities (like FRIB, CETUS, and HabEx) to associate specific physics with specific sites of the r-process.


  • Thursday, April 23 [Zoom]
    • Kate Alexander, Northwestern University
      Cosmic Extremes: Time-Domain Astrophysics in a Multi-Messenger World

      Time-domain astrophysics provides a unique opportunity to study the most extreme physical processes in the Universe, including the deaths of massive stars, the creation and merger of compact objects like neutron stars and black holes, and the tidal disruption of stars by supermassive black holes. I will discuss my recent and ongoing work to reveal the formation and structure of relativistic jets and outflows in the most extreme classes of astrophysical transients, including gamma-ray bursts (GRBs) and tidal disruption events (TDEs). I will further show that radio data provide the best constraints on the immediate environments of these tran​sients, probing models of black hole growth and accretion (TDEs) and stellar evolution models (GRBs). With the pioneering detections of gravitational waves, astronomers and physicists have gained a new, complementary tool to study compact object mergers and their associated GRBs, with implications for fields as wide-ranging as general relativity, nuclear physics, cosmology, and shock physics.

  • Thursday, April 16 [Zoom]
    • Philip Mocz
      Fuzzy Dark Matter Cosmology

      The near-century-old dark matter (DM) problem is one of the most intriguing mysteries in modern physics. We do not know the nature of 84% of matter in the Universe, yet it is thought to govern cosmic structure and hold galaxies and clusters together. In this colloquium, I will present pioneering simulations of what the Universe would look like if DM were ultra-light, in the so-called `fuzzy dark matter’ (FDM) limit where DM is a ~10^-22 eV boson. In hierarchical models of structure formation, the first galaxies form in low-mass DM potential wells, probing the behavior of DM on kiloparsec (kpc) scales. Even though these objects have not yet been observed, telescopes such as the James Webb Space Telescope (JWST) will soon offer an observational window into this emergent world.

      In this talk, I show how the first galaxies are assembled in FDM cosmology along dense DM filaments. Using first-of-its-kind cosmological hydrodynamical simulations, I explore the interplay between baryonic physics and unique wavelike features inherent to FDM. In the simulations, the DM filaments show coherent interference patterns on the boson de Broglie scale, develop cylindrical soliton-like cores, and form stars along the entire structure. The filaments are unstable under gravity and collapse into kpc-scale spherical solitons. Features of the DM distribution are largely unaffected by the realistic baryonic feedback; on the contrary, gas and stars follow DM filaments and their profiles exhibit flattened cores — smoking gun signatures of FDM. I contrast these results against first structures in cold and warm DM cosmologies.

      I will also discuss a variety of other small-scale astrophysical consequences of FDM due to its unique substructure, which place independent constraints on the FDM particle mass, and present prospects for the future to validate or rule-out FDM.

  • Thursday, April 9 [Zoom]
    • Tansu Daylan
      TESS observations of the WASP-121 b phase curve

      I will present red-optical photometry of the ultra-hot Jupiter WASP-121 b as observed by the Transiting Exoplanet Survey Satellite (TESS) and model its atmosphere through a radiative transfer simulation. Given its short orbital period of 1.275 days, inflated state and bright host star, WASP-121 b is exceptionally favorable for detailed atmospheric characterization.

      We characterize its full red-optical phase curve, including the planetary phase modulation and the secondary eclipse. We measure the day and nightside brightness temperatures in the TESS passband as 2940+38-41 K and 2190+294-146 K, respectively, and find no phase shift between the brightest and substellar points. This is consistent with inefficient heat recirculation on the planet.

      We then perform an atmospheric retrieval analysis to infer the dayside atmospheric properties of WASP-121 b such as its bulk composition, albedo and heat recirculation. We confirm the temperature inversion in the atmosphere and suggest H-, TiO and VO as potential causes of the inversion, absorbing heat at optical wavelengths at low pressures. Future HST and JWST observations of WASP-121 b will benefit from its first full phase curve measured by TESS.

  • February 24th (Monday)
      • Arpita Roy, California Institute of Technology
    Of Worlds to Come: Exoplanets with Extreme Precision Spectroscopy

    We are exoplanet hunters, anchored for now to the only habitable planet we know of, around a relatively well-behaved yellow star. Other Earths have remained elusive, hidden deep within the noise of their own stars, and the limitations of our techniques. To overcome these challenges we adopt a multidisciplinary approach encompassing stellar and planetary physics, cutting edge instrumentation, and innovative data analysis techniques. Based on our technological and scientific advancements in the last decade, the field of radial velocity (RV) exoplanet detection is now poised to enter an exciting new phase. Currently in build+commissioning phases are planet hunting Doppler spectrographs aiming at <30cm/s RV precision in the optical in quest for Earth analogs, and <1m/s in the NIR 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 software pipelines. In this talk I will outline the state of the field in the context of six new planet hunting instruments I am working on, describe the challenges we have overcome, and look forward to the rich rewards we can consequently expect — including an unprecedented understanding of stars, and the spectroscopic direct detection of planetary atmospheres from the ground.

  • February 20th (Thursday)
      • Kirit Karkare, University of Chicago
    Testing Inflation and Dark Energy with Next-
    Generation Millimeter-Wave Telescopes

    While the Lambda-CDM cosmological model is remarkably effective at describing the Universe as a whole, foundational questions remain. Where did the primordial fluctuations come from and why are they so uniform? What is causing the present-day accelerated expansion? Maps of large-scale structure at different epochs can address these questions. I will present results from the BICEP/Keck cosmic microwave background experiment, which leads the search for B-mode polarization from an inflationary expansion at the earliest moments of time. I will then discuss a promising new cosmological probe—millimeter-wave line intensity mapping—which will measure large-scale structure well into the first billion years of the Universe and constrain the nature of dark energy and the physics of reionization. These observables have benefited from recent advances in millimeter-wave technology, and I will describe how next-generation instrumentation will improve our understanding of the early Universe over the next ten years.

  • February 13th (Thursday)
      • Sarah Rugheimer, University of Oxford
    UV, Biosignatures, and Life

    When we observe the first terrestrial exoplanet atmospheres, we expect to find planets around a wide range of stellar types, UV environments, and geological conditions. Since the first exoplanets available for characterization will be likely for M dwarf host stars, understanding the UV environment of these cool stars is a vital step in understanding the atmospheres of these planets. Additionally, the atmospheres of these planets will not have been fixed in time. Earth itself offers many possible atmospheric states of a planet. We set out to examine how an Earth-like planet at different geological epochs might look around other star types. Additionally, we examine the plausibility of detecting prebiotically-interesting molecules, such as HCN, NH3, CH4, and C2H6 in an early-Earth type atmosphere around stars with very different UV environments: an M dwarf and a solar analogue.

  • February 10th (Monday)
      • Jordan Stone, University of Arizona
    Understanding the Planet Formation Process with High Resolution Imaging

    Understanding the planet formation process is a central goal of the exoplanet community. The giant-planet formation mechanism is especially important to understand because massive planets form earliest, and they dominate the dynamics and regulate material flow through planet forming disks. My work aims to inform theories of giant planet formation using high-spatial resolution techniques to isolate light from planetary surfaces from the blinding light of their host stars. Spatially resolved imaging facilitates detailed spectroscopic measurements of planetary atmospheres revealing their composition. This is important because planetary composition can be related to formation location using the fact that temperature gradients in protoplanetary disks impose chemical gradients in the gas-phase disk material as different volatile species condense to solid phases at different distances from the central star. I will discuss two projects that I lead to develop instruments for improving sensitivity to planets, including a new fringe tracking camera for the LBT to enable 23 m direct imaging.

  • February 6th (Thursday)
    • Joaquin Vieira, University of Illinois Urbana-Champagne
      The Universe, Seen in the Far-Infrared

      I will present an overview of observations, technologies, and facilities observing the evolution of the Universe in the (far-)infrared, from 2 to 2000 microns (um) in wavelength. I will begin with current efforts to study the cosmic microwave background (CMB, 1000- 4000um), the relic radiation left over from the Big Bang. I will present an overview of the rich scientific questions currently being pursued by CMB experiments, which ties together the most disparate scales possible in science: quantum mechanics and cosmology; the beginning of the universe to the present day. I will transition to studies of high-redshift galaxy evolution with the Atacama Large millimeter/submillimeter Array (ALMA 450- 3000um) and the future with the James Webb Space Telescope (2-30um). Understanding the formation and evolution of galaxies is one of the foremost goals of astrophysics and cosmology today and these two facilities are, and will be, providing exciting new insights into these key questions. The far-infrared (50-500um) portion of the electromagnetic spectrum provides a unique window into the evolution of the Universe and, while difficult, far-infrared spectroscopy is crucial for studies of the interstellar medium, galaxy evolution, and the high-redshift Universe. I will also discuss new instruments on the ground and in space which will significantly expand our discovery reach with the (far-)infrared into the coming decades.

  • February 3rd (Monday)
      • Brett McGuire, National Radio Astronomy Observatory
    New Frontiers in Cosmic Carbon

    Molecular clouds of gas and dust pervade our galaxy, and are the birthplaces of stars and planets. The temperature, density, and radiation conditions inside these clouds make them unique chemical laboratories for studying both fundamental reactions and the evolution of the molecular complexity that seeds primitive planets. We have recently discovered a new regime of unexpected low- temperature aromatic chemistry in these sources that has far-reaching implications on the lifecycle of carbon in the universe. These detections are demanding new analysis techniques to extract the maximum information content from increasingly large and complex datasets both observationally and in the laboratory. Here, I will discuss novel applications of signal processing and analysis in both arenas. Observationally, we are using Bayesian approaches combined with matched filtering techniques, while in the laboratory, we are conducting reaction screening analyses to try to understand the content and evolution of this new carbon chemistry in space. I will describe the results of our new observational analysis techniques, as well as outline several new methods we have developed for rapidly screening complex chemical mixtures in the laboratory using pump-probe rotational spectroscopy in a (semi) automated fashion to enable new molecular discovery.

Fall 2019 Schedule

  • October 3rd
    • Speaker – Romeel Dave, University of Edinburgh
  • October 10th
    • Speaker – Željko Ivezić, University of Washington
  • October 17th
    • Speaker – Rahul Kannan, Harvard University
  • November 14th
    • Speaker – John Johnson, Harvard University
  • November 21th
    • Speaker – Stephen Portillo, University of Washington
  • December 5th
    • Speaker – Clara Sousa-Silva, MIT