Title: What can the Occult do for You? Using overlapping galaxies to probe dust properties in galaxies

Abstract: Interstellar dust is still a dominant uncertainty in astronomy, limiting precision in, e.g., cosmological distance estimates and models of how light is re-processed within a galaxy. When a foreground galaxy serendipitously overlaps a more distant one, the latter backlights the dusty structures in the nearer foreground galaxy. Such an overlapping or occulting galaxy pair can be used to measure the distribution of dust in the closest galaxy with great accuracy. 

The STARSMOG program uses Hubble to map the distribution of dust in foreground galaxies in fine (<100 pc) detail. Integral Field Unit (IFU) observations will map the effective extinction curve, disentangling the role of fine-scale geometry and grain composition on the path of light through a galaxy. The overlapping galaxy technique promises to deliver a clear understanding of the dust in galaxies: geometry, a probability function of dimming as a function of galaxy mass and radius and its dependence on wavelength. I will present the recent results from Hubble and James Webb observations of the pair VV191 and prospects with the upcoming Roman surveys. 

Title: Hidden Engines: Uncovering the Workings of the Nearest Galaxy Center

Abstract: Centers of galaxies are some of the most extreme objects in our universe: They host starbursts and active supermassive black holes that can launch jets and winds far outside the compact galaxy nucleus. The effects of the interactions among stars, gas and black holes that occur here don’t just stay confined to these small regions; they have an outsized influence on the overall evolution of galaxies as a whole. 

At just 8.1 kpc away, the center of the Milky Way is unparalleled in its proximity, making it the best laboratory for detailed studies of the processes that govern and define galaxy nuclei. However, the galactic center also presents a big challenge for these studies: It is a relatively quiet environment. Few stars are forming in this region, and the black hole is not active. Clearly, it hasn’t always been this way: From the Fermi Bubbles to hundred-year old echoes of X-ray bursts, there are many relics of an active past in the center of our own Milky Way. 

We also know our galaxy center likely won’t stay quiet for long; it contains a sizable reservoir of molecular gas that is the fuel for future star formation and black hole accretion. In this talk, I will present the results of research following the gas and its properties from kiloparsec to sub-parsec scales to understand why the galactic center is so quiet right now and what the future holds. Finally, I will discuss ongoing work to increase the sample size of galaxy nuclei with parsec-scale gas measurements and what this means for putting the Galactic center in context with its more active neighbors

Dr. Itamar Kimchi, Georgia Tech

Title: Lattice defects in quantum magnets and topological systems

Abstract: Defects are always present in solid state materials. I will present our group’s recent theoretical results showing how quantum-entangled or topological systems can enable local defects to produce surprising global effects:

• First, in the Kitaev honeycomb quantum spin liquid (QSL), non-magnetic “Stone-Wales” crystallographic defects become imbued with a fluctuating magnetic chirality. The emergent Ising model for their chiralities is ferromagnetic and long ranged (~1/r^2.7), producing an instability to a topological chiral QSL at a finite critical temperature set by the defect density. 
• Second, in the 1/3 magnetization plateau of triangular lattice magnet KCSO, IR spectroscopy observed unusual satellite lines. We show that though these lines are sharp, they arise from disorder and enable its characterization as dilute vacancies. 
• Third, in the Dirac cones of the honeycomb lattice, magnetic impurities induce circulating currents with an associated topological Chern number. Surprisingly for isolated impurities, this induced magnetization and topology is reversed above a critical impurity strength with a global phase transition generated by the local defect physics.