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This Week at Physics

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Events During the Week of March 3rd through March 10th, 2019

Monday, March 4th, 2019

Special Noon Talk 3/4/19
Star Formation, etc
Time: 12:00 pm
Place: 4421 Sterling Hall
Speaker: Nia Imara, Harvard CfA
Abstract: Stars are of fundamental importance to astronomy, and how they form and shape their environments influence everything from exoplanet studies to cosmology. In this talk, I will discuss recent ALMA observations of dwarf galaxies—excellent astrophysical laboratories for exploring star formation in the early Universe—and I will consider some future prospects for advancing our knowledge in this exciting field. As a bonus, I will present a novel idea for detecting planets bound to x-ray binaries, proposing that the x-ray light curves of such systems be inspected for signatures of transiting planets.
Host: Sebastian Heinz, Astronomy Chair
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Plasma Physics (Physics/ECE/NE 922) Seminar
Ion-scale turbulence in the presence of a large magnetic island
Time: 12:05 pm
Place: 2241 Chamberlin Hall
Speaker: Lucas Morton, UW-Madison
Abstract: Understanding the effect of magnetic field geometry on turbulence is important for both tokamaks and stellarators. The introduction of a magnetic island to a toroidal plasma might influence turbulence in several ways: directly through geometric effects or indirectly through modifications to gradients that drive turbulence or flows that suppress turbulence. In an experiment on DIII-D tokamak, we studied the response of ion-gyro-radius scale density fluctuations using the Beam Emission Spectroscopy (BES) diagnostic. Our results confirm qualitative predictions of prior turbulence simulations, especially the reduction of turbulent fluctuations at the center of the island. This motivates dedicated simulations to understand the relative contribution of the various mechanisms by which magnetic geometry impacts turbulence in this situation.
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Tuesday, March 5th, 2019

Chaos & Complex Systems Seminar
Projecting future floodplains and the impacts to future risks and vulnerabilities in the United States
Time: 12:05 pm
Place: 4274 Chamberlin (refreshments will be served)
Speaker: Shane Hubbard, UW Space Science and Engineering Center
Abstract: The Space Science and Engineering Center at UW Madison is working with the Georgia Department of Natural Resources to investigate the impacts of a changing climate on the floodplains in coastal Georgia. Two communities, Hinesville and Tybee Island, are dealing with changes to their floodplains and the current impacts to their citizens. In this work we investigate the potential changes to their floodplains and the impacts associated with increased risks and damages. This work revealed that even minor changes to water volumes within the floodplain can result in damages many times greater than the expected changes in water levels. The next phase of this research is beginning and involves working directly with the community to educate and then respond to these possible future scenarios.
Host: Clint Sprott
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Astronomy Colloquium
Special Tuesday Talk
Understanding the Relationship between Dense Gas and Star Formation in Galaxy Nuclei
Time: 3:30 pm
Place: 4421 Sterling Hall, Coffee and cookies 3:30 PM, Talk begins 3:45 PM
Speaker: Betsy Mills, Brandeis University
Abstract: Star formation in environments from local molecular clouds to distant galaxies is characterized by a simple, linear relationship between the amount of gas compared to the amount of recently formed stars. However, star formation in the center of our Galaxy does not follow these universal scaling relations, suggesting that the star formation process has a more complex dependence on environment. In the Galactic center, scaling relations predict there should be a larger amount of star formation given the amount of dense gas that is present. My recent work has put new constraints on both these quantities, indicating that neither overestimates of gas density nor underestimates of star formation can completely account for this discrepancy. Instead, it is most likely that (1) in this environment there is a higher gas density threshold required for the onset of star formation and (2) the center of our galaxy is currently in a quiescent or low state in between cycles of starburst activity. This poses a challenge for using the Galactic center to better characterize deviations from star formation ‘laws’ in extreme environments, as the Galactic center is far from experiencing a global starburst. I describe two ways my recent work is addressing this challenge: firstly, by characterizing properties of dense gas and star formation in nearby starbursting nuclei at high spatial resolutions now available with ALMA. Secondly, I describe my recent studies of the Sgr B2 "mini-starburst”: an individual cloud in the galactic center that may be more characteristic of the global mode of star formation observed in nuclear starbursts. Finally, I end with a cautionary note on remaining challenges for correctly interpreting observations of dense gas tracers in more distant extreme and unresolved systems.
Host: Professor Eric Wilcots
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Wednesday, March 6th, 2019

Department Meeting
Time: 12:15 pm
Place: 5310 Chamberlin Hall
Speaker: Sridhara Dasu, UW - Madison
Department Meeting
Host: Sridhara Dasu
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Thursday, March 7th, 2019

Cosmology Journal Club
Time: 12:00 pm
Place: 5242 Chamberlin Hall
Abstract: Please visit the following link for more details:
http://cmb.physics.wisc.edu/journal/index.html
Feel free to bring your lunch!
If you have questions or comments about this journal club, would like to propose a topic or volunteer to introduce a paper, please email Ross Cawthon (cawthon@wisc.edu) and Santanu Das (sdas33@wisc.edu).
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Astronomy Colloquium
Caught in the act - witnessing the formation of the most massive galaxies across cosmic time
Time: 3:30 pm
Place: 4421 Sterling Hall, Coffee and cookies 3:30 PM, Talk begins 3:45 PM
Speaker: Chien-Chou Chen, ESo Fellow, European Southern Observatory, Germany
Abstract: More than half of the stellar mass in the local Universe is found in galaxies that are very massive yet have long ceased to form new stars. Exactly when and how these galaxies formed remain open questions. Detailed galaxy archaeology studies have suggested that these galaxies were assembled in a very dramatic fashion, with most of their stellar mass forming very early, when the Universe
was about 20% of its age, and in a strikingly short period of time. In this talk, I will discuss my efforts to uncover the progenitors of these galaxies via extragalactic submillimeter surveys, and to study their evolution and physical properties using multi-wavelength data sets. I will also present my future research plans, which focus on three novel methodologies for studying the environments and interstellar media of the progenitors.
Host: Professor Amy Barger
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Friday, March 8th, 2019

Physics Department Colloquium
How Much Time Does a Tunneling Atom Spend In The Forbidden Region?
Time: 3:30 pm
Place: 2241 Chamberlin Hall
Speaker: Aephraim Steinberg, U Toronto
Abstract: The question of the time a particle takes to tunnel through a classically forbidden region has a long history, complicated by the fact that the simplest predictions for the arrival time of a wave packet peak may be smaller than the barrier thickness divided by the speed of light. By now it is well understood that this result is not paradoxical, but it leaves open the question of how long a particle interacts with the barrier, and of whether it is sensible to distinguish between interaction times for transmitted and reflected particles.

By preparing ultracold Rubidium atoms in an atom waveguide and cooling them to approximately 1 nK, we are able to study tunneling across a 1-μm barrier formed by a blue-detuned laser beam. Using Raman coupling to generate a fictitious magnetic field, we let the spin of each atom act as a “clock” to record how long it spends in the barrier region. I will present our first results characterizing the tunneling time in this way. We analyze them in terms of the weak-measurement formalism, which makes it possible for one to discuss different “histories” for particles which end up in different final states. I will spend some time discussing this formalism more broadly, including problems it resolves and puzzles it raises.
Host: Mark Saffman
Video: https://vod.physics.wisc.edu/media/2019_03_08.m4v
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