Astronomy Colloquia
Astronomy and Astrophysics Colloquia - Usually Wednesdays, 3:30 PM, ERC 161, unless otherwise specified. Reception starts at 4:30 PM in Astro Lounge; persons with a disability who believe they may need assistance, please call the departmental secretary in advance at 773-702-8203 or email See also the list of KICP Wednesday Colloquia which alternate with the Astronomy and Astrophysics Colloquia.

Current & Future Astronomy Colloquia
DateTalk TitleSpeaker
February 10, 2016IceCube: The Discovery of High-Energy Cosmic NeutrinosFrancis Halzen, University of Wisconsin, Madison
February 24, 2016Two studies in planetary dynamics: (i) Impact seasons on Mars, (ii) The mass function of planets in the GalaxyRenu Malhotra, University of Arizona
March 9, 2016TBATsevi Mazeh, Tel Aviv University

Past Astronomy Colloquia
DateTalk TitleSpeaker
January 27, 2016The Universe's most extreme star-forming galaxiesCaitlin Casey, University of Texas at Austin
January 13, 2016New Frontiers in Simulating Black Hole Accretion and JetsAlexander (Sasha) Tchekhovskoy, UC Berkeley

New Frontiers in Simulating Black Hole Accretion and Jets
January 13, 2016 | ERC 161 | 3:30 PM
Alexander (Sasha) Tchekhovskoy, UC Berkeley

Black holes are responsible for a wide variety of astrophysical phenomena. They devour stars, eject relativistic jets, affect star formation and galaxy evolution, and enrich the Universe with heavy elements. In the next several years, the Event Horizon Telescope will produce resolved images of infalling gas and jets on the event horizon scale that promise to revolutionize our understanding of black hole physics. However, until recently, no first-principles models to quantitatively interpret these observations existed. I will present the first such models, the simulated spectra and images, and the constraints on the near event horizon physics coming from the comparison to the observations of the supermassive black hole at the center of our galaxy. I will then use simulations to constrain black hole physics in several other astrophysical contexts. I will finish by making connections to my future research plans.

The Universe's most extreme star-forming galaxies
January 27, 2016 | ERC 161 | 3:30 PM
Caitlin Casey, University of Texas at Austin

Dusty star-forming galaxies host the most intense stellar nurseries in the Universe. Their unusual characteristics (SFRs=200-2000Msun/yr) pose a unique challenge for cosmological simulations and galaxy formation theory, particularly at early times. Although rare today, they were factors of 1000 times more prevalent at z~2-5, contributing significantly to the buildup of the Universe's stellar mass and the formation of high-mass galaxies. However, an ongoing debate lingers as to their evolutionary origins at high-redshift, whether or not they are triggered by major mergers of gas-rich disk galaxies, or if they are solitary galaxies continually fed pristine gas from the intergalactic medium. Observational evidence has been mixed over recent years; some studies clearly point to chaotic kinematic histories and fast gas depletion times (~<100Myr), while other work may demonstrate secular (though active) disks can sustain high star-formation rates over long periods of time. Similarly, some works argue such extreme star-formers contribute very little to cosmic star-formation, while others find quite the opposite. Furthermore, their presence in early protoclusters, only revealed quite recently, pose intriguing questions regarding the collapse of large scale structure. I will discuss some of the latest observational programs dedicated to understanding their origins and frequency at early times, their context in the cosmic web, and future long-term observing campaigns that will reveal their relationship to `normal’ galaxies, thus teaching us valuable lessons on the physical mechanisms of galaxy growth and the collapse of large scale structure in an evolving Universe.

IceCube: The Discovery of High-Energy Cosmic Neutrinos
February 10, 2016 | ERC 161 | 3:30 PM
Francis Halzen, University of Wisconsin, Madison

The IceCube project has transformed one cubic kilometer of natural Antarctic ice into a neutrino detector. The instrument detects more than 100,000 neutrinos per year in the GeV to PeV energy range. Among these, we have recently isolated a flux of high-energy cosmic neutrinos. The high cosmic neutrino flux observed indicates that a significant fraction of the radiation in the non-thermal universe, powered by compact objects from neutron stars to supermassive black holes, is generated by proton accelerators. We will discuss the IceCube instrument, the analysis of the data, and the significance of the discovery of cosmic neutrinos.

Two studies in planetary dynamics: (i) Impact seasons on Mars, (ii) The mass function of planets in the Galaxy
February 24, 2016 | ERC 161 | 3:30 PM
Renu Malhotra, University of Arizona

I will present results of new calculations of the asteroidal impact flux on Mars. Mars' orbit is significantly eccentric and the planet orbits near the inner edge of the asteroid belt where the space density of asteroids has a large radial gradient. The correlated secular dynamics of Mars and the asteroids plays a significant role in modulating the impact flux on this planet. At the present epoch, this leads to a large variation -- of about a factor of three -- in the impact flux when Mars is near aphelion versus perihelion; significantly, the integrated annual impact flux is lower than would be expected in the absence of correlated secular dynamics.

The second part of the talk will describe some deductions about the planet mass function from the observational data of exoplanets and theoretical considerations of long term stability. I will describe analysis of the observational data from the Kepler space mission which indicates that planetary orbital separations have an approximately log-normal distribution. Adopting some plausible ansatzs for the dynamical stability of N-planet systems to relate orbital separations to planet masses, it appears that the planet mass function is peaked in logarithm of mass, with the most probable value of log m/M⊕ ∼ (0.6 − 1.0); a modest extrapolation indicates that Earth mass planets are about ~1000 times more common than Jupiter mass planets, and that the most common planets in the Galaxy may be of lunar-to-Mars mass.

March 9, 2016 | ERC 161 | 3:30 PM
Tsevi Mazeh, Tel Aviv University