Description:

CIERA Colloquia feature distinguished external visiting researchers, and are a regularly occurring colloquia series held throughout the Fall, Winter, and Spring academic quarters. Colloquium speakers are invited to spend several days at CIERA.

Description:

CIERA Colloquia feature distinguished external visiting researchers, and are a regularly occurring colloquia series held throughout the Fall, Winter, and Spring academic quarters. Colloquium speakers are invited to spend several days at CIERA.

Description:

Colloquium abstract: Tidal disruption events, or TDEs, are a dramatic type of electromagnetic transient.  TDEs are triggered by the death throes of unlucky stars that pass too close to massive black holes (MBHs).  Stellar tidal disruption initiates a complex chain of events that culminates in the production of super-luminous, multi-wavelength flares of radiation, rising and decaying over a period of weeks to months.  TDEs hold exceptional promise for measuring the bottom end of the MBH mass function, potentially answering old questions about the origins of supermassive black holes.  However, this promise can only be achieved with larger samples of TDEs and better theoretical models that are robust for parameter estimation.  I will discuss how near future time domain surveys, particularly Rubin and ULTRASAT, will expand our TDE sample by two orders of magnitude.  I will then describe my group's recent and ongoing efforts to solve the challenging "circularization problem" for TDEs, a longstanding theoretical bottleneck that has finally been cracked open with the use of moving-mesh radiation-hydrodynamics.  I will conclude with a discussion of late-time TDE emission, a phase of the TDE that is harder to observe but which may facilitate MBH mass measurement.

Speaker: Nick Stone, Assistant Professor, University of Wisconsin-Madison

Host: Fred Rasio

Description:

Abstract : In dense stellar environments, stars can physically collide. Stellar collisions play a key role in shaping the stellar population at the centers of stellar clusters and galactic nuclei. Depending on the collision's kinetic energy, different outcomes arise. When the collision kinetic energy is smaller than the binding energy of the colliding stars, the two stars merge into a more massive star, which can manifest as a group of stars appearing younger and bluer than their surroundings — known as blue straggler stars. Conversely, if the collision kinetic energy is larger, as in nuclear stellar clusters around a supermassive black hole, the colliding stars may be partially or completely destroyed, producing expanding ejecta. This process can generate bright flares, making these events promising nuclear transients. In this talk, I will discuss the properties of collision products in these two regimes. In the first part, I will explore magnetic field amplification in low-velocity collisions as a potential solution to the "angular momentum" problem for blue straggler stars. I will also present results for binary mergers and stable mass transfer in the context of blue straggler formation. In the second part, I will discuss the observables of high-velocity collisions involving both giants and main-sequence stars and those of ejecta’ accretion of onto the supermassive black hole. 

Speaker: Taeho Ryu, MPA fellow, Max Planck Institute for Astrophysics

Host: Fred Rasio

Description:

Over the next decade, large galaxy surveys will map billions of galaxies and probe cosmic structure formation with high statistical precision. This talk will outline opportunities and challenges of cosmological analyses in the presence of complex systematic effects using recent results from the Dark Energy Survey as pathfinder examples. In particular, I will describe different cosmological probes measured from photometric data and summarize the recent progress on combining galaxy clustering, weak lensing, cluster clustering and cluster abundances, as well as constraints on baryons and galaxy biasing from small scales. I will conclude with an outlook on cosmology analysis plans and challenges for future, much larger experiments such as Rubin Observatory’s LSST, Roman Space Telescope and overlapping Cosmic Microwave Background surveys.

Speaker: Elisabeth Krause, University of Arizona

Host: Lena Murchikova