Seminars

We host a mix of virtual and hybrid seminars every other Thursday from 1:30-2:30pm.  For information on how to join them, or for any other queries regarding the seminars, please contact Stacy Kim.

Spring Seminars

27 January:  Wilma Trick (MPA Garching) - virtual

Gaia & the fingerprints of the Galactic bar - A dynamical Milky Way mystery
The Milky Way's disk around the Sun is interspersed by 7 streams of stars, amongst them the famous "Hercules", "Sirius", and "Hyades" moving groups. These streaming motions are caused by some powerful perturber. Suspects are the spiral arms, satellites, and/or the Galactic bar. So far, the true culprit has been notoriously elusive.  This talk shows how stars whose orbits are in resonance with the Galactic bar exhibit distinctive patterns in action-angle space. Based on these fingerprints, the exquisite Gaia data narrows down the search for the bar's Outer Lindblad Resonance (OLR) to two likely candidates: the "Hat" and the "Sirius" moving groups. Finally pinning down the true OLR (and therefore the pattern speed of the Galactic bar) will be a milestone in uncovering the Milky Way's structure and evolution.

10 February:  Massimo Dotti (Milan Bicocca) - virtual

Dynamics of LISA massive black hole pairs in galaxy (minor) mergers
I will describe the ongoing development of a novel semi-analytical code modeling the pairing of unequal mass MBH, from ~100 kpc scales down to the (possible) formation of a bound binary. The physical prescriptions implemented so far have been validated against high-resolution N-body simulations, include the effect of strong deviations from axisymmetry (e.g. stellar bars), and will soon include a cosmologically motivated evolution of the main galaxy during the pairing. I will discuss some future perspectives and alternative applications of the algorithm

24 February:  Anna Lisa Varri (Edinburgh) - hybrid (postponed)

10 March:  Sarah Pearson (New York University) - virtual (postponed)

24 March:  Hans-Thomas Janka (MPA Garching) - virtual

Core-Collapse Supernovae: From Neutrino-driven Explosion Models to Observations
Supernova explosions terminate the lives of massive stars, produce and disseminate a major fraction of the heavy elements, play an important role as neutrino and particle laboratory, and give birth to neutron stars and stellar-mass black holes, which have recently become sources of measured gravitational waves. After more than 50 years of progressively improved computational modeling, first-principle three-dimensional hydrodynamical simulations with detailed neutrino physics have meanwhile achieved to demonstrate the viability of the neutrino-driven mechanism. The consequences of this mechanism can now be confronted with observations. In particular, 3D simulations of such explosions provide new insights into the geometrical and chemical structure of young supernova remnants, possible explosion-progenitor connections, and the natal properties (masses, kicks, spins) of the compact objects formed in stellar core collapse events.

7 April:  Bob Nichol (Surrey) - hybrid

Time-Domain Extragalactic Survey (TiDES) for LSST Rubin
Type Ia supernovae have revolutionised our view of the Cosmos leading to a Nobel Prize in Physics in 2011 for the discovery of the accelerating Universe. Cosmologists believe this acceleration in the expansion rate of the Universe is driven by a new substance called “dark energy” that dominates the energy density today. In this seminar, I will review the use of Type Ia supernovae in making such measurements and how present surveys of such supernovae have improved our understanding of our Universe. I will also review the future of transient and supernova cosmology with new experiments like the LSST on the Rubin Observatory and the Time-Domain Extragalactic Survey (TiDES) using the 4MOST instrument on the VISTA telescope.  I also review my career for the interest of early-career researchers.

21 April:  Easter break

5 May:  Axel Widmark (University of Copenhagen) - virtual

Weighing the Galactic disk using phase-space spirals
We have developed a new method for inferring the gravitational potential of the Milky Way disk, using the time-varying structure of the phase-space spiral in the plane of vertical position and vertical velocity. Our method of inference extracts information from the shape of the spiral and disregards the bulk density distribution that is usually used to perform dynamical mass measurements. In this manner, it is complementary to traditional methods that are based on the assumption of a steady state. Furthermore, this method is especially robust with respect to severe and unknown selection effects, which allows us to apply it in distant regions of the Galactic disk where dust extinction and stellar crowding would otherwise be detrimental. We have demonstrated the accuracy of our method on simulations. We have also applied our method to Gaia data, from which we inferred the local halo dark matter density and placed the most stringent constraints to the surface density of a thin dark disk.

12 May:  Kelly Hambleton (Villanova University) - virtual

Pulsations and Precision: Why Binary Stars Form the Building Blocks of Stellar Astrophysics
Thanks to precise photometry from satellites such as TESS, Kepler and CoRoT, the field of asteroseismology is advancing at an astounding rate. For many stars, it is now possible to obtain fundamental stellar masses and radii based primarily on empirical asteroseismic formulæ. In complement, the field of binary star physics has long been viewed as the cornerstone of astrophysics, providing direct measurements of fundamental stellar parameters and giving insight into the evolution of stars. In this talk, I will examine each field individually, following which I will look at what happens when these two diverse fields are combined. Specifically, I will focus on the use of pulsating stars in binary systems to validate the results of asteroseismology and tidally driven pulsations: pulsations driven by gravitational interactions between the two stars in a binary system.

30 May:  Eric Andersson (Lund University) - hybrid

Resolved stars in galaxy simulations
Hydrodynamical simulations have been hugely successful in explaining many of the details governing the formation and evolution of galaxies. The most recent advances in this fields has in large part been facilitated by parsec-resolution simulations, now reaching the scales of star forming clouds. This has provided an incredibly detailed view of how stellar feedback and chemical enrichment operate on galactic scales. With parsec resolution, galaxy simulations can now be done star-by-star, and in fact should be done this way to avoid the many restrictions imposed by the traditional IMF-averaged approach. In this seminar, I will go through the most recent advances in this field, and present my own model incorporating individual stars in galaxy scale simulations. In this context, I will focus on stellar feedback and galactic wind driving mechanics, explaining how these are crucial aspects for successfully explaining galaxy evolution. Furthermore, I will shed light on the controversy regarding massive, fast-moving runaway stars and how they can affect these aspects. Finally, I will show preliminary results, where I applied my star-by-star model to ultra-faint dwarf galaxies simulated in a cosmological context. This enables us to study the relationships between properties of specific stars and the global properties of the dwarf galaxy. This allows for a very detailed comparison to data collected in spectroscopic surveys, which are limited to giant stars due to extremely low surface brightness.

9 June:  Annika Peter (Ohio State University) - virtual

Count your halos
One of the strongest predictions of the standard cold dark matter paradigm is the hierarchy of structure down to Earth-mass scales.  However, individual self-bound clumps of dark matter--"halos"--are difficult to detect directly.  Instead, we use galaxies as lampposts for halos.  By counting galaxies, we can measure the underlying population of dark matter halos.  In this talk, I describe results that seem completely at odds with each other in measuring the population of small halos.  I argue that the resolution to the problem is a better mapping between galaxies and halos.  I will show what my group is doing so far to address the problem, and what opportunities lie ahead in the wide-field surveys of the 2020's.

16 June:  Duncan Forbes (Swinburne University) - hybrid

Ultra Diffuse Galaxies: Galaxies at the Extreme
Ultra Diffuse Galaxies (UDGs) were first identified using the Dragonfly Telescope Array in 2015. Their extreme properties (of low surface brightness, large size, and in some cases rich globular cluster systems) continue to present challenges for standard cosmological simulations. UDGs may represent galaxies with a range of properties: from puffed-up dwarfs to failed galaxies, from those with overly massive dark matter halos to some that are dark matter free. In my talk I will present new observations, and contrast these with the latest simulations, summarising our current understanding of this extreme class of galaxy

Past seminars