Stellar Dynamics seminars

Quasi-periodic eruptions are repeating X-ray transients from supermassive black holes in nearby low-mass galactic nuclei. Their regularity, timescales and energetics suggest stellar companions orbiting at about 100 gravitational radii. They may provide unusually close dynamical probes of black holes and electromagnetic counterparts to extreme mass-ratio inspirals, a target for future space-based gravitational-wave observatories such as LISA. Joheen Chakraborty reviews the thirteen known systems, including the unexpected appearance of several in accretion discs created by tidal disruption events. The talk covers observations and theoretical developments, spectroscopic constraints on emission, the use of eruption timing to investigate orbital dynamics, and what event-rate estimates imply for inspiral formation. Hybrid seminar: attend in 370 Cahill, Caltech, Pasadena, California, United States, or remotely through the public Zoom meeting.

Future extreme-mass-ratio inspiral detections could identify how compact objects reach massive black holes. This talk compares expected eccentricities, inclinations and component masses for gas-free and gas-assisted formation. It examines how population measurements might separate channels and reveal properties of active-galaxy disks, including lifetimes and turbulence, as well as accretion behaviour and stellar black-hole populations in nuclear clusters.

Subha Majumdar presents a six-dimensional Milky Way phase-space catalogue designed to map stellar motions and dark matter beyond the reach of Gaia parallaxes alone. Gaia astrometry is combined with spectrophotometric distances and radial velocities from fourteen surveys, including DESI, SDSS-BOSS, APOGEE and LAMOST. The resulting catalogue contains about 33 million tracers and detailed distance–velocity measurements for roughly half a million halo stars. It characterizes distant clusters, dwarf galaxies and stellar streams, enabling mass modeling, kinematic studies, abundance mapping, Galactic archaeology and dark-matter searches. An application constructs a continuous Galactic rotation curve extending to 250 kiloparsecs.

This lecture compares the formation and survival of open and globular star clusters. It examines rapid dispersal in galactic disks, the creation of tightly bound clusters during violent gas-rich interactions and subsequent disruption. Encounters with a galactic disk and tidal stripping connect cluster evolution to the buildup of stellar haloes.

This seminar examines how galaxies assemble through both the accretion of gas along cosmic filaments and mergers with other galaxies. It connects tidal interactions and coalescence to the history of star formation. The discussion compares sustained stellar growth with bursts associated with dynamical encounters.

How can black holes be established both as a consequence of gravitational theory and as objects in the sky? Roger Penrose examines the mathematical development of gravitational collapse and space-time singularities, explaining why black-hole formation does not depend on perfectly symmetric idealisations. Reinhard Genzel and Andrea Ghez trace the observational effort to establish a massive compact object at the centre of the Milky Way. Their accounts connect infrared observations, improvements in imaging through atmospheric distortion, and long-term measurements of stellar motions around Sagittarius A*. Together, the lectures present the theoretical and observational foundations of the 2020 physics prize: the robustness of black-hole formation in general relativity and the evidence for the Galactic Centre’s supermassive black hole.