Astronomy Colloquia at Caltech for 2021-22
Colloquia are held every Wednesday during the academic year at 4pm in the Cahill Hameetman auditorium. Wine and cheese will be served in the Cahill Foyer from 5:00-5:30pm (pending COVID restrictions).
NOTE: DURING COVID-19 PANDEMIC, ASTRONOMY COLLOQUIUM WILL BE VIRTUAL.
New general relativistic MHD models of black hole accretion flows in luminous systems (such as quasars and X-ray binaries) that include
full radiation transport will be described. These models are designed to study the steady-state structure of the accretion disk near the horizon, and the effect of radiation on the launching of relativistic jets from spinning black holes. Moreover, they enable not only the interpretation of the spectra and variability of these sources, but also predictions about the rate of growth of black holes in the early universe, and measurement of the energy and momentum feedback into the surrounding medium, a process likely to be important in galaxy formation. These calculations use a new version of the Athena ++adaptive mesh refinement code based on the Kokkos library that runs on both CPUs and GPUs. A brief description of this new code, as well as other applications and extensions underway, will also be given.
The Milky Way contains of order 10^8 stellar-mass black holes (BHs). Yet, fewer than 100 BH candidates are known, and only about 20 are dynamically confirmed. Our view of the BH population has been shaped almost entirely by observations of X-ray binaries and gravitational wave sources, both of which represent an extremely rare outcome of binary evolution. I will discuss recent efforts to uncover the (potentially) much larger population of Galactic black holes in non-interacting binaries, focusing both on interpretation of confirmed dormant BHs and on what can be learned about binary evolution from the menagerie of interacting binaries and stripped stars that have masqueraded as dormant BH binaries. Finally, I will discuss our emerging view of the BH mass distribution from observations of X-ray binaries, dormant BH binaries, BHs detected via photometric and astrometric microlensing, and BHs in gravitational wave events.
The Milky Way halo provides an unparalleled opportunity to map dark matter on small, subgalactic scales; a key regime for distinguishing competing models of dark matter. Stellar streams are particularly sensitive tracers of the halo gravitational potential. However, in addition to dark matter, baryonic objects like the Milky Way disk, bar, or its satellites, can also impact streams. Using the wealth of precise astrometric data from Gaia, we have recently been able to make progress on disentangling these by (1) discovering stream origins from their orbital histories, (2) simultaneously modeling tidal tails from multiple globular clusters to map the halo gravitational potential, and (3) uncovering evidence of stream perturbations in the outer, dark-matter dominated halo. However, due to their low mass, detecting truly dark, cold dark matter subhalos will require even more precise, ~100m/s, kinematics throughout the Galaxy. I will discuss the Via project, a full-sky Magellan and MMT survey aimed at delivering ~100 m/s radial velocities at G<20 in dozens of stellar streams (first light 2026), and ultimately discovering the nature of dark matter.
Launched in 2018, Parker Solar Probe (PSP) is the first NASA missiondesigned to directly explore the plasma environment near the Sun, with afinal orbital configuration that will bring PSP within 10 solar radii ofthe solar surface in December 2024. Although the spacecraft is dominatedby in situ plasma and field instrumentation, there is one imager on board,the Wide-field Imager for Solar Probe (WISPR). WISPR consists of twowhite-light telescopes (WISPR-I and WISPR-O) that image Thomson-scatteredlight from solar wind electrons ahead of PSP in its orbit. I will presentsome results from the PSP mission so far, with a focus on WISPRdata. WISPR has provided novel new observations of the solar windflow and transients within it (e.g., coronal mass ejections), from veryclose to the Sun. During one of PSP's encounters with Venus, WISPR alsosurprisingly became the first telescope to penetrate Venus's thickatmosphere and image the Venusian surface in optical light.