Mon, June 29, 2020
When an isolated binary black hole merges in the field of a galaxy, its gravitational-wave story is complete. But when black holes merge in a dense star cluster, their merger products can remain in the cluster, where they continue to participate in dynamical encounters, form binaries, and potentially merge again. In this talk I will describe the production of repeated mergers in globular clusters, and how the rate of mergers depends on the initial properties (e.g. spin) of black holes formed from stars. I will show how these "second-generation" black holes differ from black holes formed from stellar collapse, and how Advanced LIGO and Virgo can already distinguish these unique astrophysical populations.
Mon, June 22, 2020
In the past decade, several new jovian exoplanets at wide separations have been revealed using ground based telescopes equipped with adaptive optics systems. These planets, with masses between ~2-14 MJup, remain a puzzle for both major planet formation models. At the same time, they offer a powerful tool in the hunt for observational constraints of formation, as they can be characterized with both imaging and spectroscopy. I will describe our recent efforts to push beyond the discovery phase into the realm of detailed characterization of these planetary systems. Using OSIRIS at Keck, we have been targeting known directly imaged planetary systems for detailed mapping of their atmospheres at R~4000. I will describe our findings, including the atmospheric abundance measurements for these planets, which can potentially be used as a diagnostic of formation. I will describe upcoming instrumentation efforts that will improve our ability to obtain spectra for directly imaged planets, including spectrographs in the planning phases for Keck and the future Thirty Meter Telescope, and the discuss prospects for direct exoplanet spectroscopy in the next two decades from the ground.
Mon, June 15, 2020
The centers of massive galaxies and galaxy clusters contain hot plasma that loses its energy rapidly through radiation of X-ray photons. The energy loss is thought to be compensated by the energy input from the supermassive black holes (SMBHs) in the centers of these systems, via a process often termed as “AGN feedback”. In this talk, I will review the state of the field, and discuss what we have learned from numerical simulations in the past few years, including how AGN jets deposit their energy to the surrounding medium, and how they affect cooling and star formation. I will also talk about my recent analysis of optical and ALMA observations of multiphase filaments in cluster centers, which not only improves our understanding of AGN feedback, but also puts unprecedented constraints on microscopic transport processes in the weakly-collisional, magnetized intracluster plasma.
Mon, June 08, 2020
In the first billion years of the universe, stars and galaxies formed in the smallest dark matter halos, produced high-energy photons that reionized the intergalactic medium, and polluted the universe with the first heavy elements. Near-field cosmology probes this early era by observing nearby relic galaxies that have survived from ancient times. In particular, the elemental abundances of their old, metal-poor stars encode otherwise inaccessible information about the first stellar populations and first galaxy formation histories. Decoding these abundances requires connecting nuclear and stellar astrophysics to galaxy formation and hierarchical assembly. I will show how stellar abundances of metal-poor stars have shaped our current understanding of the rapid neutron-capture process (r-process), including how they inform future multi-messenger observations of kilonovae. The r-process can in turn be used to build our understanding of the high-redshift universe, including galaxy formation in the faintest dwarf galaxies and the hierarchical assembly of our Milky Way's stellar halo.
Mon, June 01, 2020
The existence of extragalactic fast radio bursts (FRBs) of sub-millisecond durations, originating at cosmologically significant distances, has been established. A population of repeating FRB sources is emerging, and five burst sources have been identified with host galaxies at redshifts between 0.03 and 0.66. Explaining the FRB phenomenon has proved a compelling challenge to theory, with the number of distinct models only last year being superseded by the ~100 reported events. I will review the state of the field, with a focus on recent results that are beginning to unravel the FRB mystery. I will discuss in particular the potential Galactic FRB associated with an active magnetar, and the implications for the origins of FRBs. FRBs have also opened a powerful new window into otherwise unseen matter in the Universe. I will show how large FRB samples will help assess the baryon contents and physical conditions in the hot/diffuse circumgalactic, intracluster, and intergalactic medium. Finally, I will provide an overview of FRB-related observational programs underway at the Owens Valley Radio Observatory, including the Deep Synoptic Array (DSA) and its proposed 2000-dish successor (DSA-2000).
Thu, January 16, 2020