[Seminar] [Astro] Dual Speaker Event

Lizhong Zhang (CCA, Flatiron Institute)

Title: Modeling Black Hole Accretion with Radiation GRMHD: A Parameter Survey and Connection to Observations

Abstract: Radiation and magnetic fields play crucial roles in shaping black hole accretion across a wide range of regimes. To model these systems, we solve the GRMHD equations coupled with angle-dependent radiation transfer, which enables us to capture the complex dynamics of accretion in extreme environments — from super-Eddington to sub-Eddington regimes. In the super-Eddington regime, radiative support causes the accretion disk to thermally expand, forming a narrow conical funnel through which radiation escapes, leading to low radiation efficiency. In the sub- and near-Eddington regimes, the magnetic field topology strongly influences the resulting disk structure, allowing the system to reach a steady state as either a thin disk with magnetic coronae or a magnetically elevated disk.  These simulations broadly align with observational findings — such as the soft states of X-ray binaries, ultraluminous X-ray sources, and “little red dots” — and offer predictive diagnostics for future observations, which I will discuss in detail during the talk. 

Dominika Durovcikova (MIT)

Title: Quasar lifetime constraints from IFU observations of the UV/optical nebular emission at z~6

Abstract: The billion-solar-mass black holes observed in the center of quasars within the first Gigayear of cosmic history have posed a challenge to our understanding of supermassive black hole (SMBH) growth. The problem is further exacerbated by a population of young quasars with extremely small proximity zones that have been identified at redshifts z > 6 — the imprint of their ionizing radiation on their rest-frame UV spectra seems to indicate that these quasars have grown their black holes in less than 100,000 years! If continuous, Eddington-limited accretion is assumed, it is extremely challenging to grow a billion-solar-mass black hole in such a short time. Hence, other mechanisms, such as super-Eddington accretion or obscured growth, have to be invoked to explain such rapid black hole growth. In this talk, I will present our recent progress on measuring quasar lifetimes using an independent lifetime measurement that promises to disentangle these two scenarios. We use VLT/MUSE as well as JWST/NIRSpec IFU observations to search for the rest-frame UV and optical extended nebular emission around young quasars, and measure their transverse extent to estimate the quasars’ lifetimes from their light travel time. This method yields complementary insights into the accretion history of these young quasars and, together with the proximity zone measurements, provides new insights to our understanding of SMBH growth in the early Universe.