Saiyang Zhang & Lizhong Zhang

Talk 1: Salyang Zhang, 3:00pm - 3:30pm
The imprint of primordial black holes over cosmic history
My presentation will discuss the role of Primordial Black Holes (PBHs) as one of DM components throughout cosmic history. I will specifically focus on PBH candidates in the solar mass range of approximately 10-100 $M_{\odot}$ and massive PBHs of $10^6 M_{\odot}$ as possible seeds for supermassive black holes (SMBHs).

Our research utilizes hydrodynamical and N-body simulations with the GIZMO code and semi-analytical models to investigate PBHs' impact. We find that stellar-mass PBHs, ranging from 10-100 $M_{\odot}$ and constituting a fraction of $10^{-4}$ to $0.1$ of DM, subtly influence the formation of the universe’s first stars by maintaining the standard model of star formation, while their accretion feedback shifts star formation to more massive halos. Additionally, PBHs significantly contribute to the cosmic radiation background during reionization, but do not violate existing constraints on the timing of reionization. On the other hand, more massive PBHs with $10^6 M_{\odot}$, could seed massive halos and disrupt hierarchical structure formation by engulfing newly formed halos. In contrast to the effects from stellar-mass PBHs, our recent studies reveal that the narrative of early star formation is complicated by initial perturbations and accretion feedback from these massive PBHs at very high redshifts. Our findings also suggest that PBHs provide a viable explanation for early SMBHs like UHZ-1. Future observations with JWST, Roman, and SKA will be critical in distinguishing PBH-driven structure formation from standard $\Lambda$CDM scenarios.

Talk 2: Lizhong Zhang, 3:30pm - 4:00pm
Modeling black hole accretion with radiation GRMHD: a parameter survey and connection to observations
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.