Yuan Li

Supermassive Black Hole Feedback and Star Formation in Cooling Flow Galaxy Clusters

AGN jets trigger thermal instabilities in the intracluster medium by uplifting lower entropy gas to larger altitudes (link to paper). The figure on the left is a composite mock X-ray image of my simulated cluster (link to paper). It shows both the ``positive'' and the ``negative'' sides of AGN feedback: the jets are triggering local thermal instabilities while driving shock waves that heat up the whole central region of the cluster (link to paper). The interplay between cooling, AGN feedback and star formation causes clusters to go through billion-year-long cycles (link to paper).
● Press release. ● Link to movies. ● A 3D visualization of the cold clumps.

Direct Detection of Black Hole-Driven Turbulence in the Centers of Galaxy Clusters

I use high-resolution ALMA and optical IFU data to study the kinematics of multi-phase gas in the centers of galaxy clusters. I measure the velocity structure function (VSF) of the filaments over a wide range of scales, and find that the motions of the filaments are turbulent. There is a clear correlation between features of the VSFs and the sizes of bubbles inflated by SMBHs, demonstrating that AGN feedback is the main driver of turbulence in cluster centers. The detection of turbulence near the mean free path suggests that isotropic viscosity is suppressed in the intracluster plasma. (link to paper).
Watch my 3 minute summary of the work here.
Media coverage: ● Quanta Magazine. ● LiveScience. ● Space.com.

The Fate of AGB Winds in Massive Galaxies and Clusters

Asymptotic Giant Branch (AGB) winds from evolved stars produce dust grains. Due to the fast stellar velocity, the wind is thought to form a comet-like tail, similar to Mira in the Local Bubble. I have carried out both analytical and numerical studies of the interaction between an AGB wind and the surrounding hot gas, and find that the cooling time of the tail is inversely proportional to the ambient pressure. In high pressure environments, some of the gas in the mixing layer between the stellar wind and the surrounding hot gas can cool efficiently. I speculate that instead of thermal instability, the induced condensation at the mixing layer of AGB winds may be the origin of cold filaments in massive galaxies and galaxy clusters. This naturally explains the existence of dust and PAH in the filaments. (link to paper).

Correlations between Black Holes and Host Galaxies

I have studied black hole - host galaxy correlations in the Illustris and TNG100 simulations. Both simulations are able to produce black hole scaling relations in general agreement with observations at z = 0, but with noticeable discrepancies. In Illustris, the hosts of over-massive SMBHs have formed earlier and have lower present-day star formation rates, in qualitative agreement with the observations for massive galaxies. The findings show that simulated SMBH scaling relations and correlations are sensitive to features in the modeling of SMBHs (link to paper).

ISM Turbulence in the Memory of Young Stars

My student Trung Ha and I developed a new method of probing the turbulent kinematics of the ISM using stars. We have analyzed the motions of young stars in the Orion Molecular Cloud Complex, using the full 6-dimensional measurements of positions and velocities provided by the APOGEE and Gaia surveys. Specifically, we compute the velocity structure functions (VSFs) of the stars in six different groups within the Orion Complex, and find that the motions of stars in all diffuse groups exhibit strong characteristics of turbulence. Our VSFs also show features supporting local energy injection from supernovae (link to paper). In (paper II), we compare turbulence traced by young stars and gas in several star forming regions in the Milky Way. We find that different tracers can show different levels of turbulence.
Watch us discuss our work on AAS Journal Author Series. Read more on UNT press release.

Thermal Instability in the Interstellar Medium

The interstellar medium (ISM) is thermally unstable and naturally develops different phases of gas as a result of heating and radiative cooling. Michael Jennings, an undergraduate student at UC Berkeley, investigates how thermal instability is affected by the initial perturbations and transport processes with me. The study is carried out using a set of high-resolution idealized numerical simulations. We examine the effects in both the linear and the non-linear phases of thermal instability, and how the cold dense clouds fragment or coalesce under different conditions. We find that transport processes, which are often ignored in simulations of galaxy evolution, have a huge impact on the final outcome of ISM thermal instability, and our study has interesting implications on the observed clouds in the ISM as well (link to paper).