Yuan Li

Assistant Professor
Department of Physics, University of North Texas

CV publications LinkedIn

About Me

I am a theoretical/computational astrophysicist, but occasionally work on observational data as well. My research interests include galaxies, galaxy clusters, supermassive black hole accretion and feedback, star formation and stellar feedback, the interstellar medium, the intra-cluster medium and the circum-galactic medium.

I grew up in a small city of three million people in China, and obtained my PhD from Columbia University under the supervision of Prof. Greg Bryan. My thesis title is "AGN Feedback in Cool-Core Galaxy Clusters".

Please email me at yuan.li *at* unt.edu if you would like to get in touch. You can also find me in room 305 of the UNT Physics Building.

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. It shows both the ``positive'' and the ``negative'' sides of AGN feedback: the jets are triggering local thermal instabilities while driving volume-filling 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).

Turbulence in the Interstellar Medium Traced by Stars

The interstellar medium (ISM) is turbulent on all scales from the diffuse interstellar phases to the dense star-forming gas. My student Trung Ha and I recently 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-2 and Gaia DR2 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). UNT press release.
Watch us discuss the work on AAS Journal Author Series.

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).

Research Group

Current Graduate Students at UNT: Trung Ha

Current Undergraduate Students at UNT: Alia Naciri, Mirielle Caradonna

Current TAMS students at UNT: Rushi Yalamanchili, Audrey Hopkin, Anish Goel

Other Students I have worked closely with: Wesley Deloach (UNT), David Downing (UNT), Kendra Hamilton (UNT), Jeremy Gingrich (UNT), Daniel Rangel (UNT), Michael Jennings (UC Berkeley => JHU), Brockton Stover (UC Berkeley), Richard Pan (UC Berkeley=>Tufts University), Yihuan Di (SHAO), Corey Brummel-Smith (Georgia Tech), Cassandra Lochhaas (OSU => STScI), Yu Qiu (Georgia Tech => KIAA), Chaoran Wang (Umich => UCSB).

Teaching

Fall 2021: Physics 4980 Galaxies and Cosmology
Spring 2021: Physics 1210-002 Conceptual Physics

Outreach

Current Outreach Activities
I am a guest speaker of the Ask an Astronomer streaming program at UNT. More about the UNT Astronomy Outreach program.

Previous Outreach Activities
Astronomy on Tap Ann Arbor
I founded AoT Ann Arbor along with grad students Aleksandra Kuznetsova and Erin May, and UM Astronomy faculty Keren Sharon. Check out the official website, facebook page, and the AoT paper.

Columbia Astronomy Outreach Program
I was an active member of the Columbia Astronomy Outreach Program while working on my PhD. My public lecture on black holes was featured in Columbia Spectator . I also participated in “Family Astro”, which included hands-on activities with children, and “Sidewalk Astronomy” where we carried 6" Dobsonians to the Adam Clayton Powell Plaza in Harlem and introduced astronomy to people on the street. I was also a member of the Middle School Outreach Program organized by Prof. Mary Putman, which brought astronomy, in particular galaxies, into classrooms throughout New York City through in-class presentations. I also volunteered at various other one-time events including the Intrepid Sea, Air & Space Museum Space Shuttle Opening and the World Science Festival.