Zhihui Li

About Me

Hello! I'm Zhihui, a 6th-year astronomy graduate student at Caltech.

I am an astronomer dedicated to bridging the gap between cutting-edge theoretical models and groundbreaking observational data. My research has been focused on studying the "atmospheres" of galaxies, namely the circumgalactic medium (CGM). The CGM is a vast, diffuse region of gaseous matter beyond the stars and interstellar medium of a galaxy and within the virial radius of its dark matter halo. It plays a pivotal role in governing crucial aspects of galaxy evolution, such as star formation, chemical enrichment, and feedback mechanisms. To better understand the properties of the CGM, I have utilized state-of-the-art numerical simulations, developed novel semi-analytic models, and conducted thorough analyses by comparing them with high-resolution, spatially-resolved observational data acquired from the world's largest ground-based telescopes.

In my spare time, I enjoy playing and watching basketball, reading, listening to music and spending time with my family. Feel free to shoot me an email if you are interested in my research, or astronomy in general!

Research

In Li et al. (2020), I conducted an extensive analysis of a comprehensive suite of idealized CGM simulations. I explored a wide range of cloud parameters, including cloud size, velocity, ambient temperature, and density. I also identified five distinct physical regimes, each corresponding to different fates for the clouds, ranging from their destruction due to instabilities induced by the hot ambient medium to their survival and growth through the accretion of cooled material onto their surfaces. Furthermore, I derived and calibrated an empirical scaling relation between the cloud lifetime and various cloud physical parameters, which has been widely employed and extensively discussed in numerous follow-up studies (e.g. Kanjilal et al. 2021; Farber & Gronke 2022; Abruzzo et al. 2022a,b, 2023). I concluded that radiative cooling and conduction are the most important physics that affect cloud survival, whereas magnetic fields, turbulence, and viscosity are likely to have a minor effect.

Lyα emission line is one of the most powerful tools for studying the CGM due to its luminous nature. However, Lyα emission is a resonant transition with a large cross-section, rendering its radiative transfer (RT) process notoriously difficult to model. Traditionally, the prevailing model for Lyα radiative transfer is the "shell model," which is a spherical, expanding or contracting shell of neutral hydrogen (HI). It is motivated by the observational evidence that starburst-driven galactic winds can produce such HI shells. Although the shell model has managed to reproduce the Lyα profiles of a wide variety of Lyα-emitting objects, recent works (e.g. Orlitova et al. 2018) have reported several significant discrepancies between the best-fit parameters derived from the shell model and additional observational data. These discrepancies arise from the overly simplistic nature of the shell model - a monolithic shell of HI with a single column density and outflow velocity that is inadequate for describing a multiphase, clumpy CGM. Therefore, in Li & Gronke (2022), I systematically explored the connection and difference between a more physically realistic multiphase, clumpy model and the shell model. I elucidated how the best-fit parameters derived from shell model fitting should be interpreted as certain properties of a multiphase, clumpy medium.

In Li et al. (2021) and Li et al. (2022), I pioneered the modeling of spatially-resolved Lyα emission spectra obtained by the Keck Cosmic Web Imager (KCWI) in SSA22 Lyα Blob 1 and 2, two of the earliest discovered Lyα blobs (LABs). These enigmatic objects are giant gaseous nebulae with immense Lyα luminosities at high redshifts. The physical origins of LABs remain a subject of intense debate, with various hypotheses proposed, including photo-ionization by central energetic sources, starburst-driven galactic outflows, cooling radiation from the accretion of cold gas streams, among many others. By modeling the spatially-resolved Lyα emission line profiles observed at different locations within LAB1 and LAB2, I found that for both LABs, the dominant powering mechanism involves central sources producing Lyα photons that are subsequently scattered outward by high-velocity galactic outflows.

More recently, in Erb et al. (2023), I developed a new approach to self-consistently reproduce the spatially-extended Lyα emission originating from the CGM of twelve extreme emission line galaxies at z ~ 2. Our deep KCWI observations unveiled a striking pattern among these objects - they typically exhibit double-peaked Lyα profiles that span their entire Lyα halos, characterized by three intriguing trends: as the distance from the galactic center increases, the flux ratio of the blue peak to the red peak increases, the separation in velocity between the two peaks decreases, and the flux at the line center increases. By separating all the scattered Lyα photons in the multiphase, clumpy model into three radial bins according to their escaping impact parameters and comparing them with their corresponding observed Lyα profiles, I managed to find the best-fit RT model that simultaneously reproduces the Lyα spectra observed at three different impact parameters and all three radial trends. Furthermore, I found that all three radial trends of spatially-resolved Lyα emission can be explained by considering the different RT behaviors of photons in the inner and outer regions of the halo. This work stands as the first successful attempt to self-consistently reproduce spatially-varying Lyα emission via comprehensive RT modeling within the framework of a physically realistic CGM model.

In addition to Lyα emission, low-ionization state (LIS) metal absorption lines such as SiII λ1260 and CII λ1334 also serve as valuable tracers of the cool gas in the CGM due to their similar ionization potential. In Li et al. (2023), I independently developed a novel semi-analytic model named ALPACA to simulate the LIS metal absorption line profiles originating from a clumpy CGM. Traditional models for analyzing the LIS absorption lines primarily adopt empirical or phenomenological approaches; in contrast, ALPACA is a physically motivated model that properly accounts for the interaction between the photons and the turbulent, outflowing, and clumpy gas in the CGM. It not only rapidly and reliably predicts absorption line profiles for a wide range of cool gas parameters, but also serves as a powerful tool for fitting observed absorption line profiles and extracting the underlying properties of the cool gas. In the paper, I presented the general framework of the ALPACA model and showcased its successful application in modeling the CII λ1334 absorption line profiles of star-forming galaxies at z ~ 2 - 3. My modeling with ALPACA revealed an intriguing picture of the cool gas in the CGM: the absorption observed at a particular velocity stems from non-volume-filling clumps that simultaneously exhibit outflowing and random motion across a considerably broad range of radii.

Papers

The full list of my publications is available at ADS and Google Scholar.

Teaching

I have served as a teaching assistant for the following courses at Caltech:

Contact

Elements

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