RESEARCH

The circumgalactic medium (CGM) is the diffuse gaseous halo surrounding galaxies, with a baryonic mass comparable to the stellar mass of the galaxy itself. As the interface between the interstellar medium (ISM) and the intergalactic medium (IGM), the CGM regulates the baryon cycle that drives galaxy evolution — gas accretion fuels star formation, while feedback-driven outflows enrich and heat the surrounding halo.

The physical conditions of the CGM span more than four orders of magnitude in temperature, from the cool (∼10⁴ K) phase traced by UV absorption to the hot (>10⁶ K) phase revealed in X-ray emission. These multiphase properties are probed with complementary observational techniques: UV/optical absorption spectroscopy, radio HI 21cm mapping, X-ray imaging, and the Sunyaev–Zeldovich effect. Together, these probes constrain the density, temperature, kinematics, and thermodynamics of diffuse gas across the full range of physical conditions — from the gas accreting onto galaxies to the feedback-driven outflows that enrich the surrounding halo.

My research characterizes the multiphase CGM through four interconnected themes: (1) mapping the hot phase with X-ray and Sunyaev–Zeldovich observations, (2) probing the cool and warm phases with UV absorption spectroscopy and HI 21cm emission, (3) constraining the thermodynamics and turbulent kinematics that determine the fate of halo gas, and (4) developing models to connect these observations into a coherent physical picture of the baryon cycle. A full list of publications can be found on:

NASA ADS · Google Scholar · ORCID

The Hot CGM in X-ray and SZ

During galaxy formation and evolution, the gravitational energy released by the collapse of dark matter halos and the energy injected by feedback processes can significantly heat the CGM. (1) X-rays can be used to observe the hot gas in nearby galaxies. However, due to the limitations of current instruments, there is still considerable uncertainty in current research. The HUBS mission will greatly enhance X-ray observation capabilities. (2) The Sunyaev-Zel'dovich (SZ) effect, caused by the inverse Compton scattering of hot electrons on the Cosmic Microwave Background (CMB), can also be used to detect hot gas, complementing X-ray observations.

Upper panel: eROSITA all-sky X-ray map

Lower panel: Milky Way OVII emission map in XMM-Newton (Pan, Qu et al. 2024, X-LEAP I)

Relevant Works:

The XMM-Newton Line Emission Analysis Program (X-LEAP; PI: Zhijie Qu; led by Zeyang Pan): surveying the X-ray background emission and focusing on the solar wind charge exchange (SWCX) and the hot CGM of the Milky Way. Program design (X-LEAP I); temperature structures in the MW hot CGM (X-LEAP II); the solar cycle temporal variation of SWCX (Qu et al. 2022a).
An XMM-Newton View of the Andromeda Galaxy as Explored in a Legacy Survey (New-ANGELS; PI: Jiang-Tao Li; led by Rui Huang): mapping the hot CGM and X-ray stellar population of the Andromeda Galaxy (Huang et al. 2023).
The discovery of the hot bridge connecting the MW and M31 in the Local Group using the X-ray and SZ observations (Qu et al. 2021).
Resolving the SZ signal in the hot halo of nearby L* galaxies and low-mass galaxy groups (Bregman, Hodges-Kluck, Qu et al. 2022; Pratt, Qu et al. 2021; 2024).
Mapping the hot CGM in progress with Einstein Probe (EP), Diffuse X-ray Explorer (DIXE), and Hot Universe Baryon Surveyor (HUBS).

The Cool and Warm CGM Probed in Absorption and Emission

The cool gas, characterized by its higher density, could be accreted onto galaxies and is physically correlated to star formation. The cool gas could be directly accreted from intergalactic medium, condensed from the hot CGM, or ejected from central galaxies. (1) UV/optical high-resolution absorption spectroscopy provides the currently most sensitive tool to measure the cool CGM out to the virial radius (also see CUBS). (2) Integral field spectrographs can obtain the spatial distribution and velocity fields of the cool gas. (3) Radio HI 21cm emission line observations probe the densest cool gas in the CGM with the most sensitive telescope FAST (see FEASTS).

Figure: The cool gas in the M81/M82 group (Chen & Zahedy 2025).

Relevant Works:

The Cosmic Ultraviolet Baryon Survey (CUBS) and the Circumgalactic Observations of Nuv-shifted Transitions Across Cosmic Time program (CONTACT; PIs: Hsiao-Wen Chen, Gwen Rudie, and Sean Johnson): connecting the gaseous halo with galaxy evolution over the past 10 billion year (CUBS VI, the cool CGM at z=1; CUBS VII, the warm-hot CGM traced by OVI and NeVIII; CUBS IX, the CGM of dwarfs).
The FAST Extended Atlas of Selected Targets Survey (FEASTS; PI: Jing Wang): extending the HI 21cm detection to the circumgalactic space with the most sensitive FAST (Wang et al. 2025, HI column density profile down to 10¹⁸ cm^-2); HI radial profile of edge-on galaxies (Yang et al. 2025); high-definition case study of M51 (Lin et al. 2025).
The FUSE Legacy Absorption Survey (PIs: Hsiao-Wen Chen and Zhijie Qu): surveying the cool and warm-hot gas in the local Universe, implemented with multiwavelength observations (Qu et al. in prep.).
The Quasars to Understand Environments around, and STar formation (QUEST Dwarfs; PIs: Ava Polzin, Zhijie Qu, and Erin Boettcher): connecting the cool and warm gas in the circumgalactic space with star formation in low-redshift dwarfs (Polzin et al. in prep.; also see Qu et al. 2022b).
Affordable Multiple Aperture Spectroscopy Explorer (AMASE; PI: Renbin Yan; Co-chairs of CGM WG: Hsiao-Wen Chen and Zhijie Qu): mapping Ha, Hb, [N II], and [O III] of the CGM.

The Thermodynamics in the CGM

The thermodynamics in the CGM determine the ultimate fate of the gas -- whether it will be accreted, ejected beyond the halo, or remain stably within the halo. In particular, non-thermal processes, including magnetic fields, cosmic rays, turbulence, and inflows/outflows, could play significant roles in relevant processes. (1) Absorption spectroscopy can provide strong constraints on gas density, temperature, and non-thermal broadening. (2) The velocity fields observed by integral field spectrographs also offer strong constraints on non-thermal motions within the CGM.

Figure: M51 FEASTS HI 21cm map showing column density, bulk motion, and velocity dispersion (Lin, Wang et al. 2025).

Relevant Works:

Thermal-to-non-thermal energy ratio in cool CGM absorbers constrains pressure support (CUBS V); the 3D velocity structure function reveals subsonic Kolmogorov turbulence at ∼1 pc–1 kpc (Chen, Qu et al. 2023).
A continuous Kolmogorov-like turbulent cascade spanning ∼1 pc to ∼100 kpc, traced by H I and O VI absorption and driven by accretion in the halo outskirts (Qu et al. 2026).
Constraining the turbulence field using the velocity structure functions for QSO nebulae (Chen et al. 2023; 2024; Liu et al. 2025).
Resolving the velocity field in the warm gas interface between ISM and CGM in the MW (Qu et al. 2019; 2020; 2022c).
Probing the He II re-Ionization ERa via Absorbing CIV Historical Yield (HIERACHY; PI: Jiang-Tao Li): program design (Li et al. 2025); a census of CIV at z~3-5 (Yu, Qu et al. 2025).

Modeling the Multiphase CGM

Understanding the multiphase CGM requires the development of models and simulations. These models can be categorized into several types: establishing the connection between CGM and galaxy properties, constraining physical properties from observations, and building associations between different phases of gas.

Figure: an illustration of the multiphase CGM (Chen & Zahedy 2025).

Relevant Works:

Connecting the radiative cooling in the gaseous halo with the star formation in galaxies (Qu & Bregman 2018a; 2018b).
Deciphering the 3D properties of cool clouds in the CGM from the projected absorption properties (Yang, Qu et al. 2025).