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The circumgalactic medium (CGM) is the gaseous halo of baryons surrounding galaxies. The mass of this gaseous halo is comparable to the stellar mass within the galaxy itself. Because it plays a crucial role in both gas accretion and feedback processes, it is fundamental in galaxy evolution. The density and temperature of this gas exhibit significant variations within the dark matter halo from the cool gas at 10⁴ K to the hot gas over 10⁶ K. Therefore, multiple observational techniques are required to fully characterize the properties of this diffuse gas.

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 targeting on the foreground solar wind charge exchange (SWCX) and the hot CGM of the Milky Way (X-LEAP I, program design; X-LEAP II, temperature structures in the MW hot CGM; Qu et al. 2022a, the solar cycle temporal variation of SWCX).
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).

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).

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

Relevant Works:

The Cosmic Ultraviolet Baryon Survey (CUBS; PI: Hsiao-Wen Chen): 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).
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).

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.

Right panel: the cool gas in the M81/M82 group (Lin, Wang et al. 2025) .

Relevant Works:

Extracting the temperature and non-thermal motion in absorption features (CUBS V, thermal energy ratio in the cool CGM; Chen, Qu et al. 2023, 3D velocity structure function of turbulence in the absorbing gas).
Constraining the turbulence field using the velocity structure functions for QSO nebulae (Chen et al. 2023; 2024).
Resolving the velocity field in the warm gas interface between ISM and CGM in the MW (Qu et al. 2019; 2020; 2022c).

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.

Right panel: 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. submitted).