Cool Circumgalactic Medium (CGM)

• Cool CGM is the multiphase, photoionized gas at temperatures ~10^4–10^5 K in galactic halos that plays a critical role in the baryon cycle.
• It features clumpy cloud structures with declining density profiles and a bimodal metallicity distribution, reflecting both cosmic inflows and feedback processes.
• The dynamics of the cool CGM are driven by gas accretion, outflows, and turbulence, which collectively influence galaxy fueling and evolution.

The cool circumgalactic medium (CGM) consists of multiphase gas in galactic halos with characteristic temperatures $T\sim10^4$–$10^5$ K. This phase holds a significant fraction of galactic baryons and acts as the regulating interface between galaxies and their environments, governing accretion, star-formation fueling, feedback, and the baryon cycle on scales from kiloparsecs to the virial radius. Its presence is recorded via strong absorption and emission signatures, reveals marked clumpiness, and exhibits a metallicity distribution—enriched and primordial—reflecting both feedback and cosmic inflows.

1. Physical State and Structure

The cool CGM is characterized by efficient radiative cooling and is predominantly photoionized, typically at $T\sim10^4$ K, though collisional ionization or mixing layers can produce phases at $T\sim(2–5)\times10^4$ K. Hydrogen densities in cool clumps span $n_\mathrm{H}\sim10^{-4}$–$10^{-1}$ cm$^{-3}$; inside $0.3\,R_{\rm vir}$, a typical value is $n_\mathrm{H}\sim10^{-2}$ cm$^{-3}$ (Chen et al., 13 Dec 2024).

Radially, the cool CGM density obeys declining power-law or $\beta$-model profiles. For $L^*$ halos, a representative form is

$$n_\mathrm{H}(r) = n_0 \left(\frac{r}{R_{\rm vir}}\right)^{-\alpha},\quad n_0\sim10^{-2}\ \mathrm{cm}^{-3},\quad \alpha\sim2.0\text{--}2.5$$

or, explicitly as a $\beta$-model,

$$n_{\rm H}(r) = \frac{n_{\rm H,0}}{A}\left[1 + (r/r_{\rm core})^2\right]^{-3\beta_c/2}$$

with best-fit $$log\,n_{\rm H,0}/{\rm cm^{-3}} = -2.57^{+0.43}_{-0.25}$$ and $\beta_c=0.63^{+0.16}_{-0.20}$ at $r_{\rm vir}$ (Yang et al., 3 Mar 2025).

The cool phase typically occupies only $1\%$–$2\%$ of the CGM volume but can dominate the baryon budget in $M_h\sim10^{12}\,M_\odot$ halos, with inferred masses $M_\mathrm{cool}\sim 10^9$–$5\times10^{10}\,M_\odot$ (Faerman et al., 2023, Chen et al., 13 Dec 2024). The clumpy structure is encoded in the cloud number density profile (e.g., $$n_{\mathcal{N}_{cl},0}\sim6\times10^4\,{\rm kpc}^{-3}$$ at $r_{\rm vir}$, $\beta_N\sim0.65$) and an intrinsic density dispersion $\sigma_{n_H}\sim0.6$ dex (Yang et al., 3 Mar 2025).

2. Ionization, Metallicity, and Covering Properties

The dominant ionization mechanism is photoionization by the metagalactic ultraviolet background (e.g., Haardt & Madau 2012). This maintains low- to intermediate-ion metal species (e.g., Mg II, Si II, C II) with column densities $N\sim10^{13}$–$10^{14.5}$ cm$^{-2}$ for CII, SiII, and $N_\mathrm{Mg\,II}\sim10^{12.5}$–$10^{13.5}$ cm$^{-2}$, with associated HI columns $N_\mathrm{HI}\sim10^{16}$–$10^{17}$ cm$^{-2}$ (Faerman et al., 2023, Chen et al., 13 Dec 2024).

Metallicity in the cool CGM is strikingly bimodal at $z<1$, with peaks at $[{\rm X/H}]\simeq-1.6$ (metal-poor, $\sim2.5\%\,Z_\odot$) and $[{\rm X/H}]\simeq-0.3$ (metal-rich, $\sim50\%\,Z_\odot$). Both reservoirs have comparable mass and incidence, and trace metal-poor cosmic inflow (cold streams) and enriched outflows/recycling, respectively (Lehner et al., 2013). At $z\sim2$, cool CGM around quasar hosts is typically enriched $Z\gtrsim0.1\,Z_\odot$ and holds a metal mass $M_{Z}^{\rm cool}>10^8\,M_\odot$ out to $r_{\rm vir}$ (Prochaska et al., 2012, Prochaska et al., 2014).

Covering fractions for low-ion absorbers are high: $\kappa_{\rm Mg\,II}\sim80\%$ at $d<0.5\,R_{\rm vir}$ around $L^*$ star-forming galaxies, though this drops to $20\%$ in $10^{13}\,M_\odot$ quiescent halos (Chen et al., 13 Dec 2024). At $z\sim2$, the covering fraction of optically thick cool gas within $r_{\rm vir}$ is $f_c=0.64^{+0.06}_{-0.07}$ (Prochaska et al., 2012).

3. Cloud Morphology, Kinematics, and Survival

Observational and modeling constraints indicate cool CGM absorbers are morphologically clumpy, with absorber or coherence lengthscales $\ell_A > 1.9$ kpc for Mg II (Rubin et al., 2018). Cloud masses typically span $10^3$–$10^6\,M_\odot$, with a preferred $M_{\rm cl}=10^4\,M_\odot$ matching absorber statistics (Yang et al., 3 Mar 2025). For $f_V \sim 1\%$ and to reproduce $\sim3$ kinematic components per sightline, cloud sizes must be $R_{\rm cl}\lesssim0.5$ kpc (Faerman et al., 2023).

Cloud kinematics indicate sub-virial internal velocity dispersions ($\sigma\sim25$ km/s) and line-of-sight spread consistent with the observed profiles of low-ion absorbers (Yang et al., 3 Mar 2025). Large multiphase turbulence simulations show the cold-gas mass fraction ($f_{\rm cold}$) is controlled by the ratio $\tau = t_{\rm cool}/t_{\rm mix}$, with $f_{\rm cold}\sim \tau^{-1}$ for $\tau\gtrsim2$; volume filling is always small ($V_{\rm cold}/V_{\rm tot} \lesssim 1\%$) but area covering can be high ($A_{\rm cold}/A_{\rm tot} \sim 40$–$80\%$) (Mohapatra et al., 31 Oct 2025).

Survival time for larger clouds ($R\gtrsim250$ pc) exceeds 250 Myr, with thermal conduction and radiative cooling acting to suppress hydrodynamic instabilities and prolong cloud lifetimes. Destruction rates for clouds are dictated by a balance of ablation, ram pressure, and conduction, with small clouds evaporating rapidly ($t_{\rm evap}\lesssim100$ Myr), but massive/large clouds persisting over Gyr timescales (Armillotta et al., 2016, Lan et al., 2018).

4. Mass, Radial Distributions, and Evolution

For $L^*$ halos at $z\sim0.2$, the cool CGM mass is typically $M_\mathrm{cool} \sim 3\times10^9$–$5\times10^{10}\,M_\odot$ (Faerman et al., 2023, Chen et al., 13 Dec 2024, Yang et al., 3 Mar 2025), consistent with cosmological and semi-analytic models. In high-mass halos (LRGs, $M_h\sim10^{13}\,M_\odot$), $M_\mathrm{cool}\lesssim10^{10}\,M_\odot$ (Chen et al., 13 Dec 2024, Chang et al., 14 May 2024). Redshift evolution is marked by a global decrease in cool-phase mass fraction ($44\% \rightarrow 17\%$) and increase in metallicity ($Z_{\rm cool}\sim3\times10^{-2}$ at $z=0$) (Huscher et al., 2020, Chen et al., 13 Dec 2024).

Column densities and equivalent width profiles decline steeply with impact parameter, e.g.,

$$\langle N(\mathrm{HI})\rangle \sim 10^{16.5}\,\mathrm{cm}^{-2}\, (d/R_{\rm vir})^{-3}$$

with profiles flattening and converging into the IGM regime at $\sim R_{\rm vir}$ (Chen et al., 13 Dec 2024, Harvey et al., 27 Jun 2025).

5. Gas Cycling: Accretion, Outflows, Feedback

The cool CGM is dynamically maintained by a balance of accretion, condensation, outflow, and internal feedback. For $M_\mathrm{cool} \sim 3\times10^9\,M_\odot$ and $t_{\rm ff} \sim 2.3$ Gyr, the nominal accretion rate is $\dot{M}_{\rm in} \sim 3\,M_\odot\,\mathrm{yr}^{-1}$, comparable to typical SFRs (Faerman et al., 2023). Contributions from IGM accretion, condensation from the hot halo, and galactic outflows can replenish the cool phase at rates $\gtrsim10\,M_\odot\,\mathrm{yr}^{-1}$, enabling a quasi-steady state over Gyr timescales.

Outflows can enrich the CGM over $\sim1$ Mpc, especially in post-starburst galaxies, which exhibit strong Mg II absorption excesses and kinematic evidence for $v\sim1000$ km/s cool gas ejection (Harvey et al., 27 Jun 2025). In feedback-rich environments, cool gas is observed to persist even in the presence of radio-mode AGN feedback, which appears ineffective at evacuating or disrupting the cool CGM for $M_h\sim10^{13}\,M_\odot$ halos (Chang et al., 14 May 2024).

Cosmic-ray and magnetohydrodynamic (MHD) effects provide additional support for cloud survival and acceleration. CR streaming pressure can drive cloud velocities up to hundreds of km/s if the CR energy input is sufficiently strong, particularly in starbursts (Wiener et al., 2019). MHD turbulence suppresses small-scale mixing, raising cold mass fractions and extending cloud lifetimes (Mohapatra et al., 31 Oct 2025).

6. Observational Diagnostics and Modeling

Direct probes of the cool CGM include:

• Absorption: HI, Mg II, C II, Si II, Fe II, Ca II transitions trace the cool phase, with covering fraction and equivalent width statistics tightly mapped to CGM mass and structure (Chen et al., 13 Dec 2024, Faerman et al., 2023).
• Emission: H$\alpha$, [O II], [O III], and resonant Ly$\alpha$ allow for spatial mapping, though surface brightness sensitivity is limiting (Chen et al., 13 Dec 2024).
• Scattering: Radio-wave and fast radio burst (FRB) scattering provide a direct probe of sub-pc scale cloudlets and the overall volume filling factor (Vedantham et al., 2018).

Contemporary models employ semi-analytic, clumpy, or two-phase prescriptions, matching constraint sets extracted from absorption line datasets (COS-Halos, BOSS, PRIMUS, DESI). Key model parameters include cloud mass function, injection rates (inflow, outflow, or in situ), pressure equilibrium, and evaporation efficiency (Lan et al., 2018, Faerman et al., 2023, Yang et al., 3 Mar 2025, Afruni et al., 2019). Multiphase turbulence simulations identify the local ratio $\tau = t_{\rm cool}/t_{\rm mix}$ as the crucial determinant of cold-gas mass fraction and spatial structure (Mohapatra et al., 31 Oct 2025).

7. Role in Galaxy Evolution and Open Questions

The cool CGM mediates galaxy fueling—cold accretion streams contribute directly to the ISM and star formation. Simulations and data agree that “cold mode” ($T\sim10^4$ K) accretion is prevalent in $M_h\lesssim10^{12} M_\odot$ halos at high $z$, while hot halos rely on condensation and recycled gas at low $z$ (Chen et al., 13 Dec 2024, Huscher et al., 2020).

Key outstanding issues include:

• The detailed mechanisms stabilizing clouds against destruction at $R\gtrsim1$ kpc scales.
• The regulation and cycling of baryons across feedback, accretion, and recycling pathways.
• The interplay of non-thermal pressure components (magnetic fields, cosmic rays, turbulence) with the overall phase structure.
• The true cloud size distribution (from sub-pc “fog” to kpc structures) and how this governs both observational signatures and cloud fate (Vedantham et al., 2018, Rubin et al., 2018, Mohapatra et al., 31 Oct 2025).
• The efficiency and timescales of gas accretion onto the inner galaxy from an extended, potentially massive cool CGM—particularly in passive systems where the cool phase can persist at large radii but is prevented from feeding the ISM (Afruni et al., 2019).

In summary, the cool circumgalactic medium is a dynamically active, clumpy, multi-metallicity reservoir whose structure, mass, and evolution reflect a complex balance of inflow and outflow, accretion and feedback, and turbulence on a variety of scales. It is central to understanding galaxy growth, the baryon cycle, and the chemical enrichment of both galaxies and the surrounding IGM/ICM.