Multiphase Circumgalactic Medium (CGM)

• Multi-phase CGM is the extended gaseous reservoir around galaxies, featuring cold, warm, and hot gas with distinct temperatures, ionization states, and metallicities.
• Observational diagnostics such as absorption-line spectroscopy, integral field emission, and CO mapping reveal its complex kinematics, spatial distribution, and metal gradients.
• Cosmological simulations and combined analyses demonstrate the CGM’s critical role in galaxy fueling, feedback processes, and addressing the missing baryon problem.

The multiphase circumgalactic medium (CGM) is the extended, complex gaseous environment surrounding galaxies out to and beyond their virial radii. It is characterized by the coexistence of distinct phases—cold, warm, and hot gas—with fundamentally different thermodynamic properties, ionization states, spatial distributions, kinematics, and origins. Recent advances in integral field spectroscopy, millimeter-wave interferometry, and cosmological hydrodynamical simulations have established the CGM as a central component of the baryon cycle, capturing both accretion from the intergalactic medium (IGM) and feedback-driven outflows from the galaxy. The multi-phase nature of the CGM encodes the interplay between galaxy fueling and quenching, the partitioning of metals and baryons, and the unresolved "missing baryon" problem in galaxy halos.

1. Phase Structure and Thermodynamic Properties

The CGM comprises multiple phases defined primarily by temperature, density, and characteristic tracers:

Phase	Temperature	Density (n_H)	Tracers	Typical Metallicity (Z/Z_⊙)
Cold	$10^2$–$10^4$ K	$10^{-2}$–$10^{-1}$ cm$^{-3}$	H I 21 cm, Mg II, Ca II, CO, [C II]	0.1–1
Cool	$10^4$–$3\times10^4$ K	$10^{-3}$–$10^{-1}$ cm$^{-3}$	H I Lyman series, C II, Si II, Mg II	0.1–1
Warm	$3\times10^4$–$10^5.5$ K	$10^{-4}$–$10^{-2}$ cm$^{-3}$	C III, Si III, C IV, Si IV	0.1–0.5
Hot	$10^{5.5}$–$10^{6.5}$ K	$10^{-5}$–$10^{-4}$ cm$^{-3}$	O VI, N V, Ne VIII, X-ray lines	0.1–0.3

Each phase displays distinct physical and chemical properties, with significant spatial and kinematic overlap. In case studies and simulations, volume filling factors of the cold/cool phase are $\lesssim 1\%$, while the hot phase dominates volumetrically but may contain only half the total mass in L* halos (Liang et al., 2017, Ramesh et al., 2022, Liang et al., 2017). The hot and cold phases typically maintain approximate pressure equilibrium: $$P_{\rm cold} = n_{\rm cold} k_B T_{\rm cold} \simeq n_{\rm hot} k_B T_{\rm hot} = P_{\rm hot}$$ with $P\sim 10^2$ K cm$^{-3}$ near 0.2–1 $R_{\rm vir}$ (Ramesh et al., 2022).

2. Observational Diagnostics and Methodologies

Multi-phase CGM studies exploit a diversity of diagnostics:

• Absorption-line spectroscopy: Quasar sightlines reveal multiphase structure via saturated and unsaturated profiles of H I, Mg II, C II, Si II, C IV, Si IV, O VI, and beyond. The Voigt profile analysis of metals at $z=0.313$ toward Q1130–1449 indicates metal absorption over $\sim 250$ km s$^{-1}$, with associated galaxies detected at impact parameters $b=10.6$–131.3 kpc (Peroux et al., 2019).
• Emission-line integral field spectroscopy: MUSE 3D spectroscopy maps [O II], H β, [O III], [N II], and H α emission at high sensitivity ($\gtrsim$0.01 $M_\odot$ yr$^{-1}$ SFR), allowing spatial and kinematic association between ionized gas and absorption-selected systems (Peroux et al., 2019). H α emission traces regions of active star formation and the ionized CGM.
• Millimeter CO and atomic fine-structure lines: ALMA observations of CO(1–0) provide robust measurements of CGM molecular reservoirs, yielding high $M_{\rm H_2}$ with long depletion timescales, indicative of inefficiency in star-formation fuel processing in the outskirts or group environment (Peroux et al., 2019, Cicone et al., 2019).
• Simulations and forward modeling: Cosmological zoom-in (RAMSES) simulations achieve AMR resolution $<400$ pc, allowing physical modeling of CGM filamentary structure, embedded group environments, tidal debris, and prediction of low-surface-brightness features far below emission-line sensitivity limits (Peroux et al., 2019).

3. Spatial Distribution, Kinematics, and Environmental Dependence

Extensive spectroscopic and simulated mapping has established common spatial and velocity structures:

• Group environments and galaxy associations: At $z=0.313$, the CGM is not attributable to a single host but is instead embedded in a small, dynamically bound group ($M_\mathrm{vir} \sim 3\times10^{12}\ M_\odot$, $R_\mathrm{vir}$ ≈ 420 kpc) with at least 11 galaxies. Kinematic offsets between member galaxies and the absorbing system span –180 to +140 km s$^{-1}$, comparable to the overall DLA metal-line velocity width (–200 < $v$ < +50 km s$^{-1}$) (Peroux et al., 2019).
• Gas structures: The existence of tidal debris, filaments, and faint intra-group gas bridging member galaxies is observed directly in MUSE white-light images and predicted in RAMSES simulations—these features frequently escape direct emission detection but dominate HI absorption cross-section at column densities $\log N({\rm H\,I})\gtrsim 21.7$ (Peroux et al., 2019).
• CGM extent: Ionized, neutral, and molecular phases can extend to at least 300–400 kpc, with the strong HI DLA cross-section partially attributed to extremely faint ($\mu_{\rm SB} \lesssim 10^{-18}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$) phases only revealed by quasar absorption (Peroux et al., 2019, Cicone et al., 2019).

4. Quantitative Phase Budget and Chemical Composition

Combined emission and absorption analyses yield robust phase mass estimates and metallicity mapping:

• Hydrogen phases:
  • Neutral: $\log N({\rm H\,I}) = 21.71 \pm 0.07\ {\rm cm}^{-2}$ per sightline; total M$_{\rm HI} \sim 10^{10}$–$10^{11}$ $M_\odot$ over the group (Peroux et al., 2019).
  • Ionized: Compact emission knots ($M_{\rm HII} \sim 10^9\ M_\odot$) and diffuse ionized CGM ($M_{\rm HII} \sim$ several $10^{10}\ M_\odot$), with surface brightness well below current emission detection limits.
  • Molecular: For three ALMA-detected galaxies, $M_{\rm H_2} \approx (1$–$3)\times10^{10}\ M_\odot$ and depletion times $>1$ Gyr.
• Metallicity gradients:
  • Star-forming galaxies show $12+\log({\rm O/H}) = 8.10$–8.75 ($Z = 0.25$–0.85 $Z_\odot$), whereas the absorber metallicity from Zn II is $[{\rm Zn/H}] = -0.80 \pm 0.16$ ($Z_{\rm abs} \approx 0.16\ Z_\odot$), indicating non-coincident neutral and star-forming phases—neutral gas is often chemically and spatially decoupled from known galaxy disks (Peroux et al., 2019).
  • Kinematic and metallicity distinctions suggest the DLA predominantly traces diffuse, low-surface-brightness group CGM not directly associated with a single galactic ISM.

5. Modelling, Simulations, and Scaling Relations

High-resolution zoom-in simulations reproduce the observed multi-galaxy and multi-phase CGM configuration:

• AMR and phase mapping: RAMSES simulations reveal that when surface brightness detection thresholds are lowered by $\sim$4 dex, the CGM reveals continuous, filamentary, faint H$\alpha$ structures pervading intra-group regions—analogous to the observed absorber’s inferred origin (Peroux et al., 2019).
• Physical conditions from modeling: Line emission and absorption allow empirical determination of SFR via

$$\mathrm{SFR}\ [M_\odot\,\mathrm{yr}^{-1}] = 7.9\times10^{-42}L_{\mathrm{H\alpha}}\, [\mathrm{erg\,s}^{-1}]$$

and molecular gas via

$$M_{\mathrm{H}_2} = \alpha_{\rm CO}L'_{\rm CO}, \qquad \textrm{with}~\alpha_{\rm CO} = 4.6\ M_\odot\, (\mathrm{K\,km\,s}^{-1}\mathrm{\,pc}^2)^{-1}$$

where $L'_{\rm CO}$ is the CO(1-0) luminosity.

• Covering factors: The fraction of the sky covered by cold phase at high $N($H I) is high in absorption, but emission-based surveys significantly underestimate the baryon budget unless ultra-faint limits are reached (Peroux et al., 2019, Cicone et al., 2019).

6. Implications for the Baryon Cycle and Galaxy Evolution

Multi-phase CGM observations and models challenge the one-galaxy:one-DLA paradigm—strong H I absorbers typically arise in a complex interplay of galaxies, group tidal debris, and diffuse filamentary structures, not in isolation (Peroux et al., 2019).

• Baryon closure: Accounting for faint, otherwise invisible CGM phases may ameliorate the missing baryon problem—significant baryonic mass is present in low-surface-brightness or highly ionized phases undetectable without absorption sightlines (Peroux et al., 2019, Cicone et al., 2019, Ramesh et al., 2022).
• Group dynamics: The observed mix of bright emission, faint neutral absorption, and substantial molecular gas suggests that most of the absorbing H I need not be directly associated with a single galaxy, but is distributed across interconnected group structures.
• Galaxy–CGM Coupling: The observed diversity in kinematics, metallicities, and depletion timescales reflects episodic accretion, slow fuel consumption, and complex feedback-regulated baryon cycling operating on a group and filament scale, not simply within progenitor galaxy halos.

7. Open Challenges and Prospects

• Sensitivity limits: Emission-based techniques are fundamentally limited in probing the full extent of the CGM baryon and metal budget due to low surface brightness; absorption studies remain crucial for inventorying the faintest and most diffuse phases.
• Phase coupling mechanisms: The precise spatial and dynamical relationships between molecular, ionized, and neutral phases—especially in intra-group environments—remain incompletely mapped, and require coordinated emission, absorption, and kinematic studies across multiple sightlines and wavelengths.
• Simulations and multiphase stability: The persistence of multi-phase structure, especially at modest velocity differences ($\Delta v \lesssim$200 km s$^{-1}$) and its dependence on resolution, feedback, and environmental context, must be further tested with large-volume, high-resolution simulations and compared against absorption statistics.

Collectively, combined MUSE, ALMA, and simulation analyses suggest that the multiphase CGM is a dynamically regulated, chemically complex baryonic reservoir, often dominated by low-brightness interfaces between galaxies, group-scale filaments, and tidal structures. Absorption-selected H I reservoirs are frequently signatures of a much more extensive and interconnected circumgalactic environment than is revealed by galaxy emission alone.